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Mulch Chapter from Organic Management for the Professional Book

Chapter 7 - MULCH (From the book Organic Management for the Professional by Garrett, Amaranthus and Ferguson)

Walk into the woods or onto the prairies and look around; you will be in the presence of much life - plant and animal, large and small. Then look down; you will see an equal amount of death, many expired life forms covering the soil. You will find a mulch of dead things - twigs, leaves, grass, insects, manure, and even dead animals. Dig into the mulch and you will find it begging to decay—compost. The deeper you dig, the more advanced the decay until individual pieces fade into rich moist topsoil. Topsoil is the digestive system of the earth. It keeps the water and air clean and furnishes the food for all life. The quality of all life on earth—including human life--depends on the quality of the topsoil. That crucial thin layer of soil must be protected, maintained, built and nourished. The mulch cover of organic materials performs this service and much more.
To be a successful gardener in Texas one must mulch (and everywhere else also). Mulch comes from the German "molsch," meaning "soft," and refers to any loose, generally soft material that is laid down on top of the soil to protect a plant's roots or spread lightly over the plant itself.
Mulch is not a soil amendment; it is a covering or surface layer used to protect the topsoil. Nature does not allow bare ground hence neither should we. Mulching is considered to be the most important step in any gardening program. All natural or organic mulches will improve the soil but like all things, they vary in quality and effectiveness. Good mulch lets air (oxygen) and water enter the soil and allows carbon dioxide to escape. Good mulch will readily decompose releasing the stored nutrients and will provide microorganisms and earthworms a good home and food source.
Mulches add to the aesthetic value of a garden while protecting the base of shrubs and trees from injury by mechanical equipment. Mulches reduce the energy reflected on to the walls of your house lowering the amount of energy required to cool it (less energy used means less pollution is produced in its production). Mulches prevent soil compaction and provide a home for beneficial insects and animals. Mulches are so important, in fact, that in 1989 the State of California passed a bill that among other things required the use of mulches! 
A recently revised definition of the word mulch states: "Mulch is anything that will facilitate the improvement of the microorganisms in the soil."
For years mulches were divided into two basic types, organic and inorganic of which organic mulches are the most valuable. Mulches, like many other products, range from excellent to very poor except in one important aspect, the highest quality and most beneficial mulches often cost less. Recently scientists have begun to recognize a third category of mulches called "Living Mulches" and studies of their usage and benefits are now being published. Today's gardener has access to more types of mulches than ever before.
Now the question is which mulch to use, when, where and how to use it. Mulches can be broken into 3 general categories:
Organic Mulches are composed of or made from material that was once alive but is now dead and which will decompose or rot over time. Organic mulches may be totally natural (unprocessed) like leaves or pine needles or they may be processed like newspaper.
Inorganic Mulches are made of materials that were never alive and are generally inert. Materials like stone, crushed rock, plastic sheeting, glass, etc. are inorganic mulches. Except for specific cases and locations, inorganic mulches do not allow for natural systems and cycles to work, hence gardeners will tend to have more disease and pest problems.
Living Mulches are a new class of mulches that are composed of living plants that are planted for a specific purpose in covering the soil. Living Mulches are an extension of an agricultural practice called "cover cropping".
Organic Mulches - Organic mulches offer the most benefits, often at lower cost, and improve   the fertility and health of the soil.  A three inch layer of organic mulch can lower soil temperature about 25-300F which reduces plant stress and water requirements. Bare soil can easily reach 100-1350F which speeds evaporation and dries out the soil, which stresses the plant resulting in wilting, more insect and disease problems and in most plants and eventual death. The higher soil temperature reduces a plants roots ability to absorb moisture (even if it is there) and the higher temperature kills beneficial microbes that help feed, water and protect plants roots. In addition soil nitrogen (N) decreases as soil temperatures increase. For every 100 C increase in soil temperature, soil N will decrease 2-3 times.
Studies in Austin and San Antonio Texas during the 1990’s found that lawns mulched with 1/2 inch of compost each year, save $50-$200 per month on their water bill. Two studies from Ohio State University have confirmed that plants grown organically in organically enriched soil suffer far less disease and insect problems than those grown with synthetic chemicals. Hence good organic mulch helps build up the soil, naturally increasing a plants pest and disease resistance. USDA studies on several species have found that mulched plants were often 3 times as large and with 3 times the yield of unmulched plants after several years.
Bark Mulches -The most common type of organic mulch is bark mulch. Bark mulches are made from the protective outer layer of trees and are produced as a by-product of the lumber and pulp industries. Since outer bark is designed as a protective layer for the tree it tends to be low in nutrients. Tree barks frequently contain the chemical suberin, a naturally occurring substance that waterproofs (helps bark shed water) and prevents the bark from being broken down by soil microorganisms. In addition to suberin, barks contain waxes that also help waterproof the tree. Hence, the suberin in the bark can slow or retard the growth of some plant species. Additionally, barks contain very little energy-releasing compounds used by the soil microorganisms that are extremely important to soil and plant health. 
Barks can be broken in to basic types, hardwoods and softwoods (conifers). In much of the country, hardwood bark is mostly from oak trees and softwood bark is from pine trees or other conifers. They are both a by-product of the lumber and paper industry. Since conifers tend to be a pioneer species (they grow on poor nutrient deficient soils) so they contain very little nutrients (less than hardwood bark). Barks have a very high C:N ratio that averages 450:1. Hence they require a lot of nitrogen to break down often starving nearby plants in the process.
Large pieces of bark are slower to breakdown and less likely to blow and wash away than finely ground pieces, but they are considered more difficult to work around. Barks and uncomposted sawdust from redwood, cedar, Douglas fir, larch, eucalyptus, and spruce trees, are considered toxic to many plants. Any bark that is high in tannic acids and phenols is potentially harmful unless thoroughly composted and leached. 
Fine ground pine bark packs down and prevents oxygen from reaching the soil. It is difficult to wet, sheds rain after it dries out, and prevents moisture from reaching the plants roots. Often, when trees are dying from disease they are cut for pulp or lumber and the diseased bark ends up being sold to consumers.
Research at Cornell University has shown that conifer barks release toxic volatile compounds that are harmful to plants like tomatoes. Research at the University of Arkansas has found that Marigold growth was significantly reduced in beds mulched with pine bark [HortTechnology, July-September 1997]. Many tree biologists, anatomists, arborists, soil ecologists and other experts now recommend that bark based mulches be avoided. Dr. Alex Shigo at the University of Georgia, a leading tree expert has several papers posted on the Internet about this subject. Research at the Ohio Agricultural Research and Development Center has also found that pine bark does not support many of the beneficial microorganisms that prevent disease.
The natural chemicals in pine bark tend to kill off many species of beneficial microbes that naturally attack and prey on fire ants and termites; hence problems with these insects are reported to be more common when pine bark is used.
A report presented at the Texas Association of Apple Growers convention found that bark mulches actually steal nutrients away from plants when compared with mulches made from recycled tree trimmings and brush (i.e. Native mulches). As fuel prices have increased, more and more bark is being burned for fuel reducing its availability for the nursery industry.
Cedar Mulches 
Many users report that they have had good results with cedar mulches while others report extreme failures and problems. This issue needs further research to explain the difference in results. A couple patterns that have been observed are mulches made from species with the common name of "Junipers" (Juniperus ashei, Juniperus deppeana, etc.) versus species commonly called "Cedars" such as the Eastern Red Cedar (Juniperus virginiana), Southern Red Cedar (Juniperus silicicola) and Western Red Cedar (Thuja plicata). The first group tends to grows on alkaline calcareous soils while the second group is often found growing on more acidic soils. Most reports of cedar mulches made from "Junipers" have been positive and most reports of cedar mulches made from "Cedars" have been negative.
A second issue with using Cedar mulches is the type of processing they have had. This could be fresh ground cedar from land clearing operations to de-oiled cedar flakes from industry. When de-oiled cedar flakes from industry are used they can weigh as little as 190 lbs./cy since they have been cooked at temperatures of 2250F or more to remove the oils and have almost zero moisture when they leave the mill. The low weight is good for bagging, handling and shipping purposes. After cooking the cedar flakes can absorb water after application and weigh over 1,600 lbs./cy wet. De-oiled flakes are very low in nutrients and may cause nitrogen deficiencies in the soil and other mineral tie-up problems (i.e. very high carbon:nitrogen ratio). If the cedar flakes are soaked with in an organic fertilizer such as seaweed, fish emulsion, or a good compost tea, then they can make a better mulch.
Fresh ground cedar may be used directly but is almost always more effective if composted for a while before using.
Western Red Cedar (Thuja plicata) is very rot resistant. While this characteristic makes it good for lumber, the chemicals such as resins and oils which act as preservatives are not good for plants. These chemicals resist attacks by insects and microorganisms (e.g. fungi) hence also render the wood toxic for soil or composting organisms. Any mulch from this tree should be composted for a long time first to help breakdown these chemicals.
Note: Cedar from younger trees has less of these chemicals than older trees which contain more heartwood and have accumulated more of these compounds.
Native Mulches
For years gardening experts have claimed that this was the best mulch of all and now scientific research is backing them up. "Native" mulch is made from recycled fresh green tree and brush material that was recently alive and comes directly from a grinding operation. The Texas Association of Nurserymen (TAN) recognized "Native Mulches" as a separate class of mulches from barks and other materials in their 1997 product directory. Native mulches are available as fresh ground or aged (composted) and in many variations. 
Native mulches started becoming available in recent years as society became aware of the importance of recycling brush and tree trimmings instead of burning them or placing them in a landfill. Recent research has found that mulches made from recycled native trees are the highest quality available. They are also among the lowest in cost by offering real value since they are made locally and do not have high transportation costs associated with them. 
Local native mulch is produced from a mixture of native trees (primarily hardwoods), conifers, brush and any other species growing in a given area with bark, wood and sometimes leaves included. 
Native mulches have a high percentage of buds, shoots, leaves, and cambium bark layers in them. These materials are rich in protein and other nutrients which is why deer and other animals eat them as a food source. These native mulches are many times higher in nutrients than barks. Native mulches encourage biodiversity of beneficial microbes and earthworms in the soil and feed plants as they decompose. 
Years ago consumer awareness was the only negative, since the appearance is different than pine bark or shredded hardwood bark. However as these mulches have become more available this perception has changed. Studies and market acceptance have shown that most people prefer the native mulch since it actually looks more natural than barks or other alternatives. 
Composted or Aged Native Mulch
Native mulch that is aged or composted first before application is of the highest quality. The heat generated during the composting process kills any pathogens and weed seeds that might have been present. The composting process also concentrates the nutrients contained in the raw material and stabilizes nitrogen. Additionally, the composting process breaks down the lignin and cellulose contained in the raw material rendering a less attractive home for termites and many pathogens after it is applied. The composting process allows very high levels of beneficial microbes to develop and grow in the mulch increasing its value.
Screened composted native mulch is also an excellent amendment to use in soil mixes as it supplies energy to the soil (stored in its chemical makeup) in the right form for beneficial soil organisms to use. Grinding and screening (particle size) will determine the appropriate usage. A two year study from Texas A&M University (TAMU) has found that native mulch and compost out performed all other erosion control methods. It was also the lowest cost! Research in Florida has confirmed TAMU's work. Research at the Ohio Agricultural Research and Development Center has found that plants grown in substrates rich in biodegradable organic matter (such as found in Native mulches) support microorganisms that induce systemic resistance to disease (American Nurseryman, October 1, 1997). 
As a bonus for those in the South dealing with imported fire ants, using a quality composted native mulches may reduce mound density. Many landscapers, gardeners and others have observed and reported a reduction (not elimination) in the number and size of fire ant mounds on areas where composted native mulches were used. It is believed that the native mulches increase the density of organisms that attack and prey on the fire ants reducing their numbers. This has not been confirmed by rigorous research and would be an interesting area of study for our universities.
In general, a 3-4" thick layer should be used on ornamental beds and 4-6" layer around trees and shrubs. It comes in many formulations and sizes. It is sometimes blended with shredded hardwood bark to obtain a familiar appearance (lowers quality) or with compost to increase the quality even higher. It has been used as a potting media in container grown plants, to root cuttings and often works better than bark for many species of plants.
General benefits: Economical, the composting process concentrates nutrients and stabilizes nitrogen, the heat kills weed seeds and pathogens, improves plant and soil health, sets up quickly, reported to prevent many plant diseases, encourages microbial biodiversity in the soil, reported to increase tree and plant growth rates. Subject to less freight cost and less transportation that causes pollution hence much more environmentally sensible. The usage of native mulch also saves valuable landfill space or air pollution from burning since it is made from recycled materials.
Using native mulch also helps reduce greenhouse gasses.  When organic materials are placed in a landfill they undergo anaerobic decomposition producing methane which contributes 23 times more to global climate change than carbon dioxide.  Also since native mulches are made from recycled materials they qualify for points in the Sustainable Sites Initiative (future LEED landscaping guidelines).
Note: Course ground and unscreened composted (aged) native works best from a physical, chemical and biological perspective. However, a screened version is more cosmetically appealing and works better as a soil amendment. Sometimes available in a double ground form that looks similar to some shredded barks.

Native Mulch (freshly ground)
This mulch comes directly out of the grinder. It does not have any processing or screening on it.  It tends to be inexpensive and useful in special applications.  As in all native mulches it is a mix of whatever species came into the mulch/composting recycling facility.  
If one is not in a hurry it is one of the best ways to naturally break up heavy clay soils and suppress weeds. To suppress weeds it is often applied 4-6 inches thick and sometimes rolled or watered down (weight).  The mulch smothers the existing plants essentially killing them. This mulch becomes very active biologically (microbes working) and they will use the nitrogen stored in the dead and dying weeds to help break down the mulch.  The microbes will also break apart clay particles creating a looser soil (see soil chapter). 
