By Deborah J. Benoit
Editor’s note: Deborah J. Benoit is a UVM Extension master gardener from North Adams, Massachusetts, who is part of the Bennington County Chapter.
Are you looking for a low-labor, chemical-free method to help eradicate weeds and soil-residing pests in the garden? Solarization and occultation, often referred to as “tarping,” may be just what you’re looking for.
Simply put, tarping covers the soil with plastic, clear or opaque, depending on which method you’re using. Solarization utilizes clear plastic sheeting. Occultation uses black plastic sheeting or opaque tarps.
Solarization uses the sun’s rays to heat soil, mulch and other organic matter to temperatures that can kill certain pests and soil-borne diseases. It works best on hot days when the sun is shining brightly. Solarization is unlikely to work well in shaded areas.
Occultation blocks the sun’s rays. While it does heat the soil somewhat, it stops plant growth by blocking the sun’s light and photosynthesis. This method works well in sun or shade.
The tarping process for both solarization and occultation is simple. Trim any plants growing in the area close to the ground. Using a rake, level the soil surface. Give the soil a good soak.
Lay plastic sheeting over the area as close to the surface of the ground as possible. Bury the edges in the soil or use heavy objects to hold the edges down, creating a closed environment.
Solarization can take anywhere from two weeks under optimal conditions (clear, hot days at the peak of summer) but more likely four to six weeks. If conditions result in less heat generated by sunlight and temperature, the process can take longer.
Solarization can heat the upper 12 to 18 inches of soil, with its greatest benefit in the top six inches. When temperatures reach above 110 degrees Fahrenheit, solarization can help control soil-borne diseases such as early blight (Alternaria sp.).
Solarization can also be useful in combating pests such as the invasive Asian jumping worms (Amynthas agrestis, Amynthas tokioensis and Metaphire hilgendorfi) many of us have encountered in our gardens. These worms live and multiply near the soil’s surface, making solarization a good method to help control their spread.
Organic materials such as mulch, compost and topsoil can be solarized before being introduced into the garden. Simply spread the material on a tarp in a smooth, even layer 4-6 inches thick. Cover with clear plastic sheeting. Tuck the plastic sheeting under the tarp’s edge securely to enclose the material and prevent any worms from escaping.
It will take a minimum of three days of internal temperatures of at least 104 degrees Fahrenheit to kill jumping worms and the worms’ cocoons (eggs). After that time, the mulch or other material should be jumping worm free.
The occultation process is similar to that of solarization, but opaque plastic (black sheeting or dark colored tarp) is used instead of clear plastic. Follow the steps outlined above for tarping.
Occultation can work well to rid an area of grass or weeds, but any heat generated will not raise the soil temperature sufficiently to kill soil-borne diseases or pests. For the same reason, it will work best to rid an area of annual weeds but will be less successful with perennial weeds and weed seeds hidden beneath the soil’s surface.
For more information on solarization and occultation, visit: go.uvm.edu/tarping. For more information on Asian jumping worms, visit: go.uvm.edu/jumping-worm.
