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How to Solarize a Garden
Weeds vary in their susceptibility to high soil temperatures. Winter annual grasses, barn-yardgrass, black nightshade, cocklebur, chickweed, field bindweed, hairy nightshade, prickly lettuce, red-root pigweed, shepherdspurse, velvetleaf and wood-sorrel have been reported to be controlled by solarization. Bermudagrass, johnsongrass, nutsedge, purslane and established field bindweed are only slightly affected, however. The effects of solarization on soil biota and on crops are not well understood. Although many pathogens and the Rhizobium symbionts which allow legumes to fix nitrogen are killed, there is some evidence that beneficial soil bacteria and fungi are less sensitive than pathogens to solarization. Some beneficial organisms actually increase following soil solarization, while others decrease but seem to recover rapidly. Crop plants often grow faster and more vigorously in solarized soil, even if no known major pathogens were present. Although not well understood, this growth stimulation is most often attributed to increased availability of soluble nutrients. Examples of successful use of solarization for control of annual weeds in the south include experiments in Texas and Mississippi on strawberry fields. Experiments at Auburn University in Alabama found that applying chicken manure at a rate of 8 tons per acre and then covering the soil with plastic controlled morning glory and sicklepod. General Solarization Guidelines 1. Solarization is most likely to be effective during long days of high temperatures and no wind. 2. Clear 1 mm thick plastic should be adequate and is not expensive, but in windy areas, thicker (1.5 to 2 mm) plastic may be necessary to prevent tearing. Any tears in the plastic should be repaired with clear tape. If the plastic will be used as mulch with the following crop, it must have UV inhibitors. Otherwise, sunlight and high temperatures start to break the plastic down after 4 to 6 weeks. This is particularly true for the cheaper grade builder's plastics. Polyethylene (PE) and polyvinyl chloride (PVC) performs similarly. PE is cheaper but does not heat the soil as much since it is more permeable to long wave radiation. This means it has less 'greenhouse' effect in warming the soil than the PVC film. 3. The soil-plastic contact should be as tight as possible in order to raise the soil temperature as high as possible to as great a depth as possible. Thus, the area to be solarized must be leveled and free of weeds and large clods of soil that could raise the plastic off the ground. 4. Moist heat is more lethal to pathogens than dry heat. The soil can be moistened just before the plastic is applied, but irrigating with drip lines placed under the plastic is usually more effective. 5. Plastic must completely cover the soil surface. There is an edge effect of up to 24 inches in which temperatures will be cooler and solarization less effective so solarization in narrow strips is not likely to be effective. If the area to be solarized is wider than the plastic available, 2-to-3-foot wide strips can be joined with heat-resistant glue or tape. 6. Plastic must be left in place for 4 to 6 weeks. Any tears or sections where the plastic breaks down should be patched and replaced. 7. Deep cultivation should be avoided before planting the next crop to avoid bringing non-solarized weed seeds to the surface.
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In the hottest, sunniest months of the year, covering the soil with clear plastic film for 4 to 6 weeks can kill nematodes, weed seeds and pathogens in the top layers of moist soil. During solarization, soil temperatures should reach 140 degrees F at a depth of 2 inches and 102 degrees F at a depth of 18 inches according to researchers who have developed models to predict soil temperatures under clear mulch based on atmospheric conditions. The main limitation to soil solarization is finding a time between crops when temperatures under the plastic are high long enough to be effective.
