Bull thistle (Cirsium vulgare (Savi) Ten.) is  a common weed in the Midwest, Ontario and Eastern United States.   (For Milk thistle)

As an edible wild plant
The first year's leaves of C. vulgare, C. arvense and C. edule of  these biennials may be eaten after cutting away the spines.  Young stems and first year roots may also be steamed, fried or sauteed.  (see)
Canada thistle, Cirsium arvense (L.) Scop., Irregular spiny, lance-shaped leaves, hairless above and white-hairy below.  Small heads of disk flowers only.  Bull thistle has much larger head, and Canada thistle has green stems without spiny wings.  (More)