Radiant energy from the sun, the basis of life on Earth, can be captured and amplified by home gardeners for use as a soil pest-control tool. In a simple technique called soil solarization, gardeners lay a clear plastic tarp atop garden soil, concentrating the sun’s energy in the top 12 to 18 inches. The heat trapped below the plastic can reach highs of 140°F in the top 6 inches, killing weed seeds, insects, nematodes, and many fungal and bacterial pathogens, including those that cause verticillium wilt, fusarium wilt, potato scab, damping off, crown gall disease, and phytophthora root rot. The beneficial effects from solarization are greatest near the soil surface and decrease with depth.
James J. Stapleton, Ph.D., an Integrated Pest Management plant pathologist at the University of California’s Kearney Agricultural Center Academic Unit in Parlier, California, has studied solarization since he was a graduate student in the late 1970s. Probably the biggest advantage of solarization for home gardeners, he says, is weed control, “but solarization is a general biocidal treatment that mimics the effect of soil fumigation with a fumigant chemical, and will kill pests in the soil.”
While the benefits can be big, solarization will put the garden out of commission for 4 to 6 weeks during the sunniest, hottest time of the year, Stapleton says—the peak growth period for many garden crops. “In the West—and especially here in California—the growing season is fairly long and you can solarize in the summer and follow it with a fall garden crop,” he says. “In other places, if your garden is big enough, you could solarize part of it and actively garden the rest, or if you have a bad problem with soil pests, you might be willing to forgo gardening for a year and solarize instead.”
Solarization is a simple, four-step process, Stapleton says. Step one: Cultivate the soil, and collect and dispose of dead plants and other debris that might harbor pests. Step two: Level and smooth the soil surface. Step three: Irrigate the soil very well to at least 12 inches deep to increase its heat conductivity. Step four: Lay a clear plastic tarp on the soil surface. (1 to 4 mil painter’s plastic works well and is readily available.) Anchor the edges of the tarp with soil. The closer to the soil surface the plastic is, the better the heating. Remove the tarp after 4 to 6 weeks and resume gardening. Don’t keep the plastic on much longer than that, or it will begin to break into pieces.
The heat that is concentrated under the tarp often kills pest organisms more readily than beneficials, Stapleton says, because soilborne pests tend to die out more easily in the absence of plants, while beneficials are more adaptable. Mobile pests—nematodes and insects in the juvenile or adult stages—may be able to escape the heat by moving out from underneath the tarp or burrowing deeper into soil.
Solarization is most effective when the weather is hot and cloudless. In areas that don’t get very warm or don’t get much sun, solarization may not work—and can even create a greenhouselike setting that actually stimulates weed growth. Gardeners in the far north and cool, foggy coastal regions should find out from a local extension agent if others have had success solarizing in their area.
A side benefit of solarization is an increase in nutrient availability, Stapleton says. As irrigated soil starts to heat up, the organic matter in it undergoes a mild cooking process. “Just like when you make soup, you add solid material to water and that solid material starts breaking down, creating a nice, nutrient-rich liquid. After solarization, those rich nutrients are available to your crops, usually at higher rates than they would be otherwise.”
Originally published in Organic Gardening Magaine April/May 2013.