Fay's Wildflower Meadow was completely covered with impermeable plastic sheeting this August as the first step in solarizing the soil. Solarization is a low-cost, non-toxic method for weed control, in which the area is mowed, watered and covered with clear plastic for several months during the summer. The heat of the sun turns the trapped moisture to steam which kills pathogens and weed seeds. This is a great example of using natural processes to maintain a landscape instead of chemicals.
Five Steps to Weed Control Through Solarization
1. Prepare the soil. Pull any weeds or old crops. Turn in any soil amendments and then rake the surface smooth. It's important to remove any stones or clumps that might raise the plastic and create air pockets that could cause uneven heating.
2. Water thoroughly. This creates 100 percent humidity under the plastic, which acts with the heat to kill all those unwanted critters. If you must walk on the surface while placing the plastic, the soil should not be too wet or it will compact badly.
3. Dig a trench all around the bed or plot 6-8 inches deep.
4. Lay a clear plastic sheet, 3 - 6 mm thick, over the area, overlapping the trench on all sides. Fill in the trench, weighing down the plastic while pulling it as tight as possible.
5. Sit back, relax and wait. Although cloudy weather will slow things down by cooling the soil under the tarp, a few weeks of sunshine will improve your soil dramatically, easily and inexpensively. If you live in an area with cool or cloudy summers, or if you just don't want to wait all season, you can speed up the process by adding a second sheet of plastic.Using the hoops commonly used to elevate row covers or bird netting,raise the second sheet of plastic over the ground-level sheet. The airspace between acts as a temperature buffer zone during cloudy weather and the combination of the two sheets of plastic serves to raise the soil temperature an additional 6 degrees.
Regarding types of plastic appropriate for the solarization process: you want a plastic that won't degrade into tiny pieces in the soil, can be pulled tightly over the surface, and is resistant to tearing. A little surprisingly, it has been found that clear plastic is better than black plastic. The sunlight passes through the clear plastic, heats up the soil and the water, in effect steaming the soil. It produces a mini-greenhouse effect on your soil. It is important to keep the plastic close to the soil, without too much air beneath the plastic.
For more information check out the University of California Soil Solarization informational website.