Canada Thistle is one of our worst weeds. It is a perennial by virtue of an extensively creeping root system and rhizomes. It forms large patches wherever it becomes established. The plant is dioceous, which means that the male flowers are on one plant and the female flowers are on a different plant. Usually a patch of Canada thistle is one or the other. The male plants do not form seeds. The seed is readily carried on the wind by the plumose bristles fastened to it. There is a bacterium which invades the plant and turns it bone white. But it doesn't as yet seem to be an effective control.