I just identified a mess of problems for a local gardener. Root-Knot nematodes and fungal diseases galore.

How does a gardener fix those problems before next spring?

It can be done with a seldom used practice called soil solarization.

Soil solarization is a simple, safe, and effective control of plant parasitic nematodes, soil borne plant pathogens (diseases), and some weed pests. It offers an alternative to some organic and non-organic pesticides as well as the lengthy crop rotations now needed to control many damaging soil pests. In addition, this procedure may give good 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 eradicated from the treated area, but their numbers in the plow layer (top 6 to 8 inches) will be greatly reduced, allowing successful production of a crop.

The soil to be solarized must be worked up to seed-bed condition–that is, 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 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 mulch by debris.

Make sure 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 will increase 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 seed 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 seed. 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 beneficial microbes, populations of beneficial, growth-promoting and pathogen-antagonistic bacteria and fungi quickly recolonize solarized soil, adding a biological control component to soil solarization.

If you have plans for a fall garden, this will interfere and give you a late start. But the results of letting our hot summer work for you can be fantastic.