Q: I want to sterilize a 100′ x 50′ soil area to plant wildflowers. The spot is now covered with coastal bermudagrass. I’d like to use a plastic covering on the ground to kill unwanted plants.
Will this sterilize the soil? Any difference between black and clear plastic? How long should I leave it covered?
A: The technique you refer to is called solarization because solar energy is used to heat the soil. Since the object is to heat the soil to more than 120 degrees, it is best done in summer using clear plastic.
The process is simple: till the spot and rake it smooth, then irrigate until the soil is wet, but not soggy, to a six inch depth. Cover with clear plastic, which captures solar rays better than black plastic. Anchor the edges of the plastic with soil. For better results, cover the area with a second layer of plastic, held apart from the first layer with bricks or upended plastic pots.
Leave the plastic in place for four to six weeks in summer. Over time, heat will penetrate to a six inch depth and most pest organisms will be killed.
Most is the operative word here. Bermudagrass is so tough that I doubt it will be killed by solarization. I would instead spray glyphosate (Roundup, etc) three times at four week intervals during the summer. You can plant the wildflower seed in fall.