Canada Thistle (Cirsium arvense)
Division - Magnoliaphyta
Canada thistle can be differentiated from musk and bull thistle by the fact that it has no spiny bracts or spines on the stems. In appearance, it falls between bull thistle and a centaurea. Its spineless bracted flower-head resembles that of a centaurea.
Canada thistle is a common and pervasive perennial weed that forms colonies by sending up new shoots from root nodes, not rhizomes. This tends to create bushy colonies. Cirsium arvense easily generates new shoots out of any root bits left in the ground after an attempted eradication and if gets established in an area, it is very difficult to remove entirely and will come back repeatedly. When Canada thistle does infest an area, eradication is not a pleasant task, though not nearly as daunting a task as removing Cirsium vulgare and Carduus nutans. It does not need spines to protect itself as much as those biennial relatives, relying more on regeneration from root-bits. The biennials need two years to complete a flowering cycle. Canada thistle can just hang in there, whether it flowers or not.