Soil solarization takes advantage of the sun's heat, trapped under clear plastic sheeting, to control many kinds of weed seeds as well as harmful fungi, bacteria, and some nematodes. The process is carried out in summer and works best in regions that have hot, sunny weather for 4 to 8 weeks straight; daytime temperatures above 80 degrees F/27 degrees C are ideal. Solarization isn't very effective in coastal climates with summer fog, nor does it work well in very windy areas.
Plan to solarize areas you intend to use for fall vegetables, ornamental beds, or lawn. Follow these steps:
1. Cultivate soil, clearing it of weeds, debris, and large clods of earth. It is important to get rid of growing weeds, because clear plastic ― unlike black plastic ― doesn't halt growth of plants in the soil beneath it.
2. Make a bed at least 2 1/2 feet wide (narrower beds make it difficult to build up enough heat to have much effect). Carve a small ditch around perimeter and rake to level surface.
Soak soil to a depth of 1 foot: moist soil conducts heat better than dry soil and initiates germination of weed seeds, which will then be killed by heat.
3. Cover soil with 1- to 4-mil clear plastic; use UV-resistant plastic if it's available, since it won't break down during solarization. Stretch plastic tightly so that it is in contact with the soil. Bury the edges in the perimeter ditch. An optional second layer of plastic increases heat and makes solarization more effective; use soda cans as spacers between the two sheets.
Leave plastic in place for 4 to 6 weeks (8 weeks for really persistent weeds); then remove it. (Don't leave it down longer than 8 weeks, or soil structure may suffer.) You can now plant. After planting, avoid cultivating more than the upper 2 inches of soil, since weed seeds at deeper levels may still be viable.