Musk thistle is a large, spiny biennial with rose-purple flower heads up to 2½ inches wide. The stems are commonly winged with spiny leaf tissue. The plant forms basal rosettes during its first year, which have deeply lobed leaves that are up to 10 inches long and 4 inches wide, and have a prominent, nearly white center vein. During the second year, an upright flowering stalk grows that has smaller, very spiny leaves. The flower heads are mostly solitary and nod at the branch tips. At the base of each flower head are numerous, spine-tipped bracts, 1/8 to 3/8 inch wide, that curve away from the heads. Blooms June through October. A single plant can produce 11,000 seeds, spread by silky parachutes.
Similar species: Our native thistles have strongly whitened undersides to the leaves, whereas this and two other exotic thistles have both sides of the leaves the same green color. The exotics also tend to have heavily branched stems and more numerous flower heads per stem. Our native thistles are a valuable part of our native flora, providing food for monarch butterflies and food and nesting material for goldfinches.