A tall, thistle-like plant with stout, straight, prickly, branching stems. Grows from a basal rosette. During the rosette stage, leaves vary from somewhat egg-shaped to large and oblong leaves that are quite hairy with age. The taproot may be more than 2 feet long and 1 inch in diameter at the crown. Blooms June through October. Flowers are very small, massed on a cylindrical head, each with a tubular corolla, lilac to lavender, with stiff, narrow, pointed bracts longer than the flowers. The flower heads form at the tops of the stems. Leaves on flowering plants are large, oblong, opposite, stemless, and form cups (which may hold water) and are prickly, especially on the lower midrib. The stems are prickly. Leaves on upper stems are lance-shaped, with scalloped (but not lobed) edges and without spines except on the midvein beneath.
Similar species: Cut-leaved teasel (D. laciniatus) has white flowers, blooms July through September, and has irregularly pinnately lobed upper stem leaves.