Put Suns Heat To Good Use

Daniel Gill, Merrill, Thomas A.  |  4/19/2005 10:28:54 PM

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The extraordinary heat we experience in summer can be put to good use in the vegetable garden.

Through solarization – a process of using sunlight and a clear plastic covering to heat soil – summer heat can be used to reduce the harmful effects of pathogenic fungi and nematodes and to kill weed seeds in the soil without the use of 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 and are often particularly bad where the same type of vegetable is planted into the same area year after year.

Solarization can only be used to treat empty beds located 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, since large clods could insulate nematodes and microorganisms and allow them to survive.

After turning the soil, incorporate a 2-inch 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 the clear plastic, it produces gases, 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. Lay the edge of the plastic 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 – 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 degrees and 130 degrees F – 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 little as 30 days proved beneficial in increasing crop production. Current research recommends the treatment last for about six weeks. So 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, because this could 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 mulch those beds that are direct seeded as soon as the seedlings are large enough. Good mulching materials include pine straw, leaves, chopped leaves and dried grass clippings.

Unfortunately, populations of soil disease organisms and weed seeds eventually will build back up. The beneficial effects of solarization generally don’t last beyond a season or two, but there are some things you can do to keep from reinfecting the beds.

For example, thoroughly clean all garden tools before you use them in treated beds. It is very easy to carry organisms from infested areas to other areas in the soil that clings to your garden tools.

Also, keep beds and surrounding areas well weeded, and never allow weeds to make seeds.

And, as much as possible, rotate your vegetable crops. Avoid planting the same crop in the same area year after year, since that encourages the buildup of disease organisms that will attack that crop.

Solarization can be an important part of an Integrated Pest Management (IPM) approach to controlling pests in your vegetable garden. Gardeners who practice IPM use cultural practices, variety selection, nonchemical pest control techniques and the least toxic pesticides to manage pests sensibly and safely.

Get It Growing is a weekly feature on home lawn and garden topics prepared by experts in the LSU AgCenter. For more information on such topics, contact your parish LSU AgCenter Extension office or visit our Web site at www.lsuagcenter.com.  A wide range of publications and a variety of other resources are available.

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Contact: Dan Gill at (225) 578-2222 or dgill@agcenter.lsu.edu 
Editor: Tom Merrill at (225) 578-2263 or tmerrill@agcenter.lsu.edu 

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