It is also used for temporary road beds, erosion control, soil improvement, garden paths, land reclamation, filtration of storm water runoff, and any other application where large volumes are required and cost is an issue.
Wood Chip Mulches (single species)
Sometimes recyclers will grind up one single species of tree into chips that can be used as mulch.  Research has shown that these types of mulches may retard the growth of some species of plants. One study by Bartlett Tree Laboratory, U. K. compared 6 species of pure wood chip mulch of Beech Beech (Fagus sylvatica), Hawthorn (Crataegus monogyna), Silver Birch (Betula pendula), Cherry (Prunus avium), Evergreen Oak (Quercus ilex), and English Oak (Quercus robur) taken when the trees were dormant. 
Horse Chestnut (Aesculus hippocastanum)
Bare root trees of Beech (Fagus sylvatica) considered transplant sensitive and Hawthorn (Crataegus monogyna) considered transplant tolerant were planted into a general tree compost mix into containers.  Mulches were then applied to a depth of four inches and results were recorded after one growing season. 
Container trials showed that mulch type has a substantial affect on tree survival rate.  The un-mulched control survival was 10% while the mulch increased survival rates 20-70% with Hawthorn giving the best results in all cases (survival, growth and appearance).  The Hawthorn trees had 100% survival in all cases and the ones with the Hawthorn mulch had 20-30% higher dry weights than non-mulched controls. Cherry worked second best and all mulched trees did better than non-mulched. 
Field trials of Conference Pear and Gala Apples were mulched to a depth of four inches and grown without fertilization or irrigation. Hawthorn and Cherry again produced the best results increasing crown volume by 100-150% and fruit yields by 400-600%. The worst mulch increased crown volume by 20% and fruit yields by 50% compared to the non-mulched controls.
The breakdown of various mulches releases chemicals that affect plant growth.  Other studies have shown that Cypress mulch slows the growth or a range of woody plants (e.g. hydrangea, spirea, and viburnum) compared to pine bark. 
Eucalyptus grandis mulch has been found to be phototoxic to the germination of a range of seedlings. Black Walnut (Juglans nigra) inhibits growth and even kills some plants.  The Tree of Heaven (Ailanthus altissima) has been found to contain the allopathic chemical Ailanthone which is known as an herbicide. 
Some single species mulches can stimulate the growth of trees. Application of sugars (ex. dried or liquid molasses) has been shown effective at increasing root vigor and reducing transplant stress and increasing survival rates Both Hawthorn and Cherry wood are high in sugars such as sucrose and sorbital. Also some pure mulches such as Hawthorn have been shown to increase concentrations of enzymes in the root and leaves that helps plants defend themselves from various pathogens. 

Other Organic Mulches 
Many types of organic materials can be used for mulches depending on availability and cost. They are all generally beneficial and may be the best type of mulch depending on the application and/or cost, for a given situation or plant species.
Among the other types of organic mulches, the best quality is straw, compost, and newspaper.
Straw - "Straw" is the dried stalks of grains (which are actually types of grasses) after the seed heads have been harvested. Straw often has a shiny pale gold color and is a good mulch for many purposes.  It is generally applied 4-5" deep in ornamental beds and 8-10" in vegetable beds.
It protects soil, improves soil as it breaks down, good for winter protection. Researchers have found that Colorado potato beetles had a much more difficult time finding potatoes plants mulched with straw as compared to un-mulched plants (other research has shown significantly less eggs and larva also). The whitish reflective nature is also beneficial. It is reported to protect tomatoes against soil dwelling diseases.
Hay - A mixture of grasses (and sometimes clover), that is cut dried and baled with the seed heads intact (includes any weeds present). Hay is often a dull brownish-green color. Hay can be used as a mulch but the seeds in it often germinate becoming weeds.  Apply 3-4" deep in ornamental beds and 5-6" in vegetable beds.  It will protect the soil and improves soil as it breaks down, good for winter protection. Many hay fields are now sprayed with a herbicide called picloram which is sold under the brand names GrazonTM and TordonTM. This herbicide is used to kill broadleaf weeds and persists in the environment on the hay. It is reported that if hay treated with this herbicide is later applied as mulch, the treated hay will still kill many plants (even trees) years after application. To test the hay to see if it is safe to use as mulch, place some in at least a 1 gallon bucket and soak in water for a few hours. Next pour the liquid on any broad leafed plant and see if they become stressed or die. Peanuts and beans are very sensitive hence make good indicator plants. When contaminated hay is used on plants they will have more insect and disease problems even if they are not killed outright.
Newspapers - works best if shredded first and apply 4-6" thick.  It is generally free and can protect plants from frosts. Research at the University of Vermont has found a 6" layer of shredded newspaper exceptionally good at suppressing weeds for up to nearly 2 years. Best used as a special purpose mulch.  Note: Avoid colored paper as some inks may still contain toxins and heavy metals. Most colored inks used on newspapers are now safe and biodegradable however unless you know for sure it's better to be safe.
Compost - Compost is a very high quality mulch when applied 3-5" deep.  It has a high nutrient content, improves soil fertility, stimulates plant growth and general health, does not wash out in rain, weed free if made correctly, fair resistance to compaction, excellent resistance to blowing away in wind, contains and stimulates the growth of beneficial soil life (microbes, worms, insects, etc.), suppresses the growth of many weed species (often better than dangerous chemical herbicides), research at Ohio State University has found 1" thick layer of compost is an effective disease control as any synthetic chemical on the market. A good compost can be expensive, most gardeners cannot get enough; limited supply in some areas, large swings in quality from excellent to very poor. Many products are labeled and sold as compost and they are not.
Notes: Very green or partially decomposed (immature) compost is best as a mulch, more composting time or maturity is effective to mix into soil layer. Use 1-2" compost directly on soil with 2-3" composted native mulch on top is best combination possible. Good compost is free of plastic, rocks, trash and other contaminants.
Compost should have an index of at least 5 on the SolvitaTM compost maturity test for use as a  mulch. If used as a soil amendment, compost should have an index of 6 or higher.
Compost is like all other products, it can range from extremely good to very bad. For example, Municipal Solid Waste (MSW) compost while generally very inexpensive often contains pieces of glass, plastic, metals, and other contaminants. Over time, after repeated applications, these materials tend to rise to the surface and become very unsightly.
Biodegradable Weed Barriers - New biodegradable weed barrier mulches are entering the market that are made from recycled paper and cardboard. Better ones contain holes for air and water penetration, and will last about one growing season.  A few examples are:
Hydraulic Mulches: These are a group of special purpose mulches often used commercially for hydro seeding and vegetation establishment. These are often used in a water based slurry and mixed with tackifiers (works like a glue) to help hold seed and fertilizer pellets to the soil for vegetation establishment. They can be applied with spraying equipment to cover large areas quickly. Often used to prevent erosion while vegetation is being established. There are dozens of brands and variations produced by many companies.
Wood Cellulose Fiber - derived from trees and sometimes recycled newspaper or cardboard. Sometimes polyester or other synthetic fibers may be added for greater strength. Also used in combination with straw, hay or other organic mulch. For many years this was the accepted way of stabilizing soil and planting grass to prevent erosion. Several new studies have found that recycled mulches and/or compost work better and at lower total cost. 
Erosion Control Blankets:  These are special purpose mulches made from various types of organic material (fibers) held in place by a mesh (often plastic or polyester fiber). They are used commercially to prevent erosion on steep slopes and disturbed soils. The fiber may be straw, shredded wood, coir, cotton, hemp, or other organic material. These are made into blankets that are shipped in rolls. To apply, the blanket is unrolled at the bottom of a slope and is pinned to the soil. The second roll is applied above the first with a small overlap of the material like shingles on a house.  Sometimes the blankets will also contain seed and fertilizer.
Inorganic Mulches
This class of mulches includes any material that was never alive. It includes rocks, gravels, plastic, stepping stones, bricks, pavers, etc. In general these mulches are best used in special circumstances such as decorations, pathways or erosion control. The color of these types of mulches will affect plant growth. Light colors tend to reflect more sunlight (energy) particularly the photosynthetically active or growth promoting radiation. As a result air temperatures around the plant are much lower than from darker colors. Dark colors can raise the air temperature around plants 350F or more, which could make the plant more susceptible to damage from sudden cold weather. The dark colors absorb the energy better hence soil temperatures are much higher and root growth is decreased.
Plastic Mulches
Inorganic mulches are falling out of favor with experienced landscapers and horticulturalists due to the problems they create and the new research that has demonstrated the benefits of organic and living mulches. Clear or black plastic can be used to warm the soil in spring but should be removed to prevent the growth of fungus and other pathogens in the soil. Methane and other gases produced by the anaerobic conditions can build up damaging plant roots. The better the soil (more fertile) or the higher the clay content, the greater the problems become with plastic mulches. Inorganic mulches, such as gravel, may perform well in special situations or certain very dry areas of the country (West Texas, New Mexico, Arizona, etc.) or in special situations like rock gardens.
Inorganic mulches often increases heat index around plant too much, many pathogens grow better since air flow is reduced, often the cause of root dieback and other fungal diseases. Plastic mulches breaks down (becomes brittle) when exposed to ultra violet radiation in sunlight. The plastic fragments can create a mess when it starts to break down. Plastic is difficult and expensive to dispose of and generally unsightly. Shallow root systems are often created by plastic, and during drought periods the plants may not survive the stress. Lower crop yields frequently result than other types of mulch and crops often require more water. Solid films prevent CO2 from escaping the soil reducing the benefit from localized concentration to plants. Requires frequent monitoring for best performance and requires the use of a costly drip irrigation system for best performance. Studies have shown that plastic increases soil erosion in areas between plastic mulched rows. Best success requires chemical fumigation (costly and dangerous). Many reports have found that increases in problem nematode populations. Plastic mulches also kill earthworms by preventing them from reaching the surface to feed. One report stated that melons under plastic mulch require more water (higher heat index and larger plants). Since plant parts often stick to the plastic, reuse of the plastic tends to spread plant diseases unless the plastic is cleaned and sterilized. Plastic films tend to breakdown in the second and subsequent years allowing cracks to form that  weeds can grow through. To be economic, plastic mulches requires specialized tractor drawn equipment to apply mulch. For effective weed control, it also requires a smoother seedbed than organic mulches which requires additional labor (i.e. costs) and it heats the soil too much for some cool season crops like lettuce thus reducing yields. If used in early spring, it should be removed after air temperatures warm up. Plastic mulch is most effective on chemical programs with poor abused soil that is chemically and biologically out of balance.
Cover Crops
Living mulches are showing greater and greater promise as we begin to understand natural systems better. Living mulches have many positive environmental aspects that we are beginning to measure and quantify, from reducing erosion, increasing beneficial insects and microbes, increasing water infiltration into the soil, increasing soil organic matter (humus), and many more. Living mulches are very cost effective for large areas as seen in agriculture such as orchards and vineyards.
Living mulches are sometimes called cover crops or green manures and they serve very similar purposes hence; many functions and benefits overlap. In general Living mulches are to be mowed and left on top of the soil or left standing, while green manures are to be tilled in.
An appropriate cover crop (or living mulch) planted in late fall will:
  • keep the garden green all winter, 
  • prevent erosion, 
  • prevent soil compaction, 
  • control winter weeds,
  • add large amounts of organic matter to the soil, 
  • control certain insect pests,
  • reduce erosion, 
  • reduce surface water pollution (natural filtration system), 
  • some species fix large amounts of nitrogen into the soil, 
  • improve soil structure and tilth, 
  • store and recycle nutrients, 
  • increase soil productivity and carrying capacity.
  • many types attract beneficial insects which help control pests
  • reduce pest species
  • biofumigation to reduce pathogens
New studies have shown that some living mulches (cover crops like crimson clover) can reduce weed seed germination by 27% even after being tilled into the soil. The studies also showed that if the nitrogen supplied by the clover was supplied by ammonium nitrate chemical fertilizer then weed seed germination increased by 75%! Penn State University has been researching living mulches since 1975. New data indicates that living mulches tend to reduce frost damage on many species of plants.
Studies have shown that even a grass cover crop can add over a dry ton or organic matter per year to the soil just from the root mass. In some case this can reach over 5 tons/acre per year with another 1-2 tons from the above ground leaves and stems. Many living mulches can add several times these amounts of organic matter.
For years many people have sworn by living mulches (cover crops, groundcovers, etc.). Research at the USDA in Beltsville, Maryland [HortScience 32(4):659-663 1997] have done studies comparing hairy vetch (Vicia villosa, a winter hardy legume) and plastic. The vetch plots had a longer season and produced up to twice as many tomatoes. Vetch is less expensive, more environmentally friendly, and enriches the soil by adding organic matter and nitrogen. Note, ARS scientists found that on tomatoes, growers using vetch had an average increase in profits of 65% compared to those growers using plastic. Other crops that had a strong positive response to vetch mulch were melons, snap beans, peppers, and eggplants. For southern gardens crimson clover (Trifolium incarnatum) is a good option (also attracts several beneficial insects). Additional research at the USDA in Beltsville, Maryland [HortScience 31(1):65-73 1996] has shown higher yields and nitrogen levels in tomatoes when a living mulch was used when compared to black plastic even when 2 the amount of fertilizer was used.
The USDA has found that snap beans yield in mulches from mixed annual winter legumes were comparable to those when synthetic fertilizer was used and for over 3 years, the yields were higher than conventional tillage systems (HortScience, December 1997). Additionally, the living mulch systems required no water, herbicide, fertilizer or other treatment until they were mowed. Other benefits from the mulch system included no runoff or erosion (HortScience, Vol.32 (7):1191-1193, December 1997).
Research at Kansas State University has shown that yields from muskmelons were much higher when beef manure was combined with living mulches (hairy vetch, Austrian winter pea, alfalfa, and winter wheat) than by using synthetic nitrogen fertilizer (HortScience, Vol. 31(1):62-64, February 1996).
