Involucre bracts are divided like teeth on a comb and tipped with a slender spine that makes them sharp to the touch. Sometimes the bracts are dark-tipped or spotted like spotted knapweed. 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.