For Release On Or After 06/21/13

The extraordinary heat we experience in summer can be put to good use in the vegetable garden. Through solarization – a process of heating soil in beds under clear plastic using sunlight – summer heat can be used to reduce the harmful effects of pathogenic fungi and nematodes and kill weed seeds in the soil without using toxic chemicals.

Soil-borne fungi are responsible for such diseases as root rots, crown rots, fruit rots and wilts. Nematodes are microscopic round worms that attack plant roots, causing plants to be stunted and sickly. These soil-borne problems are not easy for the home gardener to control. They’re often particularly bad where the same type of vegetable is planted in the same area year after year.

Solarization can only be used to treat empty beds in full sun. As you remove vegetable crops that have finished, or if you have beds that are currently empty, you will have the perfect opportunity to solarize the beds over the next few months.

Water the beds to be solarized thoroughly a day or two before you begin the process. Then thoroughly cultivate the soil to a depth of 6-8 inches, breaking up all the large clods. Try to get the soil as fine as you can because large clods could insulate nematodes and microorganisms, allowing them to survive.

After turning the soil, incorporate a 2- to 4-inch layer of organic matter, such as grass clippings, leaf mold, compost or rotted manure. Adding the organic matter appears to enhance the solarization process, possibly because when the soil and organic matter mixture heats up under clear plastic, it produces gasses, some of which are toxic to soil pests – such as fungi and nematodes – and to weed seeds.

Next, dig a trench several inches deep around the bed and place clear plastic over the bed, laying the edges down in the trenches. Fill in the trenches with soil to tightly seal the edges of the plastic all around each bed. Don’t use black plastic because it will not heat the soil as well. The idea is for the sunlight to shine through the plastic and the heat to be trapped underneath B just like in a car with the windows rolled up on a sunny summer day.

The clear plastic traps the heat of the sun underneath, raising the temperature of the soil surprisingly high. The upper 6-8 inches of soil in covered beds will gradually reach temperatures between 100 and 130 degrees B high enough to kill nematodes, fungi and weed seeds. The highest temperatures occur in the upper 2-3 inches.

In some studies, the plastic cover was left on for 98 days from June to September. But other research indicates that allowing the clear plastic to remain over the bed for as few as 30 days proved beneficial in increasing crop production. Current research recommends the treatment last for about six weeks.

Keeping your beds covered from June or early July until fall planting in late August or early September should allow plenty of time for the treatment to be effective.

When you get ready to plant the treated beds, pull back the plastic and let the beds cool down for a few days. Do not turn the soil in the bed prior to planting. This can bring up weed seeds from deeper in the soil that may not have been killed by solarization. Weed seeds will not germinate if they are not close to the soil surface. If you need to apply fertilizer to the bed, sprinkle it on the surface and lightly scratch it into the upper inch of soil.

When it comes to planting beds that have been solarized, you have a couple of choices. After the soil has cooled, you can direct-seed vegetables or plant transplants. Mulch beds planted with transplants immediately and direct-seeded beds as soon as the seedlings are large enough. Good mulching materials include pine straw, chopped leaves and dried grass clippings.

Populations of soil disease organisms and weed seeds will eventually build back up. The beneficial effects of solarization generally don’t last beyond a season or two, but you can help prevent reinfecting the beds.

Thoroughly clean all garden tools before you use them in treated beds. It is very easy to carry organisms from infested areas in the soil that clings to your garden tools. Keep beds and surrounding areas well weeded, and never allow weeds to go to seed. As much as possible, rotate your vegetable crops. Avoid planting the same crop in the same area year after year because that encourages the buildup of disease organisms that will attack that crop.

Solarization can be an important part of an integrated pest management – or IPM – approach to controlling pests in your vegetable garden. Gardeners who practice IPM use management practices, variety selection, nonchemical pest control techniques and least-toxic pesticides to sensibly and safely manage pests.

Rick Bogren