Other studies have found that the type of living mulch affects the availability of nutrients in successive crops. For example, it has been found that red clover produces twice the amount of available nitrogen to successive crops (i.e. corn) when compared to oats, rye, oilseed radish, etc. Yields of successive crops were also increased. 
New research is finding that certain plants when used as living mulches suppress soil borne diseases. For example researchers have found that Sudan grass and sweet corn suppress pathogenic fungus such as Verticillium wilt.
Many types of pathogenic nematodes are suppressed by sudan grass, rapeseed, white mustard, elbon rye, canola, etc. These plants also produce chemicals that are allelopathic or toxic to other plants, hence they can also be used for weed control. Using Tagetes sp. as living mulch has been found to reduce populations of root-lesion nematodes in a few months to a level where they do not cause significant damage. The effects have been found to persist for several years. It was also found that the seed cost for living mulch was about half that of chemical fumigation.
Other studies in the Salinas Valley of California have found that low cost cover crop methods can reduce soil nitrate leaching by 37-70% in intensive vegetable production systems without hurting yields.
In commercial agriculture other factors come into play when using living mulches. Tillage, compaction from farm vehicles, preservation of beneficial insects, etc. are all factors and are beyond the scope of this book. Many new research studies on these issues have been published in the last couple of years and can be found in journals at the local library.
Remember that in using living mulches, as in all plants, repeated use of one species, in one spot, will over time increase the chance that certain diseases may develop in the soil. It is a good practice to rotate the living mulch and even use multiple or mixed species at the same time.
Using Living Mulch
Living mulch is often used as a cover crop while the main crop is growing. The cover crop will produce old leaves, stems, spent flowers, seed and seed pods, etc. as the plants grow. These are often mowed to keep the cover crop from competing with the main crop, adding organic matter to the soil, and to help build a soil surface mulch layer. 
Hay fields, orchards, vineyards and other types of large plantings are getting good results from “Living Mulches”. Some types fix nitrogen for the primary crops, others provide a home or attract beneficial insects that control pests in addition to enriching the soil.
We need to remember that plants and their root exudates affect the microbes that live in the soil. These affects can often be carried over to the next crop. For example if the living mulch or previous crop stimulates more beneficial microbes then disease or even supplemental nitrogen requirements on the next crop can be reduced. Some of the beneficial metabolites produced can result in enhanced growth of the next crop. 
Researchers are also learning that cover crops and living mulches can alter the amount and types of pathogens in the soil. Sometimes the cover crop will release anti-microbial volatile compounds which have a biofumigation effect. This has been well documented with members of the Mustard family (Cruciferae). The gaseous chemicals produced by the plant while it is growing or when it is turned under can kill some types of soil microbes including many pathogens. Hence, when the next crop is grown it will experience less disease pressure. 
Research is beginning on different aspects of natural biofumigation. Biofumigation uses soil microbes to biodegrade a organic material (mulch, root exudates, etc.). Depending on the type of organic matter some of the breakdown products are volatile gases that adversely affect soil borne pathogens. The cruciferae family mentioned above contains compounds called gluosinolates that in the presence of the enzyme myrosinase (occurs in the tissues of microbes or produced by microbes), break down into isothiocyanate, nitriles, carbon disulfide or thiocyanate. Many of these are chemical fumigants with the potential to kill pathogens in the soil. 
For example a recent study showed that volatile compounds released from soil amended with meadow foam seed meal, completely suppressed sporulation by Phytophthora ramorum and Pythium irregulare. Soil potting mix amended with only 1% meadow foam seed meal showed striking growth enhancement of conifer seedlings. Another experiment with papaya (Carica papaya), meadow foam seed meal at 1% by volume greatly stimulated plant growth without suppressing mycorrhizal formation. 

Living Mulch is not suitable for crops or plants that are short, shallow rooted, or sensitive to low moisture or drought conditions that could be enhanced by the Living Mulch. Also if the soils are very sandy or other types of low fertility the effects of competition may make living mulches unsuitable.
If used in vegetable production, several studies have found it is best to delay planting of the living mulch until the primary crop is established. It has been found that about 1/3 of the way through the crop cycle works well for many crops. 
Some living mulches have different effects on the soil hence different benefits. One type may improve water infiltration into the soil better than another. A courser type plant material may reside on the surface longer since it decomposes at a slower rate. Living mulch with strong tough stalks (lignin) may encourage fungi in the soil while a  soft grass living mulch may decompose quickly and encourage bacteria in the soil.
Living mulch can be planted similarly to any other plant seed. A loose friable soil makes a good seedbed. For small areas after the seeds are spread around, lightly raking the soils can help cover the seed and ensure good soil-seed contact. For large areas follow standard agricultural practices for your area. Your local Agricultural Extension offices are often a great source of information.
Some living mulch can be used in winter while others are best suited for hot weather. Also, it is best to keep the living mulch mowed to prevent seeds from forming unless the land is going to lie fallow for a while.
Mulches can be applied anytime during the year when trees and shrubs are being planted. For most of the country, the best time would be in mid-spring when the soils have warmed up enough for sufficient root growth. In Houston and along the Gulf Coast our soils stay warm enough for root growth year round so the timing does not matter. However, new studies in the south are suggesting that early fall might be the best time, so as to trap the soil heat and prevent the soil from cooling off as much during the winter. Also the fall application of mulch would help protect the root zone of tender plants better. Some research has indicated that a twice a year application at a 2 inch thickness each is better than one application with a 4 inch thickness.
A good healthy organic mulch should support fungal growth (white spider web looking patches, yellow patches, toadstools, mushrooms etc.), and provide a home for insects, earthworms, and microbes. Research at the Morton Arboretum in Lisle, Illinois and at Cornell University have confirmed other studies and clearly shows increased soil moisture and root growth under mulches made from tree trimmings (i.e. what has become known as Native Mulches). Other studies in Illinois and North Carolina in disturbed soils have revealed greater foliage development on recently transplanted trees (Sugar Maples, Crapemyrtles, and Callery Pears) as compared to un-mulched controls.
Research has shown that many plant diseases occur only when a plant is under severe stress (hot soil, lack of moisture, etc.). For example, many common fungal cankers diseases of woody plants occur on stressed plants. Also repeated studies have shown that pest insects are attracted to stressed plants. Good quality and established plants (several years old) are very valuable and are expensive to replace. The traditional cure for insects and disease is to use toxic pesticides, fungicides and other dangerous chemicals that are very expensive and require lots of time to apply correctly. Also, who wants a loved child or favorite pet around plants covered with poisons? It is much cheaper, takes less work, and is far safer to just mulch your plants every year and prevent problems from occurring. Properly mulched beds also make a home look much nicer and helps add value when it becomes time to sell your home.
Mulched beds encourage root growth, conserve moisture, and provide a home for beneficial microbes, particularly in difficult environments. As a result, a good mulch naturally prevents many plant diseases.
As better diagnostic tools have become available in recent years, scientists are really beginning to understand how and why mulches work.
Seed Beds
After planting a seedbed, sprinkle a fine layer of mulch (weed free dry grass clippings, compost, etc.) over the seedbed to help prevent soil from crusting and keep soil moisture more even thus helping seeds germinate. Large seeds can germinate and grow through a thick layer of mulch while small seeds cannot. For seed beds we need to use mulch that is fine grained and easy for emerging plants to penetrate (i.e. cardboard or newspaper would not be a good choice). Plastic mulches can get too hot and cook the emerging seedlings. A new type of mulch made from recycled paper that is made into pellets is beginning to become available. Since the pellets tend to be loose they should theoretically work well for larger seeds if the pellets are not applied too thick. However, a problem occurs if these paper pellets are made from recycled paper containing aluminum (most paper currently does), then they can cause severe reduction in plant growth and seed germination. Also some of the fiber based row covers work well as a temporary mulch for seed germination.
Paths and Large Areas (Sheet Mulching)
Place sections of newspapers (layers) 6-8 pages thick on top of weeds or grass and overlap the layers by at least 25% (50% overlap is even better). Opening the folded sections to form larger sheets is far more effective than using the folded sections. It is harder for grass and weeds to grow between the layers to emerge (i.e. takes a lot more of the energy stored in the stems and roots the further they must travel horizontally before they can go up). Apply mulch 3-4" deep on top of paper and water in well. The weight of the wet mulch and paper presses down against the grass or weeds and will quickly kill them. This is done by first smothering the plants (little or no oxygen at when first applied), secondly by blocking the light required for photosynthesis and third by the phytotoxic effects of chemicals in the newspaper. If the mulched area is not thoroughly watered and packed down, some grasses like common Bermuda and nutsedge can live for months. After wetting, it is useful to compact the mulch by walking over the area to press it down tight against the ground. In a few weeks earthworms and microbes will have devoured the dead grass or weeds, and the paper will have begun to decay with both types of organic matter enriching the soil. A thick layer of fresh ground native mulch is more effective for killing grass and weeds than composted mulch. Fresh ground mulch requires nitrogen to break it down hence microbes take the nitrogen from the leaves of green grass or weeds assisting in the killing of the weeds and grass. The drawback from using fresh ground mulch, is that it may contain some weed seeds or acorns that passed through the grinding equipment undamaged and may eventually sprout.
Studies at the University of Georgia (1991) have shown that by walking over the same area (un-mulched) 27 times, we can compact that living soil to its maximum capacity. Mulches prevent soil compaction by spreading the weight over a much larger area (preventing or reducing the compaction), helping protect the soil and preventing erosion.
Mulches for top dressing paths and walkways should consist of larger pieces with very little fines (small particles). Fines break down quickly, become mushy when wet and get stuck on shoes and clothes. Larger pieces are slow to breakdown (often lasting several years), spread weight hence preventing soil compaction, they lock together better preventing erosion, and provide hiding places for beneficial insects. Mulches comprised of particles that are long and skinny generally work best. Mulches that are flat chips (pine bark nuggets, wood from chippers, etc.) tend to pack together very tightly preventing the soil from breathing and water from entering.
Weed Control
Many weed seeds require exposure to light before they can germinate. A layer of mulch will prevent light from reaching the weed seeds. Also some earthworm species eat weed seeds, hence a good mulch layer allows worms to find and consume the seeds.
Field studies at Cornell University (Watson, Greenlee, and Rakow) found that 3" of mulch suppressed almost all weed growth. It was also found that 3" of mulch resulted in more shoot growth on transplanted white pine and pin oak saplings when compared to bare ground or with 6-10" of mulch suggesting that more is not always better. It was found that 3-4" of settled mulch produced the maximum benefit in their studies. The USDA has found that a 3" layer of composted mulch made from green waste (grass, leaves, and trimmings) provided 98% control of weeds such as common purslane, Bermuda grass, and redroot pigweed.
Certain types of mulches suppress weeds better than other types, and research is starting to evaluate these weed suppression properties. Research at the University of Connecticut has found that compost suppresses weeds almost as well as leaves and straw. However, weeds that made it through a compost layer were very healthy. In general, compost that is still "green" (immature) will work better for weed control (A value of 5 or less on the SolvitaTM Compost Maturity test). 
Weed control can be enhanced by applying corn gluten meal (a byproduct of corn milling) to mulched areas (many nurseries, gardener supply catalogs, etc. now carry corn gluten meal or can order it for you). The corn gluten meal can be applied directly to the soil before the mulch is applied or mixed in with the mulch. Research at Iowa State University has shown that it is a natural herbicide and will prevent or reduce germination of many weed seeds. Additionally, it also has benefits as an organic fertilizer since it contains 60% protein (good for microbes and earthworms) and is 10% nitrogen by weight. It works by inhibiting or stopping root formation at the time of seed germination but does not affect mature grasses or plants. Use it at a rate of 15-20 pounds per 1,000 square feet. It needs to be applied in early spring just as weeds start to germinate as it has a window of effectiveness of about 3 weeks, and it is activated by moisture. The extra nitrogen will help feed the microbes breaking down the mulch thus enriching the soil. This nitrogen, stored in the body mass of the microbes and in the humus produced from the decay, is slowly released to the plants as they need it.
Research at Auburn University has shown that recycled paper made into pellets or in a crumbled form is also effective for the control of some weeds. As these are new products there is not a lot of information available, however the recycled paper in the pelletized form seems to be more effective. As mentioned earlier, part of the reduction in weed growth may be from toxicity due to Aluminum in the paper particularly on acid soils.
Annuals and Vegetables
For warm season vegetables like tomatoes, peppers, and eggplants apply mulch after soil has warmed up (60-65o F) in spring. Often in the Deep South (i.e. Houston and Gulf Coast) the problem is keeping the soil cool enough for germination. Applying good thick mulch early in the season keeps the soil cool and moist. Also different types of organic mulches can assist in disease and pest control for certain vegetables.
After the soil has warmed up in the spring all vegetables derive benefits from mulching. Using mulch in the vegetable garden has conserves water and keeps the soil cooler. It also helps prevent the spread of disease organisms by eliminating the splashing and scattering of pathogens to the leaves of other plants when a raindrop hits bare soil. While this benefit holds true for all plants from flowers to shrubs and trees it has extra value for vegetable gardens since it is common to have many plants of the same species near each other and susceptible to the same pathogens.
New research has shown that many grasses, annuals and vegetables do best in a soil dominated by bacteria. Mulches that tend to increase the beneficial bacteria lead to long term soil health. A few examples of these are, green leaves, straw, hay, and grass clippings. However, woody mulches made from tree trimmings (i.e. Native Mulches) are very useful to improve the quality of soils with high clay content and are a useful tool in vegetable gardens.
