This prickly, perennial herb is dioceous, meaning that plants are either
male or female, but do not have both parts on any one plant. Usually any
given stand is one or the other. Males of course do not produce seeds.
This plant can easily be distinguished from other thistles, because of
its smaller size. Of course, its difficult to distinguish them if you're
not used to looking at thistles. However, the most common mistake is that
people seem to misidentify it as spotted
knapweed (something I found in the official guide to the Flowers of
Grand Teton National Park). Spotted Knapweed
however does not have the same spiney nature of this plant, even thought
he flowers do look very similar.
The actual name, Canada Thistle, is a bit misleading, because it is not
native to Canada, nor anywhere else in North America for that matter.
It came over from Europe and now is one of the worst weeds in the Northern
United States. Another name for Canada Thistle is Creeping Thistle.