thistles, canada thistle, herbicides, weed control
Canada thistle (Ci.rsium arvense L.) and perennial sowthistle (Sonchus arvensis L. or S. ulignosus Bieb.) are deep-rooted perennials that spread by underground parts and by seed. All species emerge later in the spring and are less difficult to control or eliminate than leafy spurge, hoary cress, or Russian knapweed. They emerge about the same time as field bindweed. Prevention of seed production is more important for thistles than for most perennial weeds. Wind currents carry the seeds great distances. Thistles reduce crop yields. The amount of reduction depends on how thick they are. Two plants of Canada thistle per square yard reduced wheat yields 18%, and 19 plants per square yard decreased yields 36%. A heavy infestation of sow thistle caused a 69% reduction of oat yields. To control or eliminate these thistles, use intensive cultivation, soil sterilant chemicals, certain competitive crops, selective herbicides, or several combinations of cultivation, crops, and chemicals.
South Dakota State State College of Agriculture and Mechanic Arts, Agricultural Experiment Station
Derscheid, L. A. and Wallace, K. E., "Control and Elimination of Thistles" (1959). Agricultural Experiment Station Circulars. 145.