Solarize the soil to kill off nasties

How would you like to use an easy, efficient and effective method to kill every harmful insect egg and larvae in your garden soil? In addition, what if this technique offered many other benefits, like control of many weed seeds and diseases without disastrous or harmful side effects? Are you interested? If you're a gardener you are.

Soil solarization is the process of heating soil under clear plastic to temperatures that are deadly to many pests, yet seems to encourage beneficial soil organisms to thrive. Through the heating process, the structure of your soil may actually improve. Soil that has been solarized allows plants to draw on the nutrients — especially nitrogen, calcium and magnesium — more readily. Seeds germinate more quickly. Plants grow faster and stronger, often maturing earlier with substantially higher yields than in unsolarized soil. Does this sound too good to be true? It's not. It really works.

One of the few drawbacks to solarization is that it takes garden area out of production during the 6-8 weeks in the middle of summer that is usually used for the majority of our vegetable and annual flower gardening. Since the benefits last more than one season, it may well be worth the time lost if you have been having major trouble due to nematodes, verticillium wilt, fusarium wilt, Phytophthora root rot, and many annual and perennial weeds.

To be fair, solarization will not control all summer weeds that are relatively comfortable with extreme temperatures or have taproots deep enough to stay cool during treatment, such as nutsedge, but it will control many otherwise difficult-to-stop weeds.

To properly solarize a garden bed, it takes little more than the usual effort that gardeners put into spring planting preparation. The soil must be cleared of all growing matter and provide a smooth dirt surface, free of lumps and clods and stones that might impede the process of heating the soil. This is an excellent time to incorporate soil conditioners, fertilizers (especially manures which love the heating process) and other necessary improvements that would involve later digging, like sprinkler systems.

The plot must now be thoroughly watered. We want the soil to be watered as deeply and thoroughly as possible, as heat is much more effective in a moist soil. Don't neglect this important step.

Dig a trench 6-8 inches deep around the perimeter of the bed. Be sure to make a positive seal around the entire area, to trap all the heat generated under the plastic sheeting. There must be no rips, tears or uncovered edges.

The clear plastic sheeting can vary in thickness from 1 to 6 mils. The thinner the plastic, the more heat will be built up underneath it. However, it is harder to handle the thinner material. Three or 4 mils is probably easiest. The thicker plastic can be reused as mulch. Disposal of the used sheeting is the difficult aspect of this project.

Once in place, it takes four to six weeks of hot weather to thoroughly solarize the soil. During this period, temperatures in the upper 4 inches of soil profile should reach 130 degrees and at 18 inches it may approach 100 degrees. The greatest effect and benefit is at the surface and it is extremely important not to bring up dormant seeds after the completion of the process. That is why thorough soil preparation before application of the plastic is essential.

If you would like to increase the amount of heat to near pasteurization temperatures of 140 degrees, create a greenhouse effect by adding another layer of plastic elevated above the first one. You can use the hoops that are used for row covers or build a suitable frame over which to drape the plastic. Again, thoroughly seal all edges to prevent heat escaping. This will stop all but the very toughest weed seeds that are exposed to these temperatures.

June, July and August are the months that are best suited to soil solarization. The longer the exposure, the greater the effect. It is a rewarding process that can turn problem beds into productive, healthy growing areas once more. With our blistering summer heat, it is a natural for Southern Oregon.

Stan Mapolski, aka The Rogue Gardener, can be heard from 9-10 a.m. Saturday mornings on KMED 1440 AM and seen in periodic gardening segments for KTVL Channel 10 News. Reach him at

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