How to Solarize Soil

Does your vegetable garden have a stubborn weed problem or suffer from soil-borne diseases like late blight, verticilium wilt, potato scab or root rots? Don’t turn to the bottle (of chemicals, that is) for these problems. Instead, just look up. The sun produces an enormous amount of energy, and by using a simple organic gardening technique called soil solarization, you can harness this energy to dramatically heat up your soil and kill weed seeds, diseases and even insect pests. All you need to get started is clear plastic sheeting and a six-week stretch of hot, sunny days—read on for the rest.

Instructions

Difficulty: Easy

Things You’ll Need:

• 2-mil plastic sheeting

1. The basic idea behind soil solarization is this: you stretch clear plastic over a problem area in your garden and the plastic acts like a mini greenhouse raising the temperature of your soil to levels that are inhospitable to many weed seeds and diseases. This technique relies on the sun to heat up the soil, so consequently, it works best in regions with sunny, hot summers and in areas of the yard that receive full sun such as vegetable garden beds. The best time to solarize soil is during the hottest 6 to 8 week period of the year.
2. Soil solarization works best when the plastic sheeting is in close, continuous contact with the soil. So before your apply the plastic, you must first clear and level your garden bed. Start by pulling up all existing plants (including weeds) and remove any large rocks or clods of soil. Then rake the soil until it is very smooth and level.
3. Dig a 6-inch deep trench around the entire bed, reserving the soil in a wheelbarrow. You will use this trench to bury the edges of the plastic sheeting and keep it stretched tightly across the bed.
4. Water the bed until the soil is soaked to the depth of 12 to 18 inches. The water helps conduct heat through the bed and also softens up weed seeds and pest eggs, which makes them more vulnerable to the heat.
5. Grab a friend to help you stretch the plastic sheeting across the bed. Use 2-mil plastic if possible, because it stretches better than thicker plastic (which also tends to be more expensive). Stretch the plastic very tightly over the bed (the soil should actually look shrink wrapped). Anchor the edges of the plastic by burying them in the trench as you work.
6. Soil solarization works from the top down, so the upper layers of soil will heat up the most. How much your soil heats up depends on how hot and clear your days are, how well you stretched the plastic across the bed and how long you keep it in place. Leave the plastic on for 6 weeks in climates where the daytime temperature stays consistently above 80 degrees F and for 8 weeks in climates that are cooler, frequently overcast or windy.

Tips & Warnings

• Every few days check on the bed and fix any rips or tears in the plastic with clear plastic packing tape.
• After you solarize the soil, avoid deep cultivation, which can bring weed seeds up to the surface.
• Prevent weeds from colonizing a newly solarized bed during the fall and winter by either planting a winter-hardy cover crop or placing a 6-inch deep layer of leaves, grass clippings or straw over the soil.
• According to research at the University of California Davis, soil solarization can heat up the soil to a temperature of 108 to 131 degrees F at a depth of 2 inches and from 90 to 99 degrees F at a depth of 18 inches. Many soil pests, weed seeds and diseases are killed at temperatures between 86 and 91 degrees F. Use a simple soil thermometer to check on the temperature of your soil.