According to ZipCodeZoo.com, there are about 200 species of thistle found worldwide--in North America, Asia, Europe and northern Africa. Thistles bear spiny leaves and distinctive upright flowers.
In "The Audubon Society Field Guide to North American Wildflowers," botanist William A. Niering identifies three major thistle species found in North America: the Canada, bull and yellow thistles. The California Department of Food and Agriculture further notes the yellowspine and wavyleaf thistles as important species.
You can distinguish the bull thistle by its prickly stem. The Canada thistle, on the other hand, has a smooth stem. The yellowspine thistle spreads via vigorous lateral roots, while the wavyleaf thistle spreads weakly and produces thick, underground roots called tubers.
North American thistles likely belonged to a single species before the Pleistocene epoch about two million years ago. Glacier movements in the western part of the continent likely isolated groups of thistles, leading to the development of the array of species observed as of 2010, according to ZipCodeZoo.com.
- Postcode Zoo: Cirsium (Genus)
- "The Audubon Society Field Guide to North American Wildflowers"; William A. Niering; 1979
- California Department of Food and Agriculture: Cirsium Genus