Using plastic sheeting as a means to kill and control weeds, a process referred to a solarization of the soil, effectively destroys seedlings, seeds and underground roots. When soil warms from the heat of the sun's rays, some microorganisms thrive and attack harmful organisms in the soil. Vegetation and roots decay adding organic matter to the soil and release vital nutrients like nitrogen, potassium, magnesium and calcium. Soil texture improves which in turn improves both aeration and drainage. Soil solarization kills thistles and prevents their regrowth.
Mow or cut all vegetation in the selected area. Till or turn the soil to uproot thistles and other offending weeds. Remove any rocks, roots or plant debris by handpicking or raking with a garden rake. Rake the area smooth.
Saturate the soil to a depth of 2 feet for optimal performance. Moisture makes soil organisms more sensitive to heat and speeds the solarization process.
Cover the soil with thin clear plastic during June or July. Although black plastic can be used, it is less effective than clear as it does not warm the soil as effectively. Thin plastic of 1 mm thickness is most effective, but tears easily under normal conditions and is susceptible to wind damage. Select 2 to 4 mm plastic for durability.
Spread the plastic over the soil maintaining direct contact with the surface of the soil. Anchor edges by burying them in soil to prevent the plastic blowing free under high winds.
Leave the plastic in place for four to six weeks in southern regions that receive high intensity rays from the sun. Northern climates, or areas where weather is cool, may require 10 to 12 weeks to thoroughly kill offending thistles.