Male and female flowers usually contained in separate heads and borne on different plants. Seed is attached to tannish down that is easily broken off. There is also a Bull Thistle that is very similar except it is a biennial, reproduces only from seed, has a larger bloom, and the seed is straw-colored, striped with brown. Taproot large, deep, and fleshy. Stem is erect, branched, smooth, and with a milky sap.
Leaves are basal and along the stems.
Leaves on stems are smaller and either less lobed or entire. Flower head of ray flowers only, bright blue, about one inch across, formed at ends of branches and in the axils of leaves of upper part of plant. Flowers are most conspicuous in the morning and close late in the day. An annual shallow-rooted plant. Leaves nearly smooth, deeply cut into a number of lobes, mostly alternate.