Research at the Connecticut Agricultural Experiment Station has found that leaf compost can be substituted for synthetic 10-10-10 fertilizers without loss of yields. They also found that composted plots had better yields than those without compost. Also the beneficial effects of composts continued to increase in the second and third years of application. They also found that when compost was used with inorganic fertilizers only one half as much fertilizer was needed for the same yields (again saving money).
Research has also shown that plants like tomatoes when grown in an organic mulch like a layer of mown vetch compared to black plastic, live longer and develop less disease. These plants have larger more robust root systems that allow the plant to absorb nutrients better. Other benefits were higher yields even using 50% less fertilizer.
Experiments at Ohio State University have shown that for tomatoes organic mulches (i.e. wheat straw, composted bark, shredded newspaper, etc.) had the same weed suppressing, moisture conserving abilities, cost less as compared to black plastic, and had several times higher yields than bare ground.
USDA studies that mulched plants were often three times larger than un-mulched plants after several years of growth. Other researchers have found that a field mulched with compost had over 1,000 pounds more strawberries than a field treated with the toxic chemical methyl bromide and at lower cost.
Trees and Shrubs
Research has shown that most trees and shrubs grow better with thick mulch under the dripline instead of grass sod, which is difficult to grow in the shade anyway. Some species of trees like Post Oak are so severely damaged by the presence of turf it can lead to tree death. Dr. Tom Smiley (plant pathologist) reports that trees are much healthier (fewer disease and insect problems) and less soil compaction problems occur when mulch is used (April 1997, Tree Care Industry). Previous research reported in "Tree Care Industry" has shown that the microbes found living in native mulches under trees are vital to tree health. Similar results are being found for most shrubs. It is best for most plants if mulch is applied frequently, in thinner layers until the desired thickness is reached with composted mulches being the preferred choice. The beneficial mycorrhizae critical for tree and shrub health, flourish under leaf compost while grass does not. Numerous research studies have shown the importance of mycorrhizal fungi for increased tree growth and increased resistance to disease. In studies of pine seedlings, it was found that the growth rate increased 700% in some species. The native mulches stimulate the growth of these beneficial fungi while barks sometimes actually suppress their growth.
Mulch around trees has benefits not only for what it provides and keeps in but also what it keeps out. It is well known that turfgrass and trees do not coexist very well, and we are now beginning to understand why. Healthy turfgrass requires bacterial associations in the soil that dominate the microorganism population at the expense of fungal species, while trees and shrubs require soil dominated by fungal species for maximum growth and health. Mulches made from recycled tree trimmings (i.e. native mulches) greatly increase the beneficial fungi in the soil required by trees and shrubs. The requirements are exclusive, hence when we try to combine grass and trees, one or both suffer (see Dr. Elaine Ingham’s work on soil ecology for more detail). 
Research at the Morton Arboretum showed that the root density of trees (littleleaf linden, sugar maple, green ash, and red maple) was greater in a mulched environment than under bare soil and far greater than under turfgrass. Turfgrass inhibits the growth of a tree’s fibrous root system, slowing growth, reducing vigor and making the tree more susceptible to disease, insect pests, and drought. For new freshly planted young trees a ring of mulch starting a few inches from the trunk and extending out several feet can double or triple its growth rate. Studies have shown for trees and some woody plants can grow up to 6 times faster if given a 10-20’ diameter of mulch. 
In times of drought or stress the trees will out-compete turfgrass for water and nutrients in the root zone layers. As most grounds managers know, it is very difficult to maintain high-quality turfgrass beneath the canopy of mature trees. Often the best solution is to separate the species by maintaining a mulch zone out to the dripline of the tree at the outer edge of the canopy. This also has another advantage as it prevents "mower blight" (damage to the tree's bark and cambium from mowing too close).
When applying mulch to a tree or shrub, do not let the mulch come in contact with the plant’s bark. When trunks are covered with mulch, the higher moisture conditions created by the mulch often will cause the bark to rot. Bark on trunks was not designed for high moisture conditions like roots. The high moisture conditions allow fungal cankers to grow and attract moisture loving insects like carpenter ants. Hence, it is best to leave 1-2" of open area between the trunks of woody plants and the mulch layer. For larger trees, leaving 6-8" around the trees free of mulch will also reduce possible rodent damage. For most trees and shrubs a mulch layer 2-4” deep works well. Avoid hay mulches around trees as the seeds they contain can attract mice, voles and other small mammals which may damage the tree bark during their feeding after the seed is gone (also remember that hay mulches stimulate bacterial growth in the soil and trees prefer fungi).
"Native Mulches" are the best for trees, since they are made from tree limbs and leaves; hence they have the exact nutrient makeup that trees need. As the mulch decays, it acts like a perfectly balanced slow release fertilizer in addition to its other benefits. If the "Native Mulch" has been partially composted first, using large piles and long time frames, it will be colonized by many species of fungus that are beneficial to trees, shrubs and most perennials.
When planting new trees (or shrubs), the moderate release of nutrients from a compost amended backfill material and from the surface mulch will provide all the nutrients the new tree needs for the first couple of years, if a good quality compost and mulch is used. About 2-3" of new mulch should be added every year to replenish the mulch that has decomposed (remember this decomposition is naturally feeding the tree the exact nutrients it requires). 
Some experts have found that a organic fertilizer combined with a good mulch can speed the breakdown and enhance the growth in young trees. If the mulch you have available is bark, dyed wood or is freshly ground, a little fertilizer will help prevent nitrogen tie-up problems. I recommend using organic fertilizers if possible. Seaweed and fish emulsion liquids fertilizers are often a good choice since they also contain growth hormones and trace elements.
A study at the University of Maryland has found that the insect pest known as "Azalea Lace Bugs" are attracted to Rhododendron's (Azaleas) that have received supplemental synthetic nitrogen fertilizer. Many other studies have found that pest insects are attracted to plants that have received synthetic fertilizers. So, one of the best (and cheapest) methods of reducing insect problems is to fertilize naturally using compost and mulches. Additionally, the mulched area should be expanded by several inches each year as the root zone of the tree expands for maximum benefit. After a few years companion plantings may be added to enhance the appearance in the landscape. The closer we copy the way trees grow in their natural setting (i.e. a forest) the better success and fewer problems we have.
Erosion Control and Sediment Fences
Mulches can be very useful for erosion control. Erosion occurs wherever the soil surface is exposed to wind or water. Without some form of cover (mulch or plants) to protect the soil erosion occurs. Wind removes the finer fractions from the soil containing much of the nutrients and organic matter leaving behind the coarser particles. The eroded sediments may choke and pollute streams, block stream channels, cover roads or fill in lakes. Erosion causes damage to the land from which the soil is removed, to the water that transports it and to the place it is deposited, hence in needs to be controlled or prevented. 
The results of a two year study released by the Texas Transportation Institute in 1997 (Texas A&M University) found that compost and native mulches were as good or better than all other erosion control methods and much cheaper (Research Report 1352-2F). Similar results were found by the EPA (United States Environmental Protection Agency) and FHWA (Federal Highway Administration) when comparing "yard trimmings compost" to hydromulch for erosion control and vegetation establishment. The results showed that compost out performed the hydromulch with synthetic chemical fertilizer added. Several other studies have found the same results, that compost and native mulch out performs alternative methods and at lower cost.
Mulches that are woody (i.e. ground tree trimmings), with long thin pieces and frayed ends tend to work best for erosion control. These characteristics allow the mulch to physically lock or matt together. Secondly, fungal fibers will grow which also lock mulch particles both to each other and to the ground after exposure to water.
Freshly ground native mulch placed in a small pile or in long rows along a construction site is very effective at removing sediment from storm water runoff. When runoff carrying the sediment (often valuable topsoil) hits the mulch pile, its velocity is slowed. As a result, the water no longer has the energy to carry the sediment, and it is dropped inside the mulch line. Also the filtering action of the mulch, as the water passes through, removes additional sediment and dissolved minerals (nutrients). Several studies have shown that mulch is more effective than sediment fencing and often at lower cost. When the construction is finished the sediment (topsoil) that has been enriched by the decomposition of the mulch into compost can be spread on the landscape or used in the construction of flowerbeds and other plantings. This also results in better quality surface water in our rivers and lakes as less contaminants and sediments flow into them.
Developers and builders are also finding that coarse ground mulch made from ground up recycled construction wood makes good filter berms, and work areas.  When covering a work area is prevents mud from being tracked into the building under construction.  By the time the building is completed it has decomposed enough that there is no need for removal as it just adds organic matter into the soil which helps with the landscaping and turf establishment.  
Soil Development and Health
Scientists in Quebec, Canada have found that ramial wood (derived from a French term meaning twig wood) from deciduous trees that has been ground up, and their lignin is converted rapidly by certain soil fungi into valuable humus. Ramial wood is essentially native mulch made without large limbs and logs. Experiments for 20 years in Africa, Canada, and Europe have shown that a 1" layer of chopped or crushed twigs mixed into the top 2" of the soil will be broken down by aerobic fungi called basidiomycetes. Benefits found include improved soil aggregation and moisture retention, larger root systems, more mycorrhizal associations with roots, decreases in some soil pathogens, yield increase from 300% (strawberries) to 1,000% (tomatoes) compared to untreated soil. Garden prunings and twigs should be chopped or ground up but the leaves removed should be removed (the breakdown of green leaves encourages bacteria that can displace the basidiomycetes fungi). This type of mulch is best applied in the fall so the soil is ready for use by spring (temporary nitrogen tie-up may occur when first applied). This approach works well in all but wet soils. Branches from conifers (pines, etc.) should not be used since their lignin breaks down into polyphenols rather than beneficial humic and fulmic acids (this also applies to most bark products like pine and hardwood). Native Mulch produces the same benefits since it is composed primarily of the twigs and small limbs from primarily deciduous species. Other researchers have found that simply applying the mulch on top of the ground works well with less effort. 
Research into soil ecology has also found that the basidiomycetes fungi produce their sexual spores on a large fruiting body or basidium. These fruiting bodies are characteristic features of many healthy forest soils and in meadows in the spring and fall. Many of these Basidiomycotina form ectomycorrhizal associations (beneficial symbiotic relationships) with tree roots and are critical to the nutrition, growth and health of the tree. These extremely beneficial fungi only grow on Native (ramial) mulches. It is this group of fungi that convert lignin (white rot) and cellulose (brown rot) into valuable humic acid, fulmic acid, and humin that we call humus. If we apply fungicides to our trees, plants or grass we end up killing these valuable friends (sort of like getting ready to run a marathon and taking a pistol and shooting ourselves in the foot first).
Soil Types and Mulch
Clay soils have been found to rapidly improve in all aspects of soil quality and health when native mulch is applied. Sandy soils respond well to compost mixed into the sand and compost used as mulch, with a top dressing of 1 inch of native mulch. If the native mulch has been composted a few months, it will work even faster and provide additional benefits.
Many soils develop a condition called hardpan after repeated exposure to excessive use of synthetic chemicals. The soil becomes extremely tight and often very hard, hence the name. Air and water cannot penetrate the soil, beneficial microbes and animals are limited. This hard layer can occur at the surface or inches below the surface.
In general all types of organic mulches will improve all types of soils. Some just work faster than others. The amount of mulches used, how they are applied and handled, and the starting condition of the soil are all factors in soil improvement. As these materials break down they eventually become soil organic matter that we call humus. For healthy soils, the organic matter should be broken down enough that there are about 25- 30 carbon atoms for every atom of nitrogen present. We call this the carbon nitrogen ratio expressed as C:N (i.e. 30:1).
As organic materials age they breakdown into different type of chemicals that enter the soil. These basic constituents are grouped into some basic classes: cellulose, hemi-cellulose, lignin, water soluble fractions such as simple sugars, amino acids, and aliphatic acids (succinate and acetate), a protein fraction, an ether- and alcohol-soluble fraction (fats, oils, waxes, and resins). As organic material gets older, the content of the first 3 fractions increases and the latter groups decreases.
The native mulch (if composted first) makes an ideal ingredient to add to prepared soil planting mixes in small amounts. It helps lighten the soil mix, improves aeration and looseness, and most importantly provides a long term energy source (i.e. carbon) to help promote a healthy soil food web.
Erosion and Compaction
Rain drops can hit the ground with velocities between 10-20 mph (miles per hour) in normal conditions and over 60 miles per hour during severe storms. The kinetic energy associated with the raindrop increases as the square of the velocity (V2). If the soil is unprotected by a mulch layer, the impact dislodges the soil and erosion begins. Compare this to the surface runoff on near level areas where the rain water only moves about 2-1 mph. Of course on slopes, gullies and streams the water can move much faster and carry away the valuable topsoil knocked loose by the raindrop.
Another effect associated with raindrops hitting bare soil is surface sealing. This occurs when the dislodged soil particles wash down into the soil pore space and clog it up. This creates a thin compacted zone at the surface that seals off the soil, preventing rain water to soak in greatly increasing runoff and more erosion. 
When this seal dries it can become very hard and create a crust. This crust can prevent seeds from germinating and penetrating the layer. Since water cannot easily enter the soil, many roots and microbes can suffer or die from water stress or lack of water. Air flow into the soil is reduced preventing oxygen from reaching plant roots and microbes. It also allows gases to build up that are toxic to many plants and soil animals. The resulting conditions favor the growth of pathogens in the soil.
Decomposition of Mulch
As was mentioned above, there are many beneficial microbes at work breaking down mulch and organic matter into soil components. Inexperienced gardeners often say, "The mulch was not any good, it just rotted away in only one year." Mulch and plant residues (litter, leaves, twigs, branches, root detritus and exudates, etc.) provide carbon (the energy source) which is the fuel (energy) for the soil food web that cycles and stores nutrients, creates soil structure, and prevents pathogens and pests from taking over. When we burn wood logs in our fireplace, the carbon in the wood is combined with oxygen in the air releasing energy. The same thing happens in the soil. Carbon from decaying organic matter is combined with oxygen from the air, in the bodies of microbes, giving them the energy needed to create soil structure, fight pathogens and pests. If we do not feed our army of beneficial microbes then they will die and the pathogens and pests will take over. In other words, "WE WANT MULCH TO DECAY!" In a healthy soil with a good quality mulch, about b should decompose in a one year time frame. The remaining mulch will break down at a much slower rate providing other long term benefits to the soil. If the mulch does not break down, then the soil is very unhealthy, or you purchased very low quality mulch.
