Harnessing the energy of the sun may be the best way to control soil-borne plant diseases and some weeds.
Root knot nematodes, verticillium wilt fungi, weed seeds, and other potential problems can lurk in your garden’s soil. Sooner or later, when you least expect them, bam!, there they are. One way to minimize these problems is to heat the soil using the warmth of the sun. The build up of heat within the soil kills enough of them to suppress or even prevent their effects during a growing season.
Called soil solarization, heating the soil with sunlight has been shown to kill weed seeds, plant pathogenic nematodes, and some soil-borne plant diseases. It has also been shown to slow down, but generally not completely kill weedy perennial plants like Bermudagrass and nutsedge. Tested in many countries around the world, the technique has pretty much been shown to work under many different climates and against many different pest problems.
If you want to learn more about soil solarization, a quick web search on line will deliver a wealth of information at a touch of your fingers. My favorite web site for pest control information is the University of California Integrated Pest Management site. Backed by a number of years of field research, it is an excellent reference for desert gardeners and farmers. The specific reference on soil solarization can be found at http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74145.html.
Soil solarization works best against plant pathogenic fungi, destructive bacteria, and weed seeds and seedlings. Common examples of diseases controlled include Verticillium wilt, which causes problems in tomatoes and other garden plants; Phytophthora root rot, and crown gall.
Solar solarization does not work all that well with deep-rooted perennial weeds such as Burmudagrass and nutsedge. Underground stems of these weeds need to be within the top eight inches of the soil in order to be affected by solarization. Most deep rooted perennials will have roots much deeper than eight inches, but good suppression of these weedy plants can be achieved with solarization. You will probably get one season after treatment before the weeds return. When they do, solarization can be repeated.
Occasionally, root knot nematodes can be a problem in Pinal County gardens and landscapes. Nematodes are small roundworms, almost microscopic in size, that live in the soil. These are not segmented worms, like the earthworm. They are an entirely separate kind of animal. Some nematodes are beneficial in gardens because they attack insects and other nematodes. The root knot nematodes, however, are plant feeders and can cause lots of problems.
Root knot nematodes cause large, misshapen swellings, called galls, on the roots of susceptible plants. These grotesque, outsized galls slow down the movement of water and nutrients within the plants and can cause the plants to become stunted and unproductive.
Solar solarization, correctly done, can reduce populations of the root knot nematodes and other plant parasitic nematodes within the top six to eight inches of the soil. Unfortunately, because the animals are fairly mobile and somewhat heat resistant, complete control is difficult. Shallow rooted crops, however, planted right after solarization should be okay. Solarization may have to be repeated regularly from year to year to keep root knot nematode populations under control.
Soil solarization is best done with clear plastic sheets laid upon the surface of the soil. It is the sunlight penetrating through the plastic to the soil that heats up the ground underneath the plastic to levels that are lethal to the target pests.
Selection of the proper type of plastic is essential. The thinner plastics transmit heat better than the thicker sheets. However, the thicker the plastic, the better it can withstand wind and other environmental challenges. The thinner plastic films in the one mil range are commonly selected where wind and animal foot prints will not be a problem. In areas that experience more wind, plastic sheets up to two mils or higher may be needed. A mil is measured at 0.001 inch.
The length of time that a plastic sheet must be in place varies with the solar gain and with the time of year. The cooler the temperatures, the longer the plastic will need to be left in place to achieve effective control. During our hot summers, a solarization treatment time of between four and six weeks should be sufficient. In cooler times, or windier areas, it may be necessary to leave the plastic in place up to eight weeks. If left in place much longer than that, the plastic begins to decompose, lose its effectiveness, and break into pieces from ultraviolet (UV) light exposure. When that occurs, cleanup becomes more difficult.
Good soil preparation will be essential in order to achieve satisfactory results. The first step is to determine the area to be treated. In some cases where a garden may have serious infestations of disease or heavy weed seed populations, the entire garden may need to be treated. Where weed seed populations may be lighter, or where disease incidence is variable, smaller areas where there are known problems can be treated separately.
The surface of the soil must be perfectly flat and clear of rocks, soil clods, and dead plant material in order to get the best results from solarization. The shorter the distance between the plastic and the surface of the soil, the better the transmission of the sun’s radiant heat. The better the heat transmission, the deeper the heat will penetrate into the soil.
It will also be important to saturate the soil with water before laying the plastic down. Water increases the susceptibility of the target pests to heat, and also helps heat up the soil profile. A good, deep irrigation before applying the plastic is essential.
Some people have achieved better results by irrigating the soil once or twice after the plastic has been laid down. They install drip irrigation tubes or soaker hoses prior to laying the plastic. Occasional irrigations allow them to maintain soil saturation.
In order to maintain good plastic to soil contact, to prevent premature drying of the soil, and to prevent wind damage, it will be important to seal the edges of the plastic with soil. A recommended procedure is to dig a five to eight inch trench around the circumference of the plastic. The plastic is then carefully stretched over the area to be treated with the edges of the plastic laying in the bottom of the trench. The trench is then filled with soil and the plastic is secured.
Soil solarization is best done during the summer months. Certainly the greatest soil temperatures will be recorded during then. However, solarization can also be done at other times during the year, including the winter months, but it is generally understood that the plastic must be left in place for a longer time, and that the depth of heating into the soil may be less. During the cooler months, black plastic instead of the normal clear plastic may be a better choice because it will absorb more heat on a chilly day.
While not perfect, soil solarization can help control weed and disease problems in local gardens and landscapes with a minimum of work and without using chemical pesticides.
If you have questions, you can reach one of the Master Gardeners at the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande. The telephone is (520) 836-5221, extension 204. The author’s email address is gibsonrd@ag.arizona.edu.
The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.
Rick Gibson
Extension Agent, Agriculture
University of Arizona Cooperative Extension
820 E. Cottonwood Lane, Building C
Casa Grande, Arizona 85122
Voice: (520) 836-5221
Fax: (520) 836-1750
email: gibsonrd@ag.arizona.edu
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