Soil Solarization to Manage Weeds and Diseases

Soil solarization is a technique we often read about to control severe weed and soil-borne disease problems. The procedure is to cover problem soil with clear plastic and trap the sun's heat to raise temperatures high enough to kill weed seeds and plant disease organisms. 

This technique can be used somewhat effectively only in the hottest parts of our northern states. It is also important to realize that an area that is receiving this treatment will be out of production for nearly the entire growing season, and that weed seeds that are buried deeper than 2, perhaps 3 inches in the soil will not be affected by the procedure. Even so, for sites that are otherwise unusable, solarization can be an option worth trying. 

If you have found that weed or plant disease problems rendered your plot unusable and would like to try solarization, till under existing weeds to prevent seed production and make preparations to solarize in spring. It is suggested you try the method only if you live in one of the warmest parts of the state. Cover the site with a weed- and disease-free mulch until next spring. 

To solarize soil, all you need is a roll of clear polyethylene plastic. The best types come in 10-foot wide rolls and contain ultraviolet inhibitors. 2 to 4 mil thickness is most durable. The longer the plastic stays in place, the better. For maximum effectiveness, cover your soil for 14 weeks between June and September. 

The first step is to till the soil well, then rake it free of large clods, weeds and plant material. Be absolutely sure that the site is level, well-tilled, and smooth, providing maximum contact between the soil surface and the plastic. 

The second step is to use a sprinkler or drip-line irrigation lines to saturate the soil to a depth of 3 feet or more. This can take hours, but it is essential for successful solarization. Next, dig a 6-8 inch trench around the plot, then wet the soil again. To lay the plastic, place one edge in a trench, then cover it tightly with soil. Stretch the plastic tautly over the site, then bury the remaining edges. Leave the plastic in place all summer, making sure the wind does not blow or lift the plastic, allowing heat and moisture to escape. In late summer or early fall, remove the plastic. Do not cultivate the soil because that will bring viable weed seed back into the upper 2-3 inches of soil. Cover the site with a weed and disease free mulch, then plant the following spring. Remember, this method is most effective in warmer regions of the country. 

References: 

Controlling Pests and Diseases; Rodale's Successful Organic Gardening. 1994. Rodale Press, Emmaus, Pennsylvania 18098. 160 pp. 

Flint, M.L. 1990. Pests of the Garden and Small Farm. University of California. 

Oakland, CA 94608-1239. Pub. #3332. 276 pp. 

The IPM Practitioner. Bio-Integral Resource Center. Berkeley, CA 94707. 

Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997