The Thickness of Black Plastic Sheets to Kill Garden Weeds

Written by bridget kelly
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The Thickness of Black Plastic Sheets to Kill Garden Weeds
Solarisation works better on annual weeds than perennials. (Photos.com/Photos.com/Getty Images)

Gardeners generally have a running battle with weeds throughout the entire growing season. Herbicides, whether pre- or post-emergent take care of many weeds but for the organic gardener, or those attempting to avoid the use of toxic chemicals, fighting weeds is even more difficult. Soil solarisation is one method to kill weeds that not only avoids the use of chemicals, but has some added benefits as well. Heating the soil to high temperatures also kills nematodes and pathogens. Using black plastic sheeting is an ideal way to solarise the soil during the hottest part of the summer.

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Plastic

Specialists with the University of California Integrated Pest Management suggest using transparent plastic because black plastic deflects some of the heat instead of trapping it. They add, however, that black plastic may work better in coastal regions where the soil may not get hot enough to kill weeds. The black plastic blocks sunlight, thus killing the weeds. Thin plastic allows more heat to penetrate to the soil than thicker plastic, but it is also more fragile and may tear when animals walk across it or if it is subjected to high winds. Painter's plastic, 0.001- to 0.004-inch thick, is available at paint and hardware stores and can do the job.

Preparation

The plastic sheeting needs to lie right up against the soil with no air pockets, so it's important to create a smooth, level bed. Remove all rocks and other debris and use a lawnmower to cut the weeds close to the soil. Water the soil until it is saturated to a depth of 12 inches. This is an important part of the process, according to UCIPM scientists, and should be done immediately before laying the plastic.

Method

Dig a 4- to 6-inch trench around the area on which you will lay the plastic. Lay the plastic over the area, placing one side of it inside the trench. Fill in that trench with soil, burying the plastic's edge and then move to the adjacent trench. Pull the plastic so it is tight against the soil, the edges hanging into the trench. Fill that trench with soil, again burying the edge of the plastic. Repeat this procedure with the other two edges of the plastic, then walk the perimeter of the area to pack the soil over the buried areas.

Timing

Allow the plastic to remain on the soil for four to six weeks. In cooler or cloudy regions, leave the plastic on for eight to 10 weeks. If the weather cools for more than two days during this period, allow the plastic to remain for an additional week. The top 6 inches of soil needs to remain at 43.3 degrees Celsius or warmer during the entire period. After removal of the plastic, cultivate the soil no deeper than 2 inches to avoid bringing weed seeds to the surface.

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