Soil solarization: Solar energy for weed control
Steps in soil solarization:
1. Irrigate the area to be treated.
2. Spread a sheet of three- to four-mil clear plastic.
3. Seal the edges with soil.
4. Remove the plastic in four to six weeks.
5. Prepare the soil with amendments.
6. Seed to a cover crop, or
7. Repeat the soil solarization for two to three weeks, then plant.
Eradicating annual and perennial weeds for a new garden is the first step in good soil preparation. Soil solarization is a terrific nonchemical solution for this challenge. I have even been able to eliminate Bermuda grass in my flower garden in just four weeks with a single treatment. During the 20 years I have enjoyed the border, there has been no recurrence of this pesky weed.
In the period before and after the summer solstice, the foothill sun is very intense, and it takes only a few weeks for effective weed control with soil solarization. Deep-rooted perennial weeds may take longer to control than annuals.With several weeks of summer sun a certainty, the gardener may even practice a series of solarizations.
When perennial weeds (Bermuda grass, Johnson grass, bindweed… the list is long) are your challenge, the weeds need to be controlled before gardening or landscaping. Annual weeds are a problem too, with thousands of seeds each year. Hand-weeding a newly planted groundcover or vegetable garden is very time-consuming.
Water the area to be cleared. If weeds are tall, with strong stalks, first cut the weeds down, then rake off the cut plant material. If there are no seeds, this material may be added to your compost pile. As the plastic is spread, you will need to be careful not to puncture it. I frequently lay the plastic cover on uncut weeds.
Following irrigation, spread a three- to four-mil clear plastic sheet. Black plastic excludes light, but will not kill perennial weed roots. A lighter mil clear plastic may be used, but will deteriorate more quickly. Once the plastic is laid, all the edges will need to be sealed so that no air can enter under the plastic. Soil works well for sealing the edges. If your ground is very level, it works faster to lay lengths of wood to secure the plastic, then use soil or compost to finish the seal.
UNDER THE PLASTIC
First you will observe a buildup of moisture under the plastic. Annual and perennial seed on the soil surface will germinate. As the temperature increases, foliage of weeds will begin to burn. Newly-germinated seedlings will die quickly. Older plants and those with extensive root systems will take longer for a total kill.
Remove the plastic sheet as soon as all the weeds appear to be dead. If you are treating an area that has deep-rooted perennials such as dandelions, you may want to remove the plastic, water the area again, then replace the sheet, sealing the edges. If no growth shows, this step in your soil solarization is complete.
Now prepare your soil for planting. Add lots of compost (three to six inches). In my flower garden, I use a mix of two-thirds mushroom compost and one-third rice hulls. For the edible garden, I always use certified organic poultry manure. For new gardens, amendments should be incorporated into the native soil to a depth of eight to ten inches if possible.
In both gardens, natural phosphorus (colloidal phosphate or soft rock phosphate) is added at the rate of twenty pounds per hundred square feet, and oyster shell at the rate of five pounds per hundred square feet. The phosphorus is lacking in foothill soils, and is a necessary element for good root growth and flowering or fruiting. It is not water soluble, so it must be incorporated into the soil in the root zone. The oyster shell affects the soil pH, making the phosphorus more available to plants. Even those plants that prefer an acid soil should be planted in soil with a pH of 6.0-6.5. Oyster shell is water soluble, and may be added on the surface.
It is also possible to do your amending first, then soil solarize, but may be risky if you have perennial weeds that increase in number when their roots are cut into sections by a rototiller or even a shovel. Half an inch of Johnson grass root is enough to start a new plant.
If weeds have been growing in this area for several years, as soon as you disturb the soil after solarization, a newly-exposed generation of seeds awaits the ideal conditions for germination. You may not eliminate all the potential weeds with a single treatment of soil solarization.
Your next step is to decide how you are going to use the prepared garden area. If there’s no sense of urgency for planting, sowing a cover crop (rye, legumes, or “soil builder” seed) is your next best step. Since this cover crop may be rototilled into the soil for fall planting, it does not matter if a few weeds become part of the “green manure.” If no-till is your next step in gardening, cut down the green manure and add a couple of inches of compost on top.
If, however, you do rototill and want to plant the area as soon as possible, I would still recommend a repeat soil solarization for two to three weeks to germinate all the weed seeds that have been brought to the soil surface during your tilling or cultivation.
Soil solarizing a recently prepared garden area before you plant may save you hours of weeding. And it’s worth the time spent for truly effective control of the deeply-rooted perennial weeds common in the foothills.
Carolyn Singer has gardened organically in Nevada County since 1977. She is the author of the award-winning “The Seasoned Gardener, five decades of sustainable and practical garden wisdom,” and two volumes of “Deer in My Garden” (deer-resistant plants), available locally. Send your gardening questions and comments to firstname.lastname@example.org. Check out her website at carolynsingergardens.com.
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