Solarizing Soil - aka Killing Weeds with the Sun!

Weeds, insects, pathogens, and nemotodes (roundworms) can be killed without the use of chemicals and with a good plastic sheet. Radiant energy from the sun can heat soil up to 140 degrees Fahrenheit.  

Using a plastic sheet less than 1 mil thick (up to 1.5 mil is okay for windy areas), place the plastic over well-irrigated, damp soil that is exposed to direct sunlight.  For best results, do this during the hottest weeks of the year.  The plastic should be left in place for at least 4-6 weeks.  

For detailed information, visit the University of California's page Soil Solarization for Gardens and Landscapes.