Soil Solarization, Weeds
The best non-toxic approach is to scrape 2" off the surface to remove all the stolons and rhizomes. Do not till before doing the removal. Solarization with clear plastic is best.
Other remedies include: Remove the weed physically with mechanical devices. In beds cover the weeds with newspaper and mulch. Spot spray with the vinegar herbicide formula. For turf, overseed problem areas with ryegrass in the fall. Applying corn gluten meal in the spring at 20 pounds per 1,000 square feet prior to seed germination is also helpful. An important point to remember is that nutgrass (nutsedge) grows primarily in wet anaerobic soil. Applying hydrogen peroxide, the kind at the drug store, full strength to the problem areas can help. Also let the area dry out better between watering, improve soil health, improve drainage and help the soil to drain better.
By David A. Bainbridge
United States International College of Business
Alliant International University
San Diego CA 92131
Field solarization, photo from U. of Minnesota Extension
Solar energy can be used to kill weeds, weed seeds, and pathogens in garden and restoration sites. Solarization is often as effective as herbicides, fumigants, and other hazardous and expensive pest control methods. Soil solarization is ideal for a wide range of applications in locations with high summer air temperatures (greater than 85°F during the mid afternoon is preferred but not essential) and plenty of sun.
A single layer of clear plastic laid on the soil surface can greatly increase solar soil heating and provide good control of weeds and pathogens. Recent studies at San Diego State University and Tuskegee University have demonstrated the increased temperatures can be reached with a second layer of plastic (bubble pack worked well).
The influence of these high temperatures on weeds, seeds, and pathogens is complex and not fully understood. It often provides an added growth response on top of weed and disease control. Deep rooted, heat tolerant weeds with rhizomes are usually suppressed but may not be killed by a single treatment.
There may also be some surprises. One group we know found many old clover seeds germinated after solarization. Control can be improved by repeating the solarization process after retilling the soil. Adding compost and other soil amendments may improve control of the more resistant weedy species by increasing microbiological activity in the soil.
Solarization also reduces or eliminates many common plant pathogens such as Fusarium, Verticillium, and root rots. This effect has led to most of the research on soil solarization. Control of some insect pests has also been noted.
Soil solarization is well suited for preparing seed beds in open areas. In hotter areas it may be effective in open woodlands and between shrubs with moderate canopy cover. Solarization is also excellent for preparing soil mixes and seed beds for nurseries. By placing soil mixes on an insulated surface it is possible to generate much higher temperatures. Soil should be solarized during the hottest part of the year and treatment should continue for as long as possible (3-8 weeks). Soil temperatures over 100°F for several weeks are desirable. Solarization may be effective in cooler coastal areas with intermittent cloud cover if the plastic film is left in place for a longer time or if a double layer is used.
Soil thermometers are inexpensive and two or three at different depths will provide a good indication of how well the soil is heating. If soil temperatures are not reaching 95-105°F a small test plot (20 sq ft.) of doubled plastic can be monitored to determine the potential soil temperature rise. Small bags of dirt or rounded rocks used to hold the first sheet down (see below) can support a second sheet 1-2 inches above the bottom sheet. Bubble pack plastic also works well and is more resistant to tearing.
Begin by cultivating the area thoroughly, then level the surface and minimize clods, stubble, sticks, and stones that can tear the polyethylene plastic sheeting. For dry soils apply 1 inch of irrigation water just before laying the polyethylene sheeting to improve heat capacity and heat transfer.
Fertilizer and soil amendments may be applied before the plastic is laid.
Some soil amendments have improved the weed and pathogen killing effect of solarization.
Apply sheets of clear (not black or colored) 2 mil polyethylene (4-6 mil in windy areas) when it is least windy. The plastic doesn't have to lay flat on the ground for good heating, but it may flap less if it is in contact with the surface. Thinner plastic lets more solar energy through, but is more fragile. Use wide sheets to minimize joints and place the edges of adjacent polyethylene sheets in furrows and cover them with soil. Bury the free edges, and compact the soil around the sheets to reduce the loss of heated air and moisture. Place weights 2-3 m apart on the sheeting to prevent the plastic from flapping and tearing in the wind. Rounded river stones or small soil or sand filled plastic bags (fist-sized) can be gently placed on the plastic. The heating is less intense at the edges so overlap the planting area by a meter or two if you can. When planning the layout leave sufficient space for access to other areas and drainage, either to a drain channel or to other plantings. Solarization may provide double benefits when it is used to concentrate the runoff from the plastic for crops or trees.
Inspect the site regularly. If holes develop in the polyethylene sheeting they should be patched with wide clear plastic tape. You can find holes by looking for areas without condensed moisture on the bottom of the plastic. Avoid walking on the plastic unless repairs are needed and then use socks or bare feet.
Although farmers in the developed countries often plant into holes punched in the sheet, stronger plastic can be removed and reused for several seasons. If the soil is too wet when the plastic is removed let it dry to a workable texture before planting. If you cultivate after treatment, keep cultivation shallow (preferably less than 5 cm) to avoid moving viable weed seeds from the deep soil to the surface.
Soil solarization is not perfect. It does not work against all weeds and pathogens, requires the use of chemicals and energy to make the polyethylene, and leaves a plastic waste for disposal. But it is much cleaner and safer than hazardous herbicides and fungicides and often as effective.
For more information:
Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds (UC Davis downloadable PDF)