It is a noxious perennial weed that has invaded the whole northern half of the United States. The Canada thistle sends up countless new shoots when its underground parts are broken and are spread by tillage. It readily destroys pastures, as cattle dislike and avoid it. The thistles are characterized by spiny stems, alternate leaves, and terminal discoid flower heads set in spiny bracts. The flower heads vary in color from creamy white to deep purple and are from 1 to 2 inches across, consisting of compact, tubular, perfect flowers that look like a shaving brush.
Thistles have some positive attributes.
They provide nectar and pollen for bees, and the larvae of certain butterflies feed on the plants. Britton, N.L., and A. Brown. 1898. Illustrated Flora of the Northern United States and Canada, v. 3. Charles Scribner's Sons, New York. Darlington, W. 1859.