Photographer: Norman E. Rees Affiliation: USDA Agricultural Research Service Source: Bugwood.org Copyright: (CC BY 3.0)
Rhaponticum repens is creeping perennial with stems/shoot that grow up to 36 inches tall and are multi-branched. The black roots can penetrate up to 25 feet deep and new plants can bud directly off of them. Leaves alternate to one another along the stem and vary in shape. Upper stem leaves vary from narrowly lance-shaped to linear and smooth edged or even toothed; that grow about 2/5 to 1.5 inches. Lower leaves can grow to 4 inches and are deeply lobed or have wavy edges. Flowers can range from white, pink or lavender-blue and are interspersed with bristles gathered above green reduced leaf-like structures. Seeds are pale gray to white and slightly egg-shaped.
Russian knapweed contains allelopathic chemicals which can suppress other plant species, forming a monoculture of Russian knapweed stands. Infestations reduce yields of desired plants and decrease the production quality of rangelands. Horses with prolonged consumption of Russian knapweed can develop “chewing disease” or equine nigropallidal encephalomalacia (ENE); which is a permanent disease caused by lesions in the brain. Symptoms of ENE include the inability to eat or drink, aimless or awkward movement, and spontaneous activity and is fatal in severe cases. Because Russian knapweed is also toxic to horses, infested hay has lower feeding and market value, affecting farmers that grow hay for supplemental income. By overtaking and devaluing rangelands and threatening the health of horses, Rhaponticum repens has become a serious problem in the western United States.
Rhaponticum repens emerges in early spring and begins bolting in May to June and flowers through the summer into fall. Extensive root systems enable some populations to be extremely long lived (up to 75 years). Reproduction mainly happens from shoots budding from creeping roots and less often by seeds; and Rhaponticum repens seed production is generally lower than the other knapweeds. Plants along roadsides or on rangelands average 100 to 300 seeds per plant. Seed longevity is two to nine years. Once established, patches expand mainly by rhizomatous growth. Mature plants can spread radially from established plants’ rhizomes and can cover up to 39 square feet in two growing seasons.
Rhaponticum repens entered the United States around 1900 possibly as a contaminant of alfalfa seed from the former Russian Turkestan region of Asia. Once imported, Russian knapweed was easily spread by the sale of domestically produced alfalfa seed or hay containing knapweed seeds. Reported sighting of this weed started in the early 1920s in Washington, then Michigan in 1928 and Montana in 1934. In 1985, infestations were reported from South Dakota, Minnesota and Ohio. By the early 2000s, it was officially reported in North Dakota, Oklahoma, Nebraska, Texas and all western United States.
Russia, Mongolia, Iran, Ukraine, Kazakhstan and Asia Minor
U.S. Habitat: Commonly found in areas near a water source such as: fields, rangeland, cultivated sites, orchards, vineyards, roadsides, and disturbed, unmanaged places.
U.S. Present: AZ, CA, CO, IA, ID, IL, IN, KS, KY, MI, MN, MO, MT, NB, ND, NM, NV, OH, OK, OR, SD, TX, UT, WA, WI, WY
Russian knapweed resembles two other invasive knapweeds; the spotted knapweed (Centaurea stoebe) and the diffuse knapweed (Centaurea diffusa). However, the Russian knapweed differs in biology by being a haploid plant so it only has half the number of chromosomes as other Centaurea species. Also, Russian knapweed can be distinguished by the floral bracts (specialized leaves) which are somewhat hairy at the tip and green at the base.
Rhaponticum repens is more difficult to eliminate than the other knapweeds because of its rhizomatous growth pattern. Studies for biological control in Montana and Wyoming have shown a gall-forming nematode and gall fly to have some impact on this plant. The nematode, Subanguina picridis, was introduced in the United States in 1984 and only Russian knapweed is susceptible. The nematode induces gall formation and it overwinters as a larva in root galls in the upper layers of the soil. In early spring, the larvae move up into stems, leaves, and new shoots. Unfortunately, even though it is very effective in the laboratory; its effect on Russian knapweed populations are largely unnoticeable. In Wyoming the gall midge, Jaapiella ivannikovi has been released as a potential biological control for Russian Knapweed. It was initially released in 2010 after years of research this insect seem promising; but the real-world successes of this insect are still not known.
Fire is not a solution for the management of knapweeds. A single, low-intensity fire does not control spotted, diffuse or Russian