Dallas Morning News - December 30, 2021
Mulches – Good, Bad and Ugly
I've been mulching bare soil around new plantings and places where existing mulch has broken down over time. What I'm using and recommend for you is shredded native tree trimmings. Native mulches (also called arborist wood chips) are the best choice for several reasons. They insulate the soil and plant roots, prevent winter weeds, have good balance of carbon and nitrogen, are loaded with nutrients, stay in place well, breathe properly (oxygen in, carbon dioxide out) and break down well to feed microbes in the soil.
Shredded native tree trimmings are the best choice for mulching bare soil around all food crops and landscaping plants.
Native shredded mulches, made from pruned limbs of living trees, have a perfect balance of carbon and nitrogen. Wood and cellulose is the carbon portion and buds, stems, leaves and cambium layer are the source of protein and nitrogen.
Our native cedar (that is actually a juniper) is arguably the best tree for shredded mulch. It is especially good when double or triple ground and used for flooring in greenhouses. It provides CO2 for greenhouse plants, repels insects, and smells great.
Among the worst mulch choices are the unfortunately ubiquitous black and red mulches. Terrible choice. Chemical contaminates are one issue but more importantly these ugly mulches are nothing but ground up wood waste and contain no protein, thus no nitrogen. They are made by grinding dry wood scraps, mill waste, lumber and old wood pallets into chips and then dying. Wood is almost all carbon, with almost no nitrogen. This imbalance of carbon to nitrogen causes yellow plants unless lots of fertilizer is used. In addition to the nutrient imbalance, chemical contamination is a concern. For the dyes to stick to the wood, some form of chemical binder is often used. The preservatives used in the colorants may also inhibit the growth of many beneficial soil microbes that prevent disease in plants.
Plastic sheets or “weed blocking fabrics” should never be used under mulch, especially around trees. Look at the dying bald cypress trees on North central Expressway.
Old pallets ground up in the mulch may contain other dangerous chemicals depending on what the pallets were used for. The dyes are often used to mask the grayish color associated with chemically treated waste wood.
Mulch from the wood waste becomes hydrophobic when dry and will not absorb water easily, so plants can quickly dry out, become stressed, and even die. Colored mulches do not support beneficial microbes required for healthy plant growth as the shredded native mulches do.
Dyed wood mulches are used everywhere, but shouldn’t be used anywhere. They have bad chemistry for the soil and plants as well as contaminants.
Dyed mulches become more detrimental to the soil and plants when plastic sheets or so-called "weed blocking fabrics" are installed on the ground before the mulch is applied. Plastic creates a smothering effect that is bad for all plants, especially trees. It also fouls up the most important part of the soil. The soil surface just under the mulch is where the moisture, temperature, pH, tilth and biological activity should be in ideal condition. With the plastic there it can't happen – and there is virtually no weed control that results.
The only choice worse choice for mulch is ground up tires.