The rate at which mulch or any organic material breaks down is dependant on many factors. The type of material, the materials age, particle size, nutrition content such as nitrogen, soil moisture, temperature, aeration, pH, and a few others all influence the breakdown or decomposition rate.
Vertical Mulching
This is the name given to a group of related techniques used to inoculate soils and plants with beneficial microbes, like mycorrhizal fungi, to get organic matter deep into the soil and increase aeration. Numerous and repeated studies have shown that many species of microbes are essential for plant health, growth, insect and disease resistance, drought tolerance, etc. Soils that have been exposed to cultivation, pesticides, fungicides, herbicides, synthetic chemical fertilizers, etc. or that have low organic matter content are frequently deficient in these extremely valuable microbes.
Vertical mulching is achieved by drilling a narrow hole (or taking a soil core) 1-3" in diameter and 12-24" deep. Often a mixture of compost (or composted woody mulch) is mixed with soil and a substrate containing the microbes (a water holding gel, etc.). This mixture is placed back into the holes hence the name vertical mulching. Typically these holes are dug 1-2 feet apart around a plant or tree out to at least the dripline and even further is better. The compost and mulch provide food (organic matter) and energy (carbon) for the microbes as they become established with the plants roots.  Sometimes a organic fertilizer and trace minerals are added to aid in deep root feeding.
A variation of this technique can easily be done to improve poor and compacted soils and breakup hardpan. First we take a garden hose or better a hose fitted with a 2" inside diameter steel pipe to make a water drill. We turn on the water to a fast setting and press the tip of the steel pipe to the ground. The water pressure coming from the end of the pipe will bore a hole into the ground so we just keep turning the pipe and pushing down till we reach our desired depth (in most cases 12-18" deep is sufficient). The water should have created a hole 1-2" wide. We move the pipe over 12-18" and repeat the process until we have covered all the area we wish to treat. The holes can now be filled in with compost and then watered in. Using a seaweed based liquid organic fertilizer and some molasses mixed in with the water will jump start the microbes to improve the soil. A secondary benefit of using a water drill is that the water washes away the soil from the roots without destroying them while a conventional drill or soil core cuts through the roots creating stress for the plant or tree. Note: If the bottom of the pipe has small notches or teeth it will chew through compacted soil easier. Notches can be easily added with a hacksaw. 
Another variation of this technique can be used to lift water from a shallow water table or layer into the root zone, where it can be reached by plant roots. Tests have shown that a mixture of rice hull ash and good compost can have a strong wicking action and actually lift water several feet from the subsoil into the root zone. This type of mulching can be extremely valuable in areas that are hard to water or during drought conditions.
Potting Media 
For years professional growers used pine (conifer) bark as a soil less medium for growing plants in pots. When the pine bark is many years old and well rotted it worked very well and was inexpensive. Over the years as the old stockpiles of pine bark left over from the lumber and paper industry have been used up, suppliers have been forced to use fresher and fresher bark. As a result growers in many areas have been paying a lot more or have been receiving lower and lower quality materials resulting in higher costs and more disease and pest problems. Eventually these higher costs are passed on to the consumer and we have increased pollution from all the chemicals that are required.
With the rising cost of oil many lumber yards and paper companies are using the bark as fuel for their boilers and dryers. As a result they are no longer selling bark for potting media forcing many growers to look at alternative media.  To address these issues for growers, many universities have been experimenting with compost and composted wood chips or mulch as a planting and potting medium. It has been repeatedly found that these alternate products work better and at lower cost in most cases. To be successful, growers need to reevaluate their watering and fertilization systems and how they use materials. There has been so many success stories from research projects by various universities. American Nurseryman Magazine in recent years have published several articles encouraging growers to experiment with compost and composted mulches as a growing medium. 
For most gardeners, equal parts of compost, sand, and topsoil will make a good starting mix. Additionally, well rotted or aged (composted) native mulch works well in many cases.
Jason McKenzie of the Pineywoods Nursery near Conroe, Texas has found that a 50:50 potting mix of fine screened composted native mulch and fine screened compost gives superior growth and root development on many species with minimum fertilization and greatly reduced watering.
Researchers at the University of Vermont have shown that it is possible for disease transmission to occur via wood chips taken from infected trees and used around healthy landscape plants. They found that the nematode that causes pine wilt (Bursaphelenchus xylophilus) could move from infected chips to young Scotch pines if the infected chips were tilled into the soil during transplanting or applied against a trunk that had been wounded. While it is theoretically possible, in is very unlikely this type of disease transmission would occur in practice as raw wood chips are not incorporated into the soil, and the mulch should not be piled against a tree trunk (nematodes cannot move more than a couple of centimeters on their own). Additionally, the pine sawyer beetles that transport this nematode are not attracted to wood chips. This small risk can be eliminated by composting the wood chips for a few weeks before applying them. Also the same risks apply to bark mulches, since infected trees are often the first ones harvested for lumber and pulp.
Verticillum wilt is a common disease caused by a soil-borne fungus resulting in the decline or death of many shrubs and trees. It is caused by the fungus Verticillum dahliae and endures in the soil in infected plants or as flecks of scleroti, a type of fungal tissue designed for long term survival of the fungus. If diseased trees are ground up and used for mulch, it is possible that this disease can spread to mulched plants. It has been found that excess synthetic nitrogen fertilizer favors development of this disease. However, this disease is rapidly destroyed if the mulch has been composted for at least 3 days at a minimum of 130 F.
Rhizoctonia solani is another plant pathogen that causes damping-off of many types of seedlings. This pathogen is actually stimulated by fresh mulches as it feeds off the cellulose in the wood. Again, composting the mulch for a period of time before using eliminates this potential problem.
Prevention of disease is another good reason to use composted mulches wherever possible, if you can find them.
Remember that many types of mulch develop various types of fungus on its surface as it decays. A few common types are artillery fungus, bird's nest fungus, slime molds, puff balls, toadstools, mushrooms and others. Visible signs of fungus are often the fruiting spores and are beneficial to the soil and plant health. 
For example, fungi known as the Stinkhorn Fungus (Phallus impudicus) often found growing on mulches with a high carbon top nitrogen ratio. It may start as an egg shaped mass to a stalk covered with slime coated head. This fungus deserves its name as it often has a strong odor similar to that of rotten meat. The fungi produce this strong odor to attract flies and other insects. As the insects crawl on the slime they pick up fungal spores that they carry and spread to other locations. This fungus is most often found during warm moist conditions in the summer and is actually hard at work breaking down the organic matter in the mulch into a form that plants and other microbes can use.
Fungal activity is the sign that nature is hard at work releasing the nutrients and energy stored in the mulch, which is required for good plant health. If the appearance of the fungus bothers you, the visible appearance can often be eliminated by raking the mulch layer, blasting it with the water from the garden hose, or both. Also, if there is a lot of raw or fresh wood (high C:N ratio) in the mulch, the fungus can form a hard barrier that is difficult for water and air to penetrate.
In rare cases large amounts of organic matter may actually increase disease rather than suppress it. The process in which this occurs is not fully understood. The soil environment is changed by the organic material, a rapid growth of the microbial population occurs using up all the available oxygen (O) and producing large amounts of carbon dioxide (CO2) in the process. This happens when the material is compressed or so saturated with water that air movement is restricted. Under these conditions disease organisms would have an advantage for a while.
Pitfalls of Mulching
As the popularity and benefits of mulch have become better known, mulched landscapes are very common. However, using mulch is a science; not an automatic guarantee of successful gardening. Misuse of mulch, ranging from improper choices to misapplication, may lead to problems. Problems using mulches may occur when recommended horticultural practices and procedures are not followed. 
Odors: Odors are warning signs of low quality and potentially dangerous mulches and composts. 
1) Anaerobic organic acids that have a strong odor from putrefying organic matter. The odor varies depending on feedstock or material and what is going on, however they are all very bad.  These type organic acids form under conditions without oxygen (fermentation) which also produce alcohols.  Plant roots are very sensitive to alcohols as little as 1 ppm will kill most plant roots.
Acetic acid - vinegar smell, loss of N2 and P, alcohols present
Butyric acid  - sour milk smell, alcohols present
Valeric acid - vomit smell, alcohols present
Putrescine - rotting meat smell, alcohols present
2) Ammonia - implies an immature compost (phytotoxic) and a loss of nitrogen
3) Rotten egg (H2S) - implies an immature compost (phytotoxic) and a loss of sulfur
Color: Is often an indicator of potential problems with mulch or compost and other organic materials. A black color does not occur naturally in mulches or compost under good conditions, only a deep chocolate brown. However, many people believe black is good and some unscrupulous vendors like to take advantage of this idea.
Black organic materials in nature occur when materials decompose under anaerobic conditions (without oxygen). These conditions favor disease and other pathogens and use a different set of microbes to decompose the material. As a result pure “black” compost or mulch does not have good fertility, indicates anaerobic decomposition, and other problems. The sulfur is gone (out gassed H2S), nitrogen is gone (NH3) or in wrong form, and alcohols are usually present. Good compost is a deep chocolate brown when dry.
Industrial wastes are often used to blacken products for marketing purposes (ex. Even though it is illegal in some states some companies ground up old railroad ties to help darken material).
Smelter wastes are sometimes used as feedstock to blacken products. Copper sulfate (CuSO4) or other sulfur compounds may be present. As they breakdown elemental sulfur (S) may be produced which is a natural fungicide that kills the beneficial fungus. 
Boiler ash (bottom ash) is another industrial waste product used to color or blacken products. Boiler ash tends to be high in salts and extremely alkaline. The alkalinity is so strong that it will chemically burn raw wood black in a couple days. The products produced tend to be alkaline with high salt, with very high carbon to nitrogen ratios. Some ashes may contain large amounts of heavy metals that contaminate the mulch exceeding federal regulatory levels for safety. These mulch products will often turn a bleached grayish color in a few weeks after exposure to sunlight. These type products are very common in many areas. 
A question we often hear is, "What really happens if I use cheap or bad mulch?"  Another statement we often hear is, "I do not see any difference, why should I pay more?" or "I can get it cheaper down the street at your competitor." At the same time, we often here these same people complain about all the weeds they have and the excessive time they spent weeding, or how much money and time they spent at the doctor's office for illness or allergic reaction related to the herbicides they used to spray the weeds or other chemicals that may be present. 
The following guidelines will help to ensure success: 
  • Mulch choices and practices vary depending on many factors like climate, plant species, age of plants, soil type, location, watering practices and others. These factors vary regionally (state to state) but can also vary in a single backyard.
  • Heavy mulch around certain plant species, during extreme wet conditions can hold too much moisture. This adverse condition typically occurs in plants adapted to dry conditions (cactus, mesquite, etc.).
  • If mulch is applied against the bark at the base of some woody plants it can lead to stem rot. Some people recommend that for hosta's, Mulch should be applied right up to the crowns but not over them or touching them (allow a small gap for air to circulate). Remember that in a forest, the leaf and twig litter (i.e. mulch) is thick under the leaf canopy but becomes very thin at the base of the trunk, hence this is how mulch is to be applied (i.e. copy nature).
  • Covering the soil with mulch too early in the season with certain vegetable species can hold them back by keeping the soil too cool (it helps other species grow faster and produce more). When using any soil amendment (compost, mulch, fertilizer, etc.) one must understand the cultural requirement of the species of plant being grown to get the best results.
  • If some types of mulches are applied deeper than 4", feeder roots often grow into the mulch layer. Later, a disturbance of the mulch or drying out of coarse mulch layers may injure or kill these feeder roots. 
Black polyethylene roll type plastic mulches often look bad, absorb excessive heat (if not covered by an organic mulch) essentially cooking the root systems of most plants. In wet years the plastic often traps too much moisture in the root zone drowning plant roots and creating a breeding ground for disease. The perforated types often only work as a weed block if installed a certain way, and most need a large overlap of material to prevent roots/weeds from growing between layers. Also the use of plastic mulches creates indirect and hidden costs to society related to environmental issues, the direct cost of removal, collection and waste-disposal. Recent studies are finding that while plastic mulches help to obtain yields earlier in the season than bare ground, total yields over the entire season is often higher from bare ground.
Always go look at mulch before you order. Sometimes people have a visual image in their mind about what they want, and very frequently they use terminology incorrectly. Also in different areas of the country words and terminology are used differently. As a result consumers will place an order, only to be disappointed in the results when it is delivered. 
Also many dealers and producers will use incorrect or misleading terminology. Some suppliers/dirt yards sell products that use words like "Black" and "Humus" in their names. These products are often made from fresh pine bark fines, do not contain any humus, and are chemically burned to turn it black by adding very alkaline chemicals (i.e. it is mixed with boiler ash which is very alkaline and contains high levels of salts). Other dealers will grind up old pallets, scrap wood, trees, etc. and mix it with fly ash or bottom ash then sell it as a black hardwood mulch. These type products are very poor mulch choices and are often toxic to many plants. People use them, but when the plants get sick and die, they think "I just do not have a green thumb." People buy them because they are often sold at bargain prices...but they are not very cost effective.
Mulches made from almost any type of old pallets, scrap wood, tree trunks, etc. will contains very little nitrogen and even less is available for the plants. These types of materials often have carbon to nitrogen ratios of 500:1! For microorganisms to break down these type of products they must use up all the available nitrogen in the soil leaving the plants very nitrogen deficient and stressed. This condition causes the plants to become much more susceptible to insects and diseases. For comparison, good mature compost will have a carbon to nitrogen ratio of about 25:1 that plants love. There is also a risk factor that mulches made from old pallets may be contaminated with dangerous chemicals that are toxic to plants and the environment.
