Noxious Weeds of Nebraska

Canada thistle (Cirsium arvense (L) Scop.) is one of the nine species defined by Nebraska law as a noxious weed. At least 35 other states also have determined by law that Canada thistle poses a threat to the economic, social and aesthetic well-being of the residents of their states. It is estimated that Canada thistle growing in Nebraska crop and pasture land causes over $2.5 milion in lost production each year.
Once Canada thistle becomes established, plants can live and reproduce for many years. Horizontal roots spread rapidly and in a single season may grow 9 to 18 feet laterally and 6 to 9 feet deep. Cultivation or tillage through these patches can cut roots; however, root segments as small as one inch can survive and produce new plants, spreading the original infestation.

Canada thistle is classified as a long-day plant. It requires a 16-hour day length to induce shoot elongation and flowering. In Nebraska flowering generally occurs in mid-June. Canada thistle is dioecious -- male and female flowers are produced on different plants so cross pollination is necessary for seed production. Several varieties or subspecies of Canada thistle have been identified and are distinguished by differences in leaves.

Control methods include: cultural, mechanical, biological and chemical.