Diffuse knapweed is a member of the Aster family, Thistle tribe. Diffuse knapweed was introduced from Europe and is a biennial or short-lived perennial forb, which reproduces only by seed. The plant usually produces a single main much-branched stem that is1½ to 2 feet tall. A basal rosette of leaves is present in young plants with each leaf divided into narrow segments.
When leaves are young a thin nap is present. Stem leaves in the mature plant become much reduced as they ascend to the tip and alternate one per node. Flowers, mostly white, sometimes purple, and are located on each branch tip. The bracts surrounding each flower bear four to five pairs of lateral spines. One long terminal spine.
Diffuse knapweed can be found in pastures, riparian areas, roadside, and waste areas in Colorado. It is a tough competitor on dry sites and rapidly invades and dominates disturbed areas. It is widespread and increasing in the state, particularly along the Front Range.
Non-standard name: Tumble knapweed. Mature dried plants tumble across the landscape with high winds and scatter deciduous seed heads.