ALBANY, Ore. – Here's a simple, non-chemical way you can combat plant diseases, pests and weeds when the weather warms. It's called soil solarization.
That's a fancy term for a simple process: Put transparent plastic sheeting over moist tilled soil during the warmest and sunniest months of the year. As the clear plastic traps the heat of the sun, it changes the soil in physical, chemical and biological ways, explained Oregon State University crop scientist Dan McGrath.
Under plastic sheeting, the top few inches of soil can be as much as 20 degrees warmer than uncovered soil. In this extra-warm environment, disease-causing organisms tend to flounder while more heat-tolerant "beneficial" microbe species increase in numbers. Also, the heat eliminates some kinds of pests, weed seeds and seedlings. OSU researchers have measured temperatures as hot as 127 degrees in the top four inches of soil under plastic sheeting.
In the home garden, McGrath suggests trying solarization on one garden bed a year in a "summer fallow" rotation process, just as farmers do. Success will depend on the intensity of sunlight, soil moisture, weather and length of time the plastic is left on the soil.
May, June and July are the best months to start a solarizing project because the heating power of the sun is at its peak. If June is cloudy, wait until clear weather prevails. In the Pacific Northwest keep the plastic on about two months to ensure that enough heat works for an adequate time. In areas farther south, four to six weeks of summer solarization may be enough.
You will need a roll of plastic sheeting large enough to cover the bed plus about a foot and a half extra around all sides. Thin plastic works better than thick and you don¹t need UV-resistant plastic.
Remove or mow down the weeds in the area you want to cover with plastic. Rake the loose plant debris after mowing. This is the best time to add amendments such as lime, compost or fertilizer. The more you stir up the soil, the more weed seeds come up that aren't solarized.
Rototill the soil and amendments. Break up large clods and work in any loose plant debris. Then, rake the surface of the bed as smoothly as possible to ensure close contact of the plastic against the soil. Dig a trench outside and all around the edge of the bed about six to eight inches deep. Put the clods of soil outside the raised bed.
When the garden surface is smooth, place a soaker hose or sprinkler up and down the bed and soak the soil for three or four hours until wet to a depth of at least a foot. Moist soil responds well because water conducts heat. The moisture tends to "wake up" soil pathogens as well, making them more active and susceptible to the heat.
Finally, cut a piece of plastic the size of the bed, plus an extra foot and a half all around. Put the plastic over the bed and pull it tight and then anchor it snugly by weighing the edges down in the surrounding trench with dirt clods, rocks or bricks.
After eight to 10 weeks, remove the plastic and plant a green manure, overwintering crop or leave the plastic on until spring. Or plant crops and use the plastic sheeting as mulch. Just cut small Xs in the plastic and plant your starts. Take care not to mix up the soil from lower layers, as the effects of the process reach down only about a foot below the surface of the plastic.