Kill Vegetable Garden Soil Pests With Summer Sun

Introduction - Soil solarization is a practice used to manage weeds, nematodes, diseases, and insects in soil. The soil surface is covered with clear plastic, which allows sunlight to pass through and heat up the soil to temperatures that are lethal to many of these pests. If effective, solarization can reduce population levels of these pests for 3-4 months, sometimes longer.

Installation - The area to be solarized must first be cleared of existing weeds and debris. Tilling the site is helpful to increase penetration of heat into the top 6 inches of soil. Sticks, old roots, and other debris should be removed so they do not poke holes in the plastic.

Water helps to conduct heat, so best results occur if soil is moist but not waterlogged or muddy. If the soil is very dry and dusty, the solarization will not work as well. On sandy soils in Florida, the best conditions are when the soil received rain or irrigation the day before plastic is applied. If rain or irrigation occur just a short time before applying plastic, the soil can be heavy, muddy, or otherwise difficult to work with, and the clear plastic can get dirty.

At present, there are no recommendations about type or brand of clear plastic to use. Do not use black plastic or opaque plastic, which are readily available at the box hardware stores, because the soil will not reach as high a temperature as under clear plastic. Some people think thinner plastic is better, but maybe the main consideration is that the plastic should be strong enough to last for 6 weeks in the summer sun in Florida without breaking up.

The plastic should be stretched tight and the edges sealed completely by burying in soil. If edges are not completely sealed, heat will leak out and problems may result in these cooler areas. Some people recommend using two layers of clear plastic separated by spacers such as PVC pipe. This will create a buffer zone between the outer air and soil which keep soil temperatures higher, longer than a single sheet.

If the edges of the plastic come out of the ground, heat will leak out. It is important to re-bury or re-seal any exposed edges as soon as possible. Removal of a soil seal or damage to plastic will not ruin the solarization if it is caught quickly and repaired soon after it happens, preferably on the same day that the damage occurs.

The plastic should be left in place with all edges buried for at least 6 weeks. After that, the plastic can be removed, and if the procedure was successful, weeds and soil pests should be reduced for 3-4 months. Do not plant anything until the plastic is removed because the heat under clear plastic will kill seeds and plants! Disposal of used plastic can be a problem, especially if the plastic is not strong and breaks apart before or during removal.

Other considerations - A few small holes may not have much impact on solarization. However, duct tape can be used to seal small holes (< 1″ diameter). A small patch of duct tape will not affect heat conduction. Animals walking on plastic or birds pecking at the plastic can cause small holes that also can be sealed with duct tape. If the damage is extensive, the plastic may need to be replaced.

Suppose you want to cover a 10-ft-wide area and have strips of plastic only 5-6 ft wide. One possibility might be to lay the two strips side by side, overlapping in the center. However, overlap of plastic not sealed into the ground would allow a weak spot where heat could leak out. It is unknown if the two plastic strips could be sealed together successfully with tape, especially over a 6-week time period. A more reliable approach may be to seal both strips into the soil in order to have two solarized strips (each 4-5 ft wide) next to each other. But this would leave an untreated strip of soil between the two solarized strips where the edges of the plastic are buried

Raised beds are more work to prepare, but can be solarized. If water pools in low spots on the plastic, it should not be a problem as long as the water is clear, since light will pass through. But if there is dirt in the puddle it will block light, so dirt or soil in the puddle should be washed away or removed, especially if the area is large.

The edges of the bed may not receive direct sunlight if beds are positioned in an east-west direction. If the bed is positioned in a north-south direction, the east edge of the bed will receive direct sunlight in the morning and the west side will receive direct intense sunlight in the afternoon. On a bed that is positioned east-west, the top of the bed will receive direct sunlight through much of the day, but the edges may not receive much direct sunlight. As a result, the edges of the beds are cooler, and weeds may grow on the edges, particularly on the north side of the bed.

Solarization can greatly increase temperatures in the upper 4-6″ of soil. Deeper soil usually does not heat up enough to kill pests that are located there. So pests such as nematodes that are living deep in the soil will survive and eventually can move up into the solarized area. This re-invasion of the solarized soil usually takes about 3-4 months, so after that time the effects of solarization diminish. Solarization is best used for short-season crops. Pests may be reduced in a vegetable or ornamental crop planted right after solarization, but if another crop is planted immediately after that (double-cropping), the recolonizing pests and weeds may damage the second crop.

Do not bury drip irrigation tape or other irrigation plastic near the soil surface in a site to be solarized. The soil temperatures in excess of 120°F near the surface may melt some kinds of drip tape. Temperatures cool as we move deeper into the soil, so lines buried 6″ deep probably won’t be affected.

The temperatures that are lethal to pest organisms will kill beneficial organisms as well. Fortunately, beneficial bacteria and fungi seem to recolonize solarized soil quickly, so they can still break down organic materials and recycle nutrients if organic fertilizers are used.