Bighead knapweedCentaurea macrocephala, is a member of the thistle tribe (Cynareae) in the sunflower family. This perennial species is the tallest knapweed growing in the Pacific Northwest, ranging from two to five feet tall, depending on the habitat. The plant stems are upright and unbranched, terminating in a single flower head. The leaves are broadly lance shaped with toothed edges and pointed tips, and they have a rough surface. Basal or rosette leaves are stalked and they can reach 15 inches long and three inches wide. The leaves and leaf stalks are progressively smaller upward on the plant stem, with the top leaves being stalkless. The solitary flower heads are globe shaped, and one to three inches in diameter. The bracts beneath the flower head have thin, papery, fringed margins. The lower bracts show evidence of spines. The flowers are yellow.