About Solarization
This technique for weed and pathogen control can be very effective if the proper conditions exist for its success. At its most basic, solarization is simply the use of solar radiation trapped as heat under plastic sheeting raising the temperature of the soil to the point that it will kill weeds, weed seeds, plant pathogens, and other pests. Solarization is also a good way to kill unwanted turf without using herbicides!
Timing
Soil solarization is most effective in warm, sunny locations when the days are long, air temperature is high, skies are clear and there is no wind. Solarization is most effective when done during the hottest weeks of the year, generally from June to August.
Soil Preparation
A flat, smooth bed with clods and litter raked away works best for this method as it allows the plastic to lie snugly against the soil surface. Air pockets between the soil surface and the plastic sheeting can greatly reduce soil heating and promote “sailing” of the plastic in the wind.
Irrigate the Soil
Wet soil conducts heat better than dry soil and makes soil organisms more vulnerable to heat. Wet the soil to at least 12 inches deep. In larger areas, it is easiest to do this before laying down the plastic, but in smaller sites it can be done after the sheeting is in place by using a garden or soaker hose or drip tape under the plastic tarp. Otherwise, place the tarp over the area as soon as possible after water has been applied to prevent evaporation. Unless the soil gets dry, do not irrigate again as this will lower the soil temperature and lengthen the time required for successful solarization.
Plastic Tarp Choice
- Thinner is better, but is more prone to tearing from wind or animals (1mil)
- Slightly thicker plastic is better for windy areas (1.5 – 2mils)
- Small areas can use thicker plastic (4 mil)
“Painter’s” plastic works well for larger areas and will generally last the 3 to 5 weeks required for solarization before it begins to breakdown. The plastic sheets should be watched closely so that they can be removed before they deteriorate to the point where removal and disposal are difficult. In cooler climates, a second layer of plastic can be placed over the first, separated by objects like PVC pipe or plastic bottles, as this can raise the soil temperatures from 2 to 10 degrees over temperatures obtained with a single layer.
Sheet or Tarp Placement
The plastic must be held as tightly as possible against the soil. One way to accomplish this is to dig a trench 4 to 6 inches deep around the area to be solarized. Lay the plastic over the area with the edge in the trench. Cover that end with soil to hold it down. Pull the plastic tight from the other side and bury that edge in the corresponding trench. Do the same with the other sides and then walk around the perimeter of the trenched area and pack the soil down around the edges of the plastic. The closer to the soil surface the plastic is, the better the heating.
Solarizing Period
Solarization is both time and temperature dependent. The cooler the soil temperature, the longer the plastic needs to remain in place to raise the temperature to desired levels. In general, 4 to 6 weeks of soil heating during the warmest time of the year is sufficient to control most soil pests. In cooler, windier, or cloudy locations, or if there are pests that are harder to control, the plastic may need to remain in place from 6 to 8 weeks. The goal is to maintain daily maximum temperatures in the upper 6 inches of soil at or above 110 to 125 Degrees F. Use of a soil thermometer or temperature probe can verify these temperatures.
Post – Solarization (Removal of the plastic)
The plastic should be removed, taking care not to disturb the underlying soil, so as to not to bring up any viable seed from deeper soil layers. Alternatively, the plastic can be left in place and holes cut into it to allow for the planting of landscape plants; however over time, the plastic will degrade and fall apart during the growing season. If soil must be cultivated for plantings, the cultivation should be shallow, no more than two inches to avoid bringing up viable weed seed or pathogens to the surface.
