Wouldn’t it be great if our brutal summer heat could make a better vegetable garden? Even more so, what if the oppressive heat could make gardening easier?
It can, with a seldom practiced trick called soil solarization.
Soil solarization is a simple, safe, and effective control of plant parasitic nematodes, soil-borne plant pathogens, and some weed pests. It offers an alternative to some pesticides and the lengthy crop rotations now needed to control many damaging pathogens that linger in the soil. In addition, this procedure may give reasonable weed control in many situations.
Radiant heat from the sun is the lethal agent involved in soil solarization. A clear polyethylene mulch is used to trap solar heat in the soil. Over a period of several weeks to a few months, soil temperatures become high enough to kill many of the damaging soil pests and weed seed to a depth of nearly 8 inches.
None of these pests will be completely eradicated from the treated area. Still, their numbers in the plow layer (the top 6 to 8 inches) will be significantly reduced, allowing the successful production of a vegetable garden.
Just how hot can it get? Results from studies in 2011 and 2012 indicate that when the ambient air temperature reached 99 degrees F, the solarized soil reached over 170 F. That’s hot enough to cook chicken.
To be clear, none of these pests will be completely eradicated from the treated area, but their numbers in the top layer (say 6 to 8 inches) will be significantly reduced, allowing fewer problems in following crops.
The soil to be solarized must be worked up to seed-bed condition, and cultivated until it’s loose and friable with no large clods or other debris on the soil surface. A garden tiller will eliminate clods or other debris that create air pockets that reduce the heating of the soil and keep the tarp from fitting tightly over the soil surface. A clean, flat surface will also prevent the accidental puncturing of the thin plastic drop cloth by debris.
Ensure moisture levels are adequate for working the soil before laying the plastic tarp. If the soil is dry, water the areas to be solarized before laying the tarp. This is crucial because most soil pests are more sensitive to high temperatures in wet soil than in dry soil. When possible, lay a soaker hose or drip irrigation lines under the tarp to maintain moisture levels during soil solarization. Tarped raised beds may also be watered by flood-irrigating the adjacent furrows.
You can use any cheap plastic painting “drop cloth”. Using two layers of thin plastic sheeting separated by a thin insulating layer of air increases soil temperatures and the overall effectiveness of a solarization treatment. The edges of the sheets must be buried or otherwise secured to prevent blowing or tearing of the tarp by the wind.
For effective solarization, the edges of tarps laid over raised beds must be buried in the adjoining furrows. Expect some increase in pest and weed problems along the edge of the stripped mulches. Do not cultivate solarized areas, because healthy weed seeds will be brought to the soil surface.
Long, hot, sunny days are needed to reach the soil temperatures required to kill soil-borne pests and weed seeds. The longer the soil is heated, the better and deeper the control of all soil pests and weeds will be. During our hot summers, a solarization period of 4 to 6 weeks should be all that’s needed to control nematodes and soil-borne plant pathogens.
For those concerned about losing beneficial microbes, indeed you will lose them in addition to your unwanted pathogens. However, populations of beneficial, growth-promoting, and pathogen-antagonistic bacteria and fungi quickly recolonize solarized soil with the addition of compost or adding a biological control component to the soil.
Soil solarization will tie up land for a period of 1 to 3 months. If you have plans for a fall garden, this can definitely interfere. But the results of letting our hot summer work for you can be fantastic.
