Russian knapweed (Centaurea repens L.) is a member of the Aster family, Thistle tribe, and is a creeping perennial introduced from Europe. It is reproduced by seeds and creeping, horizontal roots. Roots, which are both vertical and horizontal in the soil, may or may not be black with a scaly appearance. The ridged stems are erect, rather stiff, branched and 1 to 3 feet high. Young stems are covered with soft gray hairs or nap.
The upper leaves are small and narrow with broken edges. Leaves attached midway up the stem have slightly toothed margins, while basal leaves are deeply notched. The flowers are thistle-like, solitary, terminal, 1/3 to 1/2 inch in diameter and lavender to white. The plant flowers in June to August and seed is produced in later summer to early fall.
Russian knapweed is a serious noxious weed. It is very difficult to control or eradicate once it becomes established. It grows in cultivated fields, along ditch banks, fence rows, roadsides and in waste places. It is distributed throughout Colorado and is especially prevalent on the Western Slope from 4,500 to 12,000 feet. It is responsible for numerous horse deaths each year in Colorado.