The long terminal spine differentiates diffuse from spotted knapweed. The weed produces one or more shoots that are branched and 1 to 3 feet tall. Rosette leaves can be 6 inches long and deeply lobed. Leaves are similar to diffuse knapweed. Lavender to purple flowers are solitary on shoot tips and about the same size as diffuse knapweed flowers.
Involucre bracts are stiff and black-tipped.
The tip and upper bract margin have a soft, spine-like fringe and the center spine is shorter than others. Phenology, Biology and Occurrence Diffuse knapweed seeds germinate in spring or fall. Seedlings develop and survive the first growing season as a rosette. Rosettes resume growth early in their second spring, bolt in May to June, set seed in June and July, and die. Occasionally, diffuse knapweed germinates and sets seed in the same year, growing as an annual, or produces a new shoot the year after seed production, growing as a short-lived perennial.