Mulch made from fresh ground trees can also cause problems in some cases. In fresh wood there is an abundance of soluble carbon compounds that can accumulate in the soil as the fresh wood chips break down. Many beneficial microorganisms only antagonize soil pathogens when they are stressed (i.e. must work for their food). These types of carbon compounds are like candy to the microbes. The microbes are busy eating hence do not have time to bother with pathogens. While the good microbes are busy the pathogens can build up in the soil and are allowed to gain a foothold. Fresh mulches can also become slimy, hold too much moisture, and block airflow creating conditions for disease organisms to grow. Several universities have found these type effects are worse on soil low in organic matter, new landscapes, and compacted soil. This is another reason why composted mulches are more effective and are a better value than fresh mulches.
We sometimes see mulches advertised with phrases like, decay resistant, does not attract insects, will not grow mushrooms, etc. Translated this means those mulches have been treated with toxic synthetic chemicals (herbicides, pesticides, fungicides, etc.) to produce these effects. These type of mulches defeat many of the benefits that mulches provide, and the chemicals they contain pollute the environment and endanger human health. 
Also, research from the USDA has found that if hairy vetch was killed with glyphosate (active ingredient in Round-Up TM) and then cut and used as a mulch, yields in some plants were reduced by 50% (HortScience, December 1997). Research at Michigan State University (published in same issue of HortScience) has found significant growth reduction on ornamental plants using pesticide treated grass clippings as a mulch with post emergent herbicides causing the most damage. It has been learned that most herbicides do not breakdown as previously believed, hence, we should not use mulch from plants or grass treated with herbicides (or pesticides and fungicides). Research at North Carolina State University has shown that herbicide treated grass, when used as mulch, substantially reduces plant growth (80% for cucumbers, 65% for marigolds, 34% for salvias, etc.). Michigan State University has also published research on using pesticide treated grass clippings as a mulch and found significant growth reduction in all species planted (HortScience, Vol. 32(7):1216-1219, December 1997). Purdue University has expanded these studies and found that even growth regulators will persist for months and cause harm when these plants or leaves are used as mulch (Journal of Environmental Horticulture, vol. 15, no.4, December 1997).
Some media articles have talked about the danger of organic materials in agriculture and horticulture. While not a pitfall of mulches or compost, it is related to poor management practices in raising chickens and other livestock. Certain new types of diseases may be present in some types of organic matter and particularly in manures. A new strain of Escherichia coli, a bacterium that is normally benign or beneficial has been discovered that is extremely toxic. It has already caused severe illness and death across the country. It is known as E. coli 0157:H7. As in all E. coli strains, it is easily destroyed by heat. This is another advantage of using any heat composted mulches (i.e. Composted Native), as E. coli is destroyed in a hot compost pile.
A problem that sometimes occurs in mulch is called the "Toxic Mulch Syndrome" or "Sour Mulch." This most often occurs with bark mulches (pine, hardwood, etc.) but can happen with almost any organic mulch. It occurs when a fine grained (small particle size) mulch is stacked over 6 feet high and remains wet for long periods of time. The material compresses and starts fermenting (anaerobic decay instead of beneficial aerobic decay) and produces chemicals (methanol, acetic acid, ammonia, hydrogen sulfide and others) that can kill annuals and damage many woody perennials. These chemicals have a strong sour acid odor versus a pleasant musky smell of fresh cut wood or compost. This problem can also occur in bagged products that have been stored in wet conditions as well as in bulk products. Using coarse ground materials reduces this risk and is better for most plants. Bedding plants and low-growing shrubs are the most vulnerable to poisoning by soured mulch with symptoms appearing in a few hours to several days after mulch is applied. Hence, avoid mulch with a strong odor of vinegar or rotten eggs. Good mulch will smell like freshly cut wood or have a rich earthy smell if it has been composted.
If mulch is piled too deep or if its texture is too fine (i.e. ground pine bark or sawdust) it will easily become compacted preventing the soil from breathing. When this occurs the soil becomes oxygen depleted causing roots and beneficial microorganisms to die. This leads to increased plant stress which we sometimes see as insect and disease problems. Studies at Cornell University have found that soil oxygen depletion under wood chips (not bark) is minimal, even when piled as deep as 10-18 inches. 
Note: Aeration is a function of particle size; coarse ground mulches will breathe better than fine ground mulches and will also allow water to soak into the soil more easily.
During the decay process various types of fungus may grow on the mulch surface. The artillery fungus (Sphaerobolus stellatus) is also known as the shotgun fungus since it can blast its spores 10-15' into the air. These spores are brown to black and very sticky, hence they can discolor light colored surfaces by sticking to them (Bird's nest fungus will also shoot its spores but not as far). If discoloration does occur, a soap and water solution will help to loosen the fungal spores so they can be scrubbed off. Most visible signs of fungus will naturally disappear as the mulch continues to decay into humus. The appearance of slime molds is distasteful to some people, however, the visible signs of this fungus is easily removed by periodically raking the mulch. These types of problems are much more common on mulch made from fresh or woody material rather than composted. They also are more common in thicker mulch layers (4-6" deep).
When using mulches we need to remember the area of the country we are in (i.e. weather and climate) and the type of soils and plants that are growing. Along the Gulf Coast a 3" thick mulch may be desired for most species, while in Arizona or New Mexico a organic mulch 3" thick may prevent the scant rainfall from reaching the soil or trap moisture around plants used to very dry soils (i.e. cactus) and increasing the possibility of disease. In drier areas perhaps only a 2-1" mulch would work better depending on the plant species and watering requirements. Another possible problem in very dry areas is the possibility of fire from a dropped cigarette or sparks from a fireplace or bar-b-que pit. Mulches like bark, shredded wood products, straw, pine needles, ground rubber and some plastics are easily ignited. 
Another factor to remember is that hot moist climates have a much faster rate of organic matter decomposition and require more frequent mulching. In cooler climates, with long winters, the mulch will break down at a much slower rate. If you are not sure, you can contact the local county extension agent's office or the horticultural department at a nearby university for detailed advice for a given locality.
A few mulches have been shown to hurt plant growth (aleopathic) these include black walnuts, eucalyptus (blue gum), tansy, wormwood, and French marigolds.  
Using Mulch for Frost Protection
Along the Gulf Coast we only have a couple nights a year where frosts are a problem. So many gardeners are tempted to grow tropical and other tender plants like citrus. There is an easy trick that I learned from a friend which can give gardeners a few extra degrees of protection on cold nights if the plants have been well mulched.
It is well known that a thick mulch layer will keep the soil warmer in the winter since the mulch acts like a blanket (an insulator) and keeps the heat stored in the soil from escaping. On the night of the cold weather, take a garden rake and remove the mulch out to the dripline of the plant, exposing the soil underneath the plant. This will allow heat energy stored in the soil to escape by both radiation and convection. IF there is no wind (or it is very light) and only a couple of degrees of protection are needed, there will be a chimney effect created around the plant from the rising warmer air. The energy that is released from the soil will keep the air around the plant a few degrees warmer and often it is enough to prevent frost from forming. If the soil has been well watered a few days before it turns cold, then even more energy will be stored and be available to help warm the plant. 
The next day after the cold weather has passed, rake the mulch back into position around the plant. This allows the heat energy from the surrounding soil to move into the area around the plant and recharge the soil for the next time it is needed.
A variation of this technique is to remove the mulch as before creating a circular berm around the plant with the mulch forming a basin. If the soil type is slow to drain, this basin can be filled with water. The heat energy stored in the water may also give some extra protection.
Another benefit of using mulch is protecting soil structure. The occasional freezing and thawing of soil under mulch if done slowly can be beneficial to the soil. However, under the conditions of rapid freezing and thawing the soil aggregates can be broken down reducing the tilth of the soil. 
The Science of Mulch
Often we hear people talk about the possibility of "nitrogen tie-up" or robbing the soil of nitrogen when using mulch thereby depriving the plants of a required nutrient. In practice this can only happen when proper mulching techniques are not followed or a low quality mulch is used. It is important to remember that even when nitrogen tie-up does occur, it is only temporary, and in time the nitrogen is returned to the soil. In fact extra nitrogen is usually returned to the soil. 
A healthy soil is balanced in the ratio of carbon atoms to nitrogen atoms (30 to 1). If we incorporate (till or mix) a carbon rich material into the soil, it can temporarily cause a nitrogen tie-up problem (i.e. sawdust or pine shavings can have a 300:1 carbon to nitrogen ratio or higher) until microbes break down the mulch into humus. The key item here is to till or mix, which means we are not using the material as mulch, but as a soil amendment. Soil amendments have a different set of rules that need to be followed for plants to be healthy. In practice, if a mulch material is placed on top of the ground, it rarely causes nitrogen tie-up problems except for special cases (see the mulch material descriptions below for more specific risk factors).
As we learn more about the importance of soil life and the role they have in producing healthy disease free plants we are also learning that these microbes are sensitive to certain chemicals. For example, the bisulfate anion (HSO3-) which is a solubility product of sulfur dioxide (SO2) is highly toxic to soil bacteria and fungi and these chemicals may be found in acid rain.
As the soil becomes acid, metals like aluminum and to a lesser extent manganese are mobilized and can become toxic to microbes or have negative effects (i.e. prevent fungal spores from germinating). Hence, one needs to be very careful with (or avoid) chemical fertilizers or additives such as Aluminum Sulfate which can lead to microbe toxicity. It is just safer and easier to use natural or organic ingredients to amend soils.
Also as a soil becomes more acid, it tends to favor and lead to an increase of fungal microbes over bacterial which may affect the growth of some plants. Remember that some plants prefer bacterial dominated soils.
A five year study at Pennsylvania State University has revealed that colored polyethylene sheeting can increase yields as much as 25% when compared to black plastic. Each crop was found to have its own preference. Plant response is believed to be related to the way each color plastic reflects light and heat (Horticulture, January 1997). Additional research has been done at Clemson University, Pennsylvania State University, the USDA, and others. Some of what is being learned is discussed below:
Peppers - performed best when mulched with yellow sheeting
Tomatoes - red plastic worked best
Strawberries - with red plastic they ripen more quickly, emit a stronger aroma (90% increase in aromatic compounds than black plastic), were almost 20% larger, have higher sugar and organic acid concentrations as compared to black plastic mulch.
Squash - blue and red plastic worked best
Silver - aphids tend to ignore plants mulched with silver plastic
Orange - turnips grow bigger
Blue - turnips had a sharper taste
Green - turnips had the sweetest taste (chemical analysis confirmed the most sugars)
White - plants have thicker wax coat hence plants use less water
Yellow - attracts the greatest amount of insects
Other research has found that the frequency or color of light reaching plants affects the growth of disease organisms as well as plant growth. The spores from the pathogen botrytis require ultraviolet light to germinate. Several fungal diseases require blue, ultraviolet or infrared radiation to multiply. Research at Disney World's Epcot center has found that red light reduces foliar fungal diseases on tomatoes, peppers and cucumbers. It also found that blue wavelengths stimulated the development of these diseases. However, in some cases the elimination of some frequencies also reduced plant growth.
Many plant responses to light reflected from colored plastic mulch is dependant on the exact species and cultivar tested. This means that a different species of pepper may do worse with a given color than another. Also, it has been found that two red plastic mulches, that look identical to the human eye, have very different plant growth responses, as they reflect differently at wavelengths invisible to humans. It turns out that plants are sensitive to radiation in the far-infrared and ultra violet wavelengths that humans cannot see. It should be noted that the performance attributes listed in the research reports only compares colored plastic mulch to black plastic mulch.
This research into the reflective properties of plastic mulch explains why straw or hay and fresh ground wood chip mulches work so well. These mulches are very light in color and reflect all colors or wavelengths of light. This allows the plant to use all the colors or whichever color is best. They also enrich the soil as they breakdown.
As scientists continue to study the effects of light on plants and diseases we will learn how to use the reflective nature of different types of mulch to its fullest. Plastic mulches have their greatest benefits when used in commercial agriculture to help warm the soil in the spring. A lot of research on plastic mulches is in progress and we will learn how to use them more effectively in the future.
Effects of Colors
This is a new area of research that we are beginning to understand. The color of mulch or other materials affects plant growth. Research at Texas A&M University studied trees that were planted in paving bricks (pavers) of three different colors; a light (blond), medium (red-brown), and dark (charcoal). The light and medium colored bricks reflected the most photosynthetically active (growth promoting) radiation. The air temperature above the plants was less for the lighter colors as compared to the darker colors. In the fall and winter on sunny days the air temperature was as much as 350F higher which could make the trees (plants) more susceptible to damage from sudden cold snaps. In addition, the darker the color the root growth was decreased in the upper portions of the soil which resulted in reduced growth above ground. This effect was more pronounced in the shallow rooted species.
Genetic Effects
The USDA has found that a plants gene activity changes with the type of mulch applied and with the type fertilizer used. Some of the research was done on tomato plants and they found at least 10 different genes were affected. For example when a organic mulch like mown hairy vetch was used instead of black plastic, the tomato plants lived longer and developed less fungal disease. When the organic vetch mulch was used two genes for plant defense (immune system) and two genes for regulation of aging greatly increased their activity. The researchers also found that vetch mulched fields receiving only half as much fertilizer produced larger yields than conventional plastic mulched fields with the full amount fertilizer. The vetch mulched fields also provided other benefits such as reduced erosion, decreased disease and the delays in plant aging. One of the genes studied produces chitinase an enzyme that dissolves the walls of attacking fungi along with osmontin another defensive compound and extra activity of receptors for cytokinins that regulate plant ageing. The mowed vetch mulched tomatoes more developed root systems that allowed for better nutrient absorption.
