Biennial thistle with the upper stems and branches winged with a wavy, prickly tissue. Flowerheads typical of thistles, reddish purple to purple, with a prominent involucre (the flowerhead base, covered by leaflike bracts), which is covered with a fine, cobweb-like silk. Spiny bracts grow right up to the flowerheads; the middle and lower spines are spreading and yellow or straw-colored. Blooms June-September. Leaves alternate, deeply lobed, hairy, spiny, the upper surface with stiff bristles.
Similar species: The quickest way to distinguish the exotic invasive thistles from our native thistles are the leaves: the leaves of exotics are typically green on both sides, while leaves of natives usually have whitened, woolly undersides. Exotic thistles also tend to have heavily branched stems and more numerous flowerheads per stem. There are nine species of Cirsium in Missouri. Bull thistle and Platte thistle (C. canescens) are the only two with stems having spiny-margined wings. Platte thistle has cream-colored flowers (rarely pale pink) and has been collected only once in Missouri, along a railroad in Jackson County in 1951.