Cooperative Extension Service 
Institute of Food and Agricultural Sciences
Hendry County Extension, P.O. Box 68, LaBelle, Florida 33975-0068  Phone (863) 674-4092

Hendry County Horticulture News

Solarization - Bake Out The Bugs

Many gardeners and other growers are looking for a soil treatment that can reduce nematodes, since they no longer have access to pesticides that are suitable. "Soil solarization" may provide that alternative.

Soil solarization is a nonchemical technique in which clear polyethylene is used to cover moist soil that is ready to be planted, so the heat generated by sunlight hitting the soil will be trapped  in it and raise the soil temperature high enough to kill many nematodes, fungi and weed seeds in the upper few inches of the bed. In Central Florida, 30 days of solarization of soil infested with root-knot nematodes and wilt fungi increased tomato yields 48% over those grown in soil that had been kept fallow for the same 30 days. Wilt affected 80% of plants in the fallow plots but only 8% of plants in the solarized plots.

Soil temperatures are magnified due to the trapping of incoming solar radiation under the clear plastic. To be effective, soils must be wetted and maintained at a high soil moisture content to increase the susceptibility (thermal sensitivity) of soil-borne pests and thermal conductivity of soil. Wet mulched soils increase soil temperatures due primarily to the  elimination of heat loss by evaporation and upward heat convection. In addition, a greenhouse effect is produced by prohibiting dissipation of radiation from the soil.

The most opportune time to include soil solarization for pest control would be during our hot, summer months, which fortunately are off-season in most peninsular Florida vegetable gardens. Unfortunately our summers are also our wettest period of the year with frequent afternoon rain showers. The most important impact of these frequent afternoon showers is to cool the soil and to allow an insulating ponding of water to occur on the surface of the plastic panel.  It has been used to reduce damage caused by a wide range of soil-borne fungi, weed seeds, and nematodes in fields in Israel, Jordan, and California. However, it can not be recommended without reservation for Florida, where it has worked well in some trials but failed in others.

Soil Solarization Procedures and Precautions.

   1.Soil should be covered at least 4 weeks, preferably longer, during a hot and sunny time of  the year. June and July appear to be most suitable in Florida, since air and soil temperatures are highest then, and day length and the angle of the sun provide for maximum solar energy hitting the soil at that time.

   2.Soil must be well tilled before covering, to destroy clods and plant debris which might interfere with uniform conduction of heat through the soil and thus protect some organisms from the full effects of the treatment. All other soil preparations such as fertilizing and placing drip irrigation tubes should be completed before covering the plastic over the bed. This will minimize the need to disturb the treated soil before planting.

   3.Soil moisture must be high when the plastic is applied. Wet soil conducts heat better than dry soil, so moistening the soil before tarping will provide for a deeper treatment. Also, most pest organisms are more susceptible to lethal effects of heat in moist soil; they may be dormant if dry.

   4.Raise the center of the bed to form a small "crown" or peak to enable the covered bed to shed water. Solarization seems to be less effective in periods of high rainfall, probably because water standing on the plastic absorbs heat from the soil.

   5.Use clear polyethylene, not black plastic mulch. Clear plastic produces higher soil temperatures faster than black plastic. Sunlight passes through clear plastic to heat the soil directly. Black plastic intercepts the light, and soil is heated primarily by conduction only where the plastic actually touches the soil.

   6.Leave the plastic in place until planting time. It has no detrimental effect on the soil, so it will only maximize the period of treatment and reduce the chance of recontamination before planting. It probably should be removed before planting any crops that require relatively cool soil for best growth. However, it may be safe to leave the plastic over the beds as a mulch for warm season plants that could benefit from higher soil temperatures in autumn.

While soil solarization is not 100% effective against nematodes, it is a useful tool in the gardeners bag of tricks for combatting pests. Try it you may like the results.  Good luck and good gardening.

Gene McAvoy is the horticulture agent with the Hendry County Extension Service.  Direct your horticulture questions to PO Box 68, LaBelle, FL 33975,  e-mail - or phone 863-674-4092 or 863-983-1598.  You are also welcome to visit the Hendry County Extension Office at 225 Pratt Blvd., LaBelle.   Office hours are from 8:00 - 5:00.

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