Composted Mulches
Composting mulch at high temperatures for a period of time is one of the best ways to increase the quality of the mulch and eliminate potential problems before they occur. However, there is best management practices that the processor needs to follow to ensure high quality. 
When large piles of fresh material are composted correctly, the temperatures in the piles rise to 140-1600F or more. These high temperature favor microorganisms known as thermophiles (primarily facultive aerobes) that love the heat (and actually produce it) that perform the initial decomposition called composting. These microorganisms die when the mulch is placed in the landscape and cooled off to 50-800F. Since they require high temperatures to survive they cannot grow or compete with soil microorganisms. If mulch is taken straight from a high temperature processing pile and applied to the landscape, the lower temperatures often will create conditions called a "biological vacuum." A biological vacuum can also occur from bagging mulch or from letting it get too dry before application. These can cause problems after the mulch is applied or bagged, allowing the wrong type of microorganisms (pathogens) to grow and colonize in the mulch. 
If the piles get too hot, then the range of beneficial microbes at work become very restrictive and phytotoxic acids are often produced. Some producers use techniques that produce very high temperatures since it speeds up the composting process but it is results in lower product quality.
Potential problems can be prevented in several ways. First, the producer can move the mulch to smaller piles and allow it to cool off for a few weeks before selling the product (a process known as "curing"). This allows the beneficial soil dwelling microbes known as mesophiles to colonize the mulch before application. Many producers screen their mulch before selling it. The screening process cools off the mulch which is then placed in smaller piles for sale. The second method is to soak the mulch with water, after application, to help it settle in and speed up the colonization by beneficial microbes. The addition of water encourages beneficial bacteria to grow and colonize the material bringing the fungal and bacterial species into a healthy balance (fungus can grow in drier conditions than bacteria). Often mulch producers allow mulch to dry out which reduces the weight, allowing more product to be delivered on the same truck reducing the delivered cost per cubic yard.
Note: We also receive a environmental benefit  when compost is used. In the United States today we have dead zones along our coastlines caused by excess nutrients from agricultural pollution. Some of these dead zones are from New Orleans to Galveston along the gulf coast, from Baltimore into Virginia, Chesapeake Bay, etc. When we compost manure and other organic materials to make mulch or compost we significantly improve the quality of water entering the nation’s watersheds as they do not leach nutrients as much as artificial fertilizers.

Playground Mulches
Many types of material are used to help reduce injury at playgrounds. These materials range from sand to gravel and wood chips. In tests done and published in the Handbook for Public Playground Safety, it was found that wood mulches perform well. It was found that a 6" layer of uncompressed wood mulch had the shock absorbing capacity to protect from a fall of 7', while sand or medium gravel would only protect from a 5' fall. Uncompressed wood mulch 9" deep would give the same protection from a 10' fall while sand or medium gravel would only protect from a 5' fall. When using any material it is best to use a little extra to give an added margin of safety. Also wood mulches tend to be a little cleaner than sand or gravel. For this type of application mulches made from fresh ground materials work best, since they take the longest to break down. Mulch for playgrounds should have the fine particles removed and the very large particles.
How to Calculate the Amount of Mulch to use
# Inches Square Mulch
Thick Feet Required
1" 100 0.3 cy
2" 100 0.61 cy
3" 100 0.91 cy
4" 100 1.11 cy
6" 100 1.85 cy
1) To calculate other areas with different square footage, multiple the number of square feet by 0.0031 (exactly 0.0030864) cy per square foot for a layer 1" thick. Then multiply that number by the number of inches of mulch desired.
2) Another way to estimate ones mulch or compost needs can be expressed as:
 square feet X depth (inches)
 Volume (cy) = -----------------------------
3) A third way is to remember that 1 cy of product will cover 324 square feet 1 inch deep.
4) Or we can calculate exactly what one needs by using the following example.  Volume is calculated by multiplying length times. Width times height. All measurements must have the same units (e.g. feet is the most common). 
Example: My flowerbed is 75' long by 6' wide and I want a 4" deep mulch layer after settling. How much mulch do I need? 
1st: We need to calculate how much mulch we will need. We do this by finding out how many square feet are in the bed. This is simply the length of the bed times the width of the bed (75' X 6' = 450 square feet).
2nd: Next we calculate the settled or compacted volume of mulch we need. This is simply the square feet of our bed multiplied by the thickness of the mulch layer we want (450 square feet X 4 inches thick). We need to remember that measurements in inches must be converted to measurements in feet before we can find the final volume in cubic yards (cy). Simply, 4 inches = 4/12 feet = 0.333 feet and our calculation becomes (450 square feet X .333 feet = 149.85 cubic feet).
3rd: We must convert cubic feet to cubic yards. Since there is 27 cubic feet in a cubic yard (3' X 3' X 3' = 27 cubic feet) we need to divide the total cubic feet by 27cubic feet.     (149.85 cubic feet) / 27 cubic feet = 5.55 cubic yards).
We also could use the conversion factor above listed above by the thickness and the square feet of the bed (0.0031 X 75' X 6' X 4" = 5.58 cubic yards).
4th: The last step is to allow for the settling that will occur after the mulch is put down (remember that almost all mulch is sold in a loose form whether from a bag or delivered in a dump truck). The amount of settling will vary depending on the type of mulch used. In general, allow for a 10-20% settling rate. For example if we choose a 10% settling factor, more mulch must be ordered. The simplest way to estimate how much mulch is needed is multiply the settled volume of mulch by 10% and order this much extra mulch
(5.58 cy X 10% = 0.558 cy). So we need to order (5.58 cy + 0.558 cy = 6.1 cy) about 6 cy of mulch.
See Appendix 6 – Mulching Requirements of a few common plants
Mulch and Food Quality
In general any type mulch may work well (or bad) for a given situation. See the tables at the end of the chapter for are some general guidelines to help one choose the best type of mulch for their situation, and the Chapter Appendix Mulching Requirements of a few common plants.
Mulches and Animals
Various mulches are often used for animals from bedding materials in stables to flooring in dog pens. Since animals are sensitive to things humans are not it is best to use natural or organic mulches.  
Traditionally mulches such as pine shavings are used as animal bedding.  However, people are finding that fresh ground native mulches that have been screened to remove the larger pieces work better than wood shavings.  Fresh ground native mulches have help absorb urine and feces from the animals and the microbes that live in them will help break down the manure and urine resulting in less odor. Hence the mulch lasts much longer.  This saves time and money as one does not have to replace it as often.  When the pens are cleaned the spent material is great for composting.  Which may save disposal costs and one gets a useful by product. Dogs and cats also like to chew on native mulches to help clean their teeth which helps reduce them chewing on plants.
Another use of screened native mulches is on horse arenas.  Several inches of mulch lasts a long time, has the benefits listed above and is much easier on the hoofs of the animals reducing problems and veterinarian costs.
Fine screened native mulch also works well as bedding from hamsters to chickens, rabbits and other smaller animals.
Many people have found that some mulches work well for animal control as it helps keep them out of flowerbeds.  The most common is cracked nut shells such as almonds, pecans, hickory, walnuts, etc.  The hard rough edges hurt the animals feet hence they stay off of the treated area.  It only requires a light sprinkling over the regular mulch.  Another mulch that has been used is ground up fresh green pine cones as the sharp barbs also hurts their feet.  For severe problems one may lay down chicken wire before sprinkling the nut shells then the animal’s claws also get tangled which they do not like.
Most organic mulches are also safe for animals to eat .  However, some may be toxic (e.g. cocoa shells and dogs).
Mulch for Bio-Remediation
Mulches contain lots of carbon, and when carbon is combined with oxygen from the air energy is released. It is this energy that allows microbes to break down many types of chemicals and clean the air, soil or water.
Greywater - As the supply of fresh water becomes scarcer and more valuable more and more communities are turning to greywater for landscape use. Greywater is all the waste water from our homes and buildings except that used for toilets or sewage disposal. It often contains soap residues and other cleaners, food particles, water softener chemicals, and much more. There is two ways to clean the water for re-use; buy an expensive treatment and purification system coupled with a wetlands type filtration system or just use mulch. 
Mulch can be used in two ways; first form a heap (as in a compost pile) and let the greywater filter through it, then we can collect and use it. The other way is to let the greywater filter through a thick mulch/compost layer directly in the flower beds. Flower beds that have been mulched with a couple inches of compost on the soil followed by a couple inches of woodier mulch on top of the compost is the best way to mulch it is also ideal for treating greywater. 
As the greywater filters down through the mulch layer the tremendous amount of microbes (fungus and bacteria) that live in this layer eat the chemicals in the greywater breaking them down into harmless components. The microbes also help buffer the soil from changes in pH due to the effects of soaps and other cleaning products that tend to be alkaline. By the time the water reaches the plants root zone it is much safer for the plants to use. 
Studies have shown that a well drained loamy soil can absorb 500 gallons of greywater per week for every 1,000 square feet of flowerbed. Since many households produce over 100 gallons per day this is enough water to meet the demands of most plants. If more greywater is produced in can be filtered through a compost pile and then used to water the lawn or released to lakes or streams.
Greywater should be treated and used as it is produced so that pathogens do not have time to grow. If it must be stored before use then some form of treatment should be considered to prevent bacteria and pathogens from growing.
Many types of chemicals can be broken down by microbes that live in compost and mulch.  Oil and grease will be absorbed by the mulch and are easily broken down.   It has been proven that Compost can bioremediate (in-situ or at facility) the following types of chemicals:
  • Polynuclear aromatic hydrocarbons found in creosote.
  • Chemicals such as carbofuran insecticide (carbamate family) and simazine herbicide (triazine family)
  • Pentachlorophenol, dioxins, cyanides, TNT, DDT, creosote, and coal tars.
  • Biodegradation of PCB and TCB, and BaP (PAH)
  • Pentachlorophenol (a wood preservative more toxic than CCA), dioxins, cyanides, DDT, TNT (explosive), creosote, and coal tar.
  • Explosive propellants (WC860 and H5010) contain nitrocellulose, nitroglycerin, dibutylphtalate, calcium carbonate, dinitrotolulene, diphenylamine, potassium nitrate, sodium sulfate, graphite, tin dioxide.
  • Hexachlorocyclohexane (HCH)
  • Alachlor, metolachlor, and 2,4,D
  • CE (trichlorethylene- used to degrease parts)
  • Explosives 2,4,6,-trinitrotoluene, hexayydro-1,3,5,-triniro-1,3,5-trizine, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine
  • Chlorophenol, PAH's (1-octadecene; 2,6,10,15,19,23-hexamethyl-tetracosane, phenanthrene,flouranthene and pyrene) and Aroclor 1232
  • Mineral oil and grease, diesel, JP-4, and gasoline 
  • Almost any hydrocarbon based material
See , “BIO-REMEDIATION - The Natural Way” for more information.
Making your own Mulch 
Over 30 states have now passed laws restricting the amount of organic materials entering landfills, Hence disposal is becoming an increasing problem in some areas.  
If one has there own shredder or chipper it is easy to chop up woody branches up to 2-3” in diameter. These chips can be used as mulch or a carbon source for making compost. These chips (Native Mulch) make one of the most beneficial mulches.  If you have a large amount of brush or large pieces from tree removal these can be stockpiled then periodically have a grinding company come in and grind the material for you as large commercial grinders are very expensive to own and operate.    
To get rid of leaves one of the easiest ways is to use a lawn mower with a mulching blade (works best if the blade is sharp).  One simply mows over the leaves and they are chopped up into small pieces that will settle between grass blades and decompose quickly improving the organic matter content of the soil.  Mowing is often cheaper and faster than raking and disposal when all costs are included.  Dry leaves can be very abrasive hence on large areas check the blade periodically to ensure that it is still sharp.
Several companies now offer leaf bagging equipment that will vacuum up the leaves which then can be used in a compost pile. Many landscapers will also pay a dump fee to get rid of their leaves and brush trimmings, hence another source of material.  Many tree service companies will also provide chipped branches and limbs.  Small amounts of material are usually not a problem, however if one accepts larger amounts or collects a dump fee, then your state regulations may require some form of permit.  Always check with your state environmental or solid waste departments first.
Mulch Naming Conventions
Often we find many vendors of mulch products using the same terminology to describe vastly different products. The consumer often ends up confused or disappointed when the product arrives. In an effort to standardize product descriptions and prevent fraudulent claims, the “Mulch & Soil Council (formerly the National Bark and Soil Producers Association)” has devised definitions listed in Appendix 8:
Pine Bark Nuggets:- products derived from conifers of genus Pinus with particle size from 1.25" to 3.50" in diameter.
Pine Bark Mini-Nuggets: products derived from conifers of genus Pinus with particle size from 0.50" to 1.50" in diameter.
Pine Bark Mulch : products derived from conifers of genus Pinus with particle size less than 1.5" in diameter.
Western Large Bark: products derived from conifer trees common to the Western region of North America with particle size from 1.75" to 3" in diameter.
Western Medium Bark: products derived from conifer trees common to the Western region of North America with particle size from 1" to 2" in diameter.
Western Small Bark: products derived from conifer trees common to the Western region of North America with particle size from 2" to 1" in diameter.
Western Pathway Bark: products derived from conifer trees common to the Western region of North America with particle size from 3 to 2" in diameter.
Western Bark Mulch: products derived from conifer trees common to the Western region of North America with particle size less than 1" in length.
Hardwood Bark Mulch: products derived from deciduous hardwood trees with particle size less than 3" in diameter.
Cypress A Mulch: products derived from trees of the genus Taxodium (i.e. species such as Bald Cypress) with particle size less than 3" in length.
Hemlock Bark Mulch: products derived from conifer of the genus Tsuga with particle size less than 2" in length.
