Tall, branching biennial or short-lived perennial. Stems hairless or sparsely hairy, sometimes with patches of white felty hairs; lacking spiny-edged wings. Flowerheads many, solitary at the branch tips, pink-purple or reddish purple, rarely white. Outer involucre bracts with a dark spot; all bracts end in a weak prickle. Blooms July-October. Leaves unlobed, toothed or wavy, with marginal prickles only (none on the leaf surface), woolly-hairy beneath. Upper stem leaves narrowly lance-shaped with longer spines.
Similar species: Missouri has nine species of Cirsium. Some are invasive exotics. Others, like tall thistle, are native. Tall thistle is closely related to field thistle (C. discolor), and the two can hybridize where they grow near each other. Most or all of the leaves of field thistle are deeply lobed. Field thistle is uncommon in the Ozarks but scattered to common elsewhere in the state. It is more common in upland habitats and disturbed places.