|Use clear plastic when solarizing soil. Photo/Jeff Norcini|
For small wildflower gardens, soil solarization is a very effective means of obtaining a weed-free planting site. But it takes time and patience. Solarization is most effective when done from June to mid-August. The sun is high in the sky, days are long, and the temperature is highest. The method works by heating slightly moistened soil to as much as 140oF near the surface, according to a University of California publication. Moist heat is every effective at killing seeds and often even tubers and rhizomes of noxious weeds like nutgrasses (Cyperus spp) that are close to the surface.
Use clear plastic for soil solarization, which allows the sun to heat up the soil via the greenhouse effect (the same reason cars get so hot in the sun). Don’t be concerned if you initially see some green plants emerge after you lay down the plastic. They will die after a few sunny days.
- Eradicate vegetation and debris from the area to be solarized.
- Till the soil 12-18 inches deep; break up clods of soil, the finer the texture the better. Remove any sticks, roots, stones, and other debris brought up to the surface as a result of tilling.
- Rake the area so that the surface of the tilled area is smooth.
- Irrigate so that the entire soil profile is slightly moist (but not soggy). Moist soil is much better than dry soil at conducting heat. Also, moist heat is very effective at killing seeds.
- Cover the site with 3 to 6 ml clear plastic. Do not use black or opaque plastic. Bury the edges of the plastic about 8 to12 inches so the plastic cover is snug; the plastic needs to be snug and buried around the perimeter to prevent wind from lifting it up. If using multiple sheets of plastic, make sure the overlap is sealed and secure so as not to compromise the efficacy of the solarization process.
- The soil solarization process takes about 6 weeks. You can leave the plastic in place until you are ready to plant.
For more information:
- Robert McSorley and K.G. Harsimran. Introduction to soil solarization.ENY-062, Entomology and Nematology Department, Florida Cooperative ExtensionService, IFAS, University of Florida, Gainesville, FL.
- Stapleton, J.J., A. Wilen, and R. H. Molinar. 2008. Pestnotes: Soil solarization for gardens & landscapes management. UC StatewideIPM Program, University of California, Davis, CA 95616.