Russian knapweed is a member of the Aster family, Thistle tribe and is a creeping perennial introduced from Europe. It reproduces by seeds and creeping, horizontal roots. Roots in the soil, may or may not be black with a scaly appearance. The rigid stems are erect, branched, and one to three 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, 1/3to 1/2 inch in diameter, lavender in color, or occasionally white. The plant flowers in June to August and seed is produced from late summer to early fall. Seed longevity is reported to be 2 to 3 years.
Russian knapweed is a serious noxious weed. It is very difficult to control or eradicate once it becomes established, due to the 20 ft. deep root system. It grows in cultivated fields, along ditch banks, fence rows, roadsides, and in waste places. It is distributed throughout Colorado, except in the eastern plains area. It is especially prevalent on the Western Slope mainly up to 8500 feet, but has been found growing as high as 11000 ft. It also tolerates 115 degree plus temperatures at2500 ft. in the desert near Tucson, Arizona. This weed is poisonous to any equine species; horses, mules and burros.
For more info and how to control, check out the La Plata County Weeds Dept. Fact Sheet