MULCH PRODUCTS - consist of 100% bark and wood products (excluding the use of any reprocessed wood products), mechanically screened and/or shredded, containing at least 70% of the named material as follows:
Pine Mulch: products derived from the conifers of genus Pinus with particle size less than 1.5" in length.
Hardwood Mulch: products derived from deciduous hardwood trees with particle size less than 3" in length.
Cypress Mulch: products derived from trees of the genus Taxodium with particle size less than 3" in length.
Western Mulch: products derived from conifer trees common to the Western region of North America with particle size less than 1" in length.
Cedar Mulch: products derived from trees of the genus Thuja with particle size less than 3" in length.
Stump & Root Mulch: products derived from the processing of tree stumps and/or roots.
MULCH BLENDS - consist of bark, wood products, or reprocessed wood products, mechanically screened and/or shredded that exceed any limits for Decorative Bark & Bark Mulch or Mulch Products by virtue of genus mix or particle size. Mulch blends must be labeled as a mix or blend. If a specific material is used in the product name, it must comprise at least 51% of the total volume. If reprocessed wood products are used in any portion of a blend, such use must be indicated on the product label and shall not contain hazardous materials above limits established by the Environmental Protection Agency (EPA).
WOOD MULCH - consists of wood, wood products, or reprocessed wood that is mechanically shredded and/or screened and shall not contain hazardous materials above limits established by the Environmental Protection Agency (EPA).
RUBBER MULCH (we do not recommend)
Rubber mulch is typically made from ground up recycled tires and has generated a lot of discussion on the benefits and risks or dangers of using it.
There seems to be two schools of thought on rubber mulch. It seems that all the studies paid for by the rubber mulch manufacturers and tire companies show benefits after their PR firms get through with them, while all the independent studies show that it is toxic and dangerous.  
Let’s look at each of these claims and see what the research says:
Doesn’t float away, doesn’t blow away, doesn’t sink into the ground
Most rubber mulches have a specific gravity greater than water hence they do not float or wash off as easily as some other materials like bark mulches.  If the soil is healthy and full of microbes and earthworms ALL materials will settle and sink over If a material is heavier (denser) than water, it will sink or settle faster than a lighter material like an organic mulch. 
Research at Texas A&M University found that the particles from shredded brush (i.e. native mulch) would physically lock together preventing erosion even in extreme weather events.  Researchers have also found that during decomposition that bacteria produce chemicals called polysaccharides that act as glue to help hold the particles together. The hyphae from many fungus species also help to lock the particles together and to the soil surface. As a result, the native mulch and compost resisted erosion (floating or blowing off) better than any other material tested.     
Doesn’t decay away, Lasts many years
Rubber mulch is broken down by microbes like any other product (remember microbes can break down granite rocks into soil), rubber is easy by comparison.  The rubber encourages species of bacteria that break down rubber and rubber like products in your home to multiply.  The additives in tires to prevent bacteria decay (which are toxic chemicals) are broken down by white and brown rot fungal species that live in soil.  This same decomposition is what releases the toxic chemicals in tires.
Doesn’t feed or house insects
Nothing eats tires except microbes.  However, the tire mulch does kill many species of good microbes that kill insects and prevent disease.  The toxic chemicals in the tires will also kill beneficial insects that help control pests.
Doesn’t smell, mat or mold
This may be a matter of opinion, but most people find that rubber mulch starts to stink as it gets hotter.  On a hot day it has a strong stench.  As tires are ground up into chips the amount of surface area is greatly increased and all the new surfaces are freshly exposed allowing for maximum odors to be released.  Many people get sick from just being in the sales area of a store selling new tires.
As rubber mulch heats up, it releases toxic gases such as volatile organic compounds (VOCs) and another class of chemicals called polyaromatic hydrocarbons (PAHs).  These gases have been found to cause irritation of the nasal and respiratory passages, central nervous system damage, depression, headaches, nausea, dizziness, eye and kidney damage, and dermatitis.  Hence, ground tires should never be used in an enclosed area or indoors.  These effects would be even worse in areas of high air pollution (i.e. Houston).
It is true that rubber mulch will not mat down as easily as organic mulches, since the beneficial microbes that create soil structure, prevent insect and disease problems, cannot live in it.    
As to mold, I have seen many tires used on piers and boat docks covered with algae.  I have also seen tires used as planters covered with what appears to be mildew and mold.  Mildew and mold will grow on about any surface if moisture is present, unless it is too toxic and something kills them. 
Impedes weed growth
In comparison studies of several mulch types in herbaceous perennials, rubber tire mulch was less effective than even raw wood chips. Other studies have found that even sawdust worked better and have found rubber mulch less effective than straw and other fibers.
Several studies have found that rubber tire mulches kill many species of plants hence the public relation specialist spin it off as “retards or impedes weed growth”.  Who wanted flowers in the first place?
Also as temperatures rises the type of plants that will survive is reduced and rubber mulch can get fairly hot (see below).  Metal toxicity also reduces the type of plants that can live and grow in rubber mulch (see below).
Water and nutrients permeate
Some researchers have found that ground up tires can absorb chemicals from fertilizers and pesticides preventing them from leaching into ground water.  Hence, fewer nutrients reach the plants. Eventually, the tire chips will degrade and the stored chemicals will be released most likely at time when one does not want or need them.
Water will run through the tire mulch as it is highly permeable.  However, the problem begins when the water reaches the surface of the soil.  Earthworms and soil microbes create soil structure allowing air and water to enter the soil.  Earthworms and microbes require decomposing organic matter as a food and energy source.  Without this food source most of the earthworms and microbes will die off and the soil structure will collapse over time.  When this happens, the soils will become anaerobic (favors root pathogens) and water and air cannot enter easily.  Hence, conditions may be created that favor disease and since the water can no longer be absorbed, it must run off.   
Safe for flowers, plants and pets
Research at Bucknell University has found that the leachate from ground tires can kill entire aquatic communities of algae, zooplankton, snails and fish.  Even at low concentrations it can cause reproductive problems and precancerous lesions.  Also, marine life from seaweeds to plankton is negatively affected.
The toxic nature of the leachate from tire rubber is due at least in part to the chemicals used in producing tires (cadmium, chromium, aluminum, copper, iron, manganese, molybdenum, selenium, sulfur, and zinc).   Of these minerals, rubber tires may contain extremely high levels of zinc even up to 2% of the tire mass.  Many plant species have been shown to accumulate zinc in their tissues to the point of death.  USDA researchers who have studied the effects of metals in sewage sludge, biosolids and compost, have found that ground rubber should not be used on any agricultural or garden soil, potting media, or compost.  Yes- some companies use tire chips and crumb rubber as a bulking agent for compost and we wonder why the compost does not work and is toxic to plants!   
Other rubber leachates have been found to cause problems from skin and eye irritation to major organ damage and even death. Long term exposure can lead to carcinogenesis and mutagenesis.  For example, 2-mercaptobenzothiazole used in vulcanizing rubber is highly persistent in the environment and harmful to aquatic life.  Ground rubber also contains a class of chemicals called Polyaromatic hydrocarbons (PAHs) that many studies have found extremely toxic to humans and the environment.  
Research has also found that the toxicity of leachate from the rubber tire mulch increases over time as the rubber breaks down.   
COLORED MULCH – (we do not recommend)
Colored or dyed mulch is a product that comes in many forms and has evolved and changed a lot over the last few years.  Many of the early problems have been overcome, however it is still a special purpose mulch, with limited uses, and must be used correctly or it may result in stunted plant growth coupled with increased insect and disease problems.
Colored mulches are made by grinding up some form of dry wood waste into chips and then dying them with a water based solution.  For the dyes (colorants) to stick to the wood it has to be dry and some form of chemical binder needs to be used to help the dye stick to the wood.
Colored or dyed mulch entered the market several years ago and created many problems for consumers.  The inexpensive early dyes often contained toxic heavy metals and other contaminates in the colorants. The natural or organic dyes were far more expensive and hard to find as they did not work as well, faded quickly and became unsightly.  Many of these problems have now been solved but risks remain.
Colored mulch is not widely available but availability is improving. The colored mulches with organic dyes are more expensive than the cheaper toxic chemical older types of dyes. Colored mulches do not support the types and variety of beneficial microbes required for healthy plants as other organic mulches such as a good native mulch or compost.
To absorb the dye the mulch has to be made from fresh dry wood.  This may be unused construction wood scraps, mill waste or old pallets.  Some vendors grind up and use the hazardous CCA treated wood so the colored mulch may be an unwanted source of arsenic (studies in Florida have found levels of arsenic in colored mulches to exceed Federal safety limits in over 75% of the samples tested).  
The preservatives used in the colorant may also inhibit the growth of many beneficial soil microbes that prevent disease in plants.  The dry wood waste and old pallets also have a very high carbon to nitrogen ratio (C:N>200) hence if supplemental nitrogen is not added, it may cause a  nitrogen tie-up in the soil.  Healthy fertile soil has a C:N ratio of only 30:1 hence nitrogen is pulled from the soil as microbes try to break down and decompose the colored mulch.  This often results in poor plant growth with increased disease and insect problems. 
If old pallets were ground up the colored mulch may contain dangerous chemicals depending on what the pallets were used for.  The colored dyes are often used to mask or hide the grayish color associated with recycled C&D waste wood.   
Researchers at Penn State have found that Artillery Fungus is attracted to and often grows on colored mulch made from old pallets. Artillery fungus causes black spots on the sides of houses, cars and other structures that are very difficult to remove.  Some colorants will delay the onset of Artillery fungus.
Mulch from the type of wood required often becomes hydrophobic when dry and will not absorb water easily, hence plants dry out and become stressed and even die.  
The vendors of some colored mulches use powdered dyes. Powders quickly rub or wear off and the customer is left with an ugly mess. These are more commonly found in the cheaper colored mulches. Dust from the powdered dyes are easily inhaled causing respiratory irritation.
Colorants are expensive and if the vendor does not use enough colorant, then the color will quickly fade and look very unsightly. In the best cases the colorants only last one year and less in warm humid climates.
Black colorant such as carbon black may be made by the incomplete combustion of petroleum products (possible carcinogenic to humans and short-term exposure to high concentrations of the carbon black dust may produce discomfort to the upper respiratory tract, through mechanical irritation). Higher temperatures increase the release of chemicals into the environment (black colors absorb more energy from sunlight hence have a higher surface temperature). Also nitrates (i.e. synthetic fertilizers) also increase the breakdown and release of toxic chemicals.
Natural carbon black from vegetable origin is used as a food coloring (in Europe it is known as additive E153) also carbon black (PBL-7) is the name of a common black pigment traditionally produced from charring organic materials such as wood or bone. Ask the mulch vendor for a MSDS sheet to be sure which one is used.  
Blue, green, and yellow are other colors that are available. Very little information of how or what they are made from has been published and no MSDS reports were found except for one yellow colorant made from basically an hydrogenated iron oxide compound that is relatively safe. A lack of published information is a clue that the manufacturers of the chemicals do not want the customer to know what is in them.  
Many of the new dyes are natural, such as iron oxide for a red color or carbon black to get a black color.  Iron is a plant nutrient and the red mulch will provide small amounts of iron as it breaks down.  
The new water soluble spreader stickers are biodegradable hence many of the earlier problems can be avoided. These are similar to the ones used to help compost tea, seaweed and fish emulsions stick to plants when applied. 
The colored mulches can be very attractive when used in the correct setting and the proper manner.  
  • Green mulches have been used on concrete floors to protect horses and other animals for rodeos and livestock shows.
  • Black mulches have been used to absorb energy from the sun (heat up) to warm animal pens. 
  • Colored mulches can be used on pathways where one does not want plants to grow as they are slower to breakdown compared to more natural mulches.  The negative effects also reduce weed growth.
  • They can be used to accent the color of flowers planted adjacent to the pathways (e.g. greenish gray or soft green mulch will make the yellow flowers of daylilies stand out). 
  • Colored mulches can give the "look" of redwood mulch and their use helps the environment since valuable redwood trees are not destroyed IF they are produced and used properly.
  • They can be used on the floor at trade shows or exhibits and similar uses for display purposes and can be very attractive when used in this manner.
  • They can be used to cover a bare area in a town home or other small space where a flowerbed is not wanted and then set potted plants or patio furniture on.
  • They are light weight as compared to other mulches hence easy to move, handle and apply. 
  • They are often available in bagged form for convenience.
  • They are offered in various colors and used for its aesthetic appeal. 
  • They may be used for the reflective properties similar to colored plastic for some plants. 
  • They are made from recycled materials and a good use of material that would have been burned or dumped using up valuable landfill space.  
Correct Usage
IF one chooses to use colored mulch for flower beds, only a thin layer of colored mulch should be used (½ -1" thick) over a 2-3 inch thick layer of good compost or native mulch. The layer of compost or native mulch acts as a buffer minimizing the negative effects of the raw wood. The good mulch layer gives the soil and plants the health benefits they require and the colored mulch gives the aesthetic look one may want.
A possible use for colored mulch is for their reflective properties but no controlled studies have been found (a future research topic).  What we have learned is that red plastic has been found to increase yields (12-20%) and sweetness of tomatoes when compared to other colors. In strawberries with red plastic they ripen more quickly, emit a stronger aroma (90% increases in aromatic compounds than black plastic), were almost 20% larger, have higher sugar and organic acid concentrations as compared to black plastic mulch.  However, shallow root systems are often created by all plastic and during drought periods the plants may not survive the stress. Other drawbacks are the same as clear or black plastic.  Colored wood mulches might give the same benefits without the negative effects.  As research into mulches continues we will get better answers in the future.  
Summary:  As with all products both the quality and price will vary for colored mulches. The old saying holds true “One gets what one pays for.” If one wishes to use colored mulches it is best to only buy from a reputable vendor and use correctly, but we strongly recommend avoiding this product. 

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