Method of preventing, for a time, growth of perennial grasses and sedges in which a halostyrene compound selected from .alpha.-(2,2,2-trichloroethyl)-m-chlorostyrene and .alpha.-(2,2,2-trichloroethyl)-m-bromostyrene is incorporated into a surface layer of the soil of a field plot infested with the tubers and/or underground stems of the said perennial grasses and sedges at a concentration in the range of about 0.25 to about 50 ppm, and more preferably about 1 to about 25 ppm within the layer, and the layer being about 1/8 to about 6 inches in thickness and the dosage rate of halostyrene compound being at least about 0.5 pound per acre and up to about 10 pounds per acre and preferably below about 4 pounds per acre. Essentially a substantial portion of the compound is incorporated in the soil above most of the tubers and/or underground stems of the perennial grasses and sedges to be controlled, or directly contacted with said tuber and/or stems.