Soil Solarization

Soil solarization is an environmentally friendly method used to rid an area of pretty much everything: weeds, fungal and bacterial pathogens, and nematodes. Solarization is particularly effective against the pathogens that cause fusarium wilt, verticillium wilt, root rot, blight, tomato canker, and crown gall disease. It is also used to eliminate a lawn and prepare it for more drought-resistant varieties of plants.

To prepare an area for soil solarization, it must be smoothed flat and free of dirt clods and visible weeds. Solarization works far better on level areas than on hillsides, but sun exposure is critical for the process to be effective. Shady areas are not suitable.

Basically, soil solarization covers an area with clear or white plastic and lets the sun cook everything to death. Now, when I say covered, I mean completely. No cracks, no bubbles, no loose edges. If gophers, ground squirrels, or birds are a problem in your area, this can be difficult.

Here are the steps to use for effective soil solarization:

1. Mow or cut down any existing plants as short as possible.
2. This is a good time to rototill the area.
3. Rake the area smooth - and I mean really, really smooth. You don’t want any air pockets between the soil and the sheeting.
4. Dig a shallow trench around the area being solarized.
5. Water the area deeply. (Wet soil heats up faster than dry soil.)
6. Apply transparent or white plastic sheeting tightly over the space, overlapping any edges and making sure the sheeting reaches the trench. (See NOTE below)
7. Fill the trench with the soil dug out of it, to hold down the edges.
8. Leave the sheeting on for 4-6 weeks.
9. Monitor the area frequently to ensure that the plastic stays tightly on top of the soil.

NOTE: Black or colored tarps should not be used unless you live in a cooler, coastal area. In this case, a black tarp will prevent weeds from growing during the early part of the process and the tarp will need to be left on longer than normal. When selecting the sheeting material, heavier is generally better:

• Thin plastic (1 mil) provides the most heating, but it tears easily
• Slightly thicker plastic (1.5 to 2 mils) is better in windy areas
• Thicker plastic (4 mil) is best for residential applications
• Rolls of 1-4 mil painter’s plastic can be found at most hardware stores

Using a double layer of plastic, separated with plastic bottles or PVC pipe, can increase the effectiveness of solarization. This has been shown to increase temperatures by 2-10°F. 

If it is done properly, the heat of the sun should reach 140°F, killing pathogens and weed seeds, without the use of chemicals. Another benefit of soil solarization is that it speeds up the breakdown of organic matter, making all the nutrients bound up in various life forms available to new plantings. Plants grown on solarized soil tend to grow bigger and faster, as a result. Of course, this process also kills earthworms and beneficial soil microorganisms.

Soil solarization can also be used on planting containers and in raised beds that have become infested with undesirables. In this case, use a double layer of plastic and place the container in the sunniest place you can.

Once solarization is complete, carefully remove the plastic and put it in the trash or recycling, depending on your region. Disturb the ground as little as possible to avoid planting newly arrived weed seeds.