Russian knapweed (Acroptilon repens) is an alien weed, unintentionally introduced from Russia many years ago as a contaminant of imported alfalfa seed. Because it has had no natural insects or diseases in North America, Russian knapweed has established itself in cultivated fields, pastures and roadsides. It is a noxious weed in all western and central states, and has infested most counties of Colorado west of the continental divide. It is spreading along the Front Range, including Larimer and Weld counties. In Colorado, Russian knapweed is on the B list of noxious weeds that requires a state wide plan to control its spread. The plant thrives in all soil types. It is also allelopathic, meaning it produces an inhibitory substance from its roots that retards other plant growth in its vicinity, thereby enabling it to develop dense stands or monocultures.

Russian knapweed is an aggressive perennial plant that spreads via its extensive root system. It is an erect, branching plant from one to three feet tall. The young stems are covered with soft gray hair or knap (hence the name). The simple leaves are alternate and covered with short stiff hairs. The flowers are in heads like some thistles, one third to one half inch in diameter and lavender to white in color. The bracts are papery and have no spines. The seeds are grayish in color, about one eighth inch long with bristles at one end. Rodents may aid in the spread of the plant as they bury the seeds in their burrows, inadvertently propagating the plant.

Unlike the noxious spotted and diffuse knapweeds, and Canada thistle for which it can be mistaken, Russian knapweed is both noxious and poisonous to horses. The toxin that is present in all parts of the plant specifically destroys the area of the brain that coordinates and controls a horse’s ability to bite and chew food. A horse must eat about 1.8 2.6 kg/100 kg body weight over a period of a month to cause brain damage. It has been calculated that horses have to consume an estimated 59 to 71% of their body weight before clinical signs develop. Horses will eat Russian knapweed mostly before it flowers, with some horses acquiring a liking for the plant. Cattle, sheep and goats graze the plant without problem, and have been used as a means of controlling the weed.

“Chewing disease”, as it is referred to in horses, is characterized by increased tonicity and incoordination of the muscles that enable chewing of food. The hypertonicity of the facial muscles produces a "wooden" expression to the face. Food is often held in the mouth unchewed, and continual chewing movements cause frothing saliva which can resemble that seen in rabies. Curling the tongue from side to side is often seen.

Some horses may wander about with their lips brushing through the grass that could be mistaken for normal grazing. Although affected horses cannot chew their food, they are still able to swallow. Some horses learn to submerge their heads far enough into a deep trough of water to allow water to reach the pharyngeal area where it can be swallowed. Weight loss and depression are common. Pneumonia resulting from inhalation of feed is a serious sequel to the disease. There is no effective treatment for the permanent damage to the brain, and it is usually necessary to euthanize the affected horse to prevent it starving to death.

Russian knapweed is difficult to control once it becomes established. Early recognition of the plant and its prompt removal is the best means of eradicating it. There are herbicides such as Roundup and Tordon that, if used correctly, will control Russian knapweed. However it is important to till and reseed with grass to help prevent the knapweed re-establishing itself. For detailed information see Fact Sheet #3.111 - Russian Knapweed

Unfortunately, another invasive weed that has recently been detected in Colorado is Yellow star thistle (Centaurea solstitialis). This noxious weed also affects horses in the same manner as Russian knapweed. Yellow star thistle is a weed that was introduced from the Mediterranean area and has become well established in California, Oregon, Idaho and Washington. If you recognize this plant anywhere in Colorado, it is important to notify your local Extension weed specialist as soon as possible.

Photographs taken by author Tony Knight

The authors have received training through Colorado State University Cooperative Extension's Master Gardener program and is a Master Gardener volunteer for Larimer County.

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This page updated:  August 27, 2007