How use solarization to raise the soil temperature enough to kill weeds.
Take advantage of the summer sun to help reduce weeds, diseases and pests in your soil.
Looking to eliminate a patch of lawn by turning it into a thriving garden bed? Solarize! Got some stubborn weeds that you are sick of pulling? Solarize! The concept when applied to a garden bed is to raise the soil temperature to a level that is high enough to kill off many of the undesirable things dwelling there.
Solarization is not difficult, but it does require a bit of patience and some clear plastic. The main advantage is that it does not involve the use of chemicals. As the soil temperature rises, weed seeds become unviable, weeds are burned out and microorganisms that are happiest in warmer temperatures multiply and grow while attacking many of the harmful soil-dwelling organisms. It’s also a great way to kill unwanted vegetation before you create new garden beds.
To prepare a bed for solarization, the following steps work best: Cut or mow all growth and remove debris and bigger clumps. Turn over the soil with a spading fork or rototiller to expose latent weed seeds. Smooth out the surface by raking to remove air pockets to ensure maximum heating of the soil.
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Next, add moisture to the soil by wetting it down. If the soil is too dry, seeds and other pathogens may not be as affected by the higher temperatures. Then place transparent plastic sheeting on the area of interest to trap as much heat as possible. Clear polyethylene about 1 to 2 millimeters thick works best. Don’t use plastic that is so thin that it easily tears, but anything above 4 millimeters is too thick. Black plastic is not recommended because it blocks much of the light that helps heat the soil.
Bury the edges of the plastic sheet in the soil and anchor it with stones, bricks or boards. A better method is to dig a trench around the outer perimeter beyond the area to be solarized by about 2 feet, if possible. Extend and tuck the excess plastic into the trench and bury it with the excavated soil. This method ensures a tight, heat-trapping seal all the way around the perimeter.
Under ideal conditions, which include full exposure to unobstructed sunlight, temperatures under the plastic will heat up to about 140 degrees F at the soil surface, and 10 degrees cooler 2 inches below. The longer the soil is heated, the better the results. Ideally, leave the plastic on for about eight to 10 weeks during the summer to get maximum solarization. During the summer months, the sunlight is powerful enough to solarize the soil, even in northern climates.
If you can’t give up your beds for two months, even a few days of solarization will kill many of the weed seeds, and in four to six weeks you’ll have killed many of the weeds themselves.
Don’t disturb the soil after solarization because the sun’s weed- and pathogen-killing heat will only penetrate a few inches. So make sure to prepare the bed ahead of time for planting. The less the soil is disturbed after the plastic is removed, the better your results. Cultivation of the soil after removal of the plastic will bring up seeds that survived below and are still viable.
While solarization helps reduce the amount of weeds, it does not eliminate all weeds. Certain weeds such as purslane are resistant to solarization. However, solarization kills weeds such as Bermuda grass, barnyard grass, chickweed, thistle, pigweed and lambsquarters. Even more, plants growing in solarized soil have been observed to grow faster and produce higher and better-quality yields due to the survival and rapid re-establishment of beneficial microbes, researchers theorize.
So now we have another eco-friendly reason to feel good about harnessing the sun’s power this summer as a way to reduce or even eliminate the chemicals in our garden.
Just be sure to save that plastic for reuse next time, too.
Joe Lamp’l, host of “Growing a Greener World” on PBS, is a Master Gardener and author. For more information, visit www.joegardener.com.