Bull thistle is a biennial with a short, fleshy taproot. The stem is 2 to 5 feet tall, bearing many spreading branches. It is green or brownish, sparsely hairy, irregularly and spiny winged. Leaves in the first year form a rosette, stem leaves are pinnately lobed, hairy and prickly on upper side and cottony underneath. Flowers are 1 1/2 to 2 inches wide, more or less clustered at the ends of branches. Involucral bracts are narrow, spine-tipped, progressively longer and narrower from outer to inner ones, flowers dark purple. Seeds are topped by a circle of plume-like white hairs.
Bull thistle is a native of Eurasia and is now widely established in North America, having been introduced many times as a seed contaminant. Pastures, fields, roadsides and disturbed sites are potential habitats for this highly competitive weed. Flowering occurs from July through September. It is possible to separate bull thistle from Canada thistle by examination to the leaves alone. Bull thistle leaves are prickly hairy above and cottony below, while Canada thistle leaves are glabrous above and glabrous or hairy below.