Source: Idaho's Noxious Weeds by Robert H. Callihan & Timothy W. Miller
BACKGROUND
Canada thistle (Cirsium arvense) is an invader from Eurasia. It was introduced to Canada probably as a crop seed contaminant before 1800. It is an aggressive weed that spreads both by seed and extensive root systems. The deep roots grow horizontally and send up shoots along their length, forming dense colonies
DESCRIPTION
Canada thistle is a perennialPlant that lives for more than 2 growing seasons plant up to 5 feet tall. Leaves are wavy marginedmargin: edge of the leaf to lobedA cut into a leaf from the edge toward the center; greater than toothed, but not quite compound, up to 6 inches long, and armed with yellowish spines. Stems are single, branched near the top, ridged, and hollow. Flower headsA group of flowers borne tightly together are borne in midsummer, ½ inch in diameter, and are not particularly spiny. Flowers are purple to lavender, occasionally white, with male and female flowers borne on separate plants. Seeds are slender, tan, 1/8 inch long, and bear fine plumesA hairlike or featherlike structure, often on a seed.t brown comblike marginThe edge of the leaf. The upper part of each bract narrows into a short, stiff spine. Seeds are brown to gray in color and are tipped by plumesA hairlike or featherlike structure, often on a seed that fall off at maturity..
DISTRIBUTION
Canada thistle is found in the northern U.S. and southern Canada. Canada thistle is present is every county in Idaho. It is one of Idaho's most widespread and damaging noxious weeds.
CONTROL
Biological control agents are available, but control is only poor to fair. Herbicides are available that control the weed if consistently used.