This is a noxious weed across cool temperate regions of North America. It was first reported from Ky. in 1914, when Gm noted: "The Canadian thistle is more often seen in newspapers than in our fields... it occurs locally in the State... but for some reason disappears again... its seeds frequently come to Kentucky in imported forage seeds..." It was rarely recorded until after 1950 (Harvill 1941), but increased greatly after 1970. Since 1990 arvense has become much more widespread than colls. indicate, but Ky. is near the southern edge of its range in North America. It has expanded towards the south, especially along major highways, but much less in Tenn. (Sudbrink et al. 2001). It remains virtually absent on the southern Interior Low Plateaus and Coastal Plain. C. arvense is an unusual thistle, spreading aggressively by perennial running roots and largely dioecious, with a small proportion of perfect flowers; 2n = 34 (versus 20 in most native Cirsium). Most or all plants may be var. horridum Wimmer & Grab. (see also W), but varieties are not generally recognized in North America.