Diffuse knapweed is a member of the Asteraceae or Sunflower family. Diffuse knapweed grows 1 to 3 feet tall from a deep taproot. Upright stems of the plant have numerous spread branches, giving a ball-shaped, tumbleweed appearance. Basal leaves, which form rosettes on a central crown, are borne on short stalks and are deeply divided into lobes on both sides of the vein. Stem leaves of the plant are stalkless and become progressively smaller and less divided higher up the stem, with the uppermost small leaves being bractlike. Flower heads are urn-shaped and 3/16 to 1/4 inch in diameter and 5/16 to 7/16 inch long, excluding spines and flowers. Flower heads are solitary or borne in clusters of two or three at the ends of the branches of the plant. Bracts surround the flower heads and are yellowish green in color with a buff or brown margin. Each bract is edged with a fringe of spines and ends with a longer spreading spine at the tip, resembling a crab like appearance. Flowers are white or purple in color. Diffuse knapweed seeds are buff to brown in color, about 1/8 inch long, and have a plume of bristle-like hairs.
Diffuse knapweed is considered a noxious weed under North Dakota state law, thus landowners are required to eradicate or control the spread of the plant.