I moved into a new house during the winter. Summer has revealed that the yard is just full of weeds. They are taking over. I want to sterilize the soil, but organically and cheaply. Any suggestions?
Hardiness Zone: 2b
By nelson2v from Saskatchewan, Canada
Yep - do soil Solarization -
Solarization is the safest and most effective means of controlling soil pest problems in home gardens. In many parts of the country where sunshine and warm temperatures are less abundant, solarization may not work. But here in Arizona, high solar radiation and lots of heat make soil solarization a snap!
Solarization consists of covering the soil with a clear plastic tarp for 4 to 6 weeks during a hot period of the year when the soil will receive maximum direct sunlight. When properly done, the top six inches soil will heat up to as high as 125 degrees. Over several weeks, that's hot enough to kill a wide range of soil inhabiting pests such as; wilt and root rot fungi, root knot nematodes and noxious weed seed. In addition, solarization stimulates the release of nutrients from organic matter present in the soil. It is especially effective for treating garden soils, where the intent is to plant vegetables, herbs, and flowers.
Here in Tucson, the best time for solarization late spring. This is when solar radiation is at it's peak, and when soil temperatures are naturally their warmest. Ideally, solarization should begin in May and extend into June. Solarization should be complete by July, when monsoon clouds and rains begin to have a cooling effect on the soil.
The first step in soil solarization is to till the area to be treated. It is necessary to break up soil clods and plant debris in order to enhance heat conduction through the soil. The soil surface should be raked smooth before covering with plastic. A fine soil surface will allow the plastic covering to be placed in close contact with the soil, with few air pockets to interfere with direct solar heating.
Before applying the plastic covering the soil should be moistened. Wet soil conducts heat better that dry soil. Moisten the soil to a depth of one foot. This will enhance heat penetration through the potential rooting profile. Also, in moist soils, pest organisms are more active and thus more susceptible to the lethal effects of heat. In dry soils most pest organisms are dormant.
Large sheets of plastic to use for solarization are available at hardware and home supply stores. Use clear polyethylene, not black plastic! 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. Some of the heat generated when sunlight hits black plastic is lost directly to the outside air.
Thin plastic (1 to 2 mil) may permit more sunlight to penetrate to the soil. It has also been reported to favor more rapid and deeper control of soil-borne fungi than thicker plastic (6 mil). However, both eventually provide equal control and thicker plastic is less likely to tear.
To install the plastic over the garden area to be solarized, first dig a trench around the garden perimeter. The trench need only be about 8 inches deep and 12 inches wide. It will be used to hold the plastic in place.
Lay the plastic out so that it covers both the garden area and the surrounding trench. As the plastic is pulled tight, hold the plastic in place by covering it with soil in the trench. As you move around the perimeter covering the plastic, keep pulling it tight. A tight fit against the soil surface allows for better heating. Burying the edges will also prevent the wind from picking up the plastic tarp and blowing it off.
Leave the plastic covering on for a minimum of 4 to 6 weeks. You can monitor soil temperatures under the plastic by placing a soil or compost thermometer through the plastic covering and into the soil. A dial thermometer with 5 inch probe can also be used. They are inexpensive and can be purchased at local hardware stores. Locate the thermometer close enough to the edge of the bed so that it can be read without walking on the plastic. Push it down five or six inches into the soil. Leave it in place during the entire solarizing process. The goal is to raise and maintain temperatures in the top 6 inches of soil to a level between 110 to 125 degrees F.
When solarizing is complete, plant your bed with seed or healthy, uncontaminated plants. Do not mix untreated soil into the solarized bed. The less disturbance of the solarized soil before, and at the time of planting, the better. Raised beds can also be solarized in the same fashion as ground beds.
Solarization is by far, the most effective way home gardeners have to reduce or eliminate soil-borne garden pests. The tangible benefits: healthier and more productive flower and vegetable gardens!
Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on
May 07, 2000
- Updated: August 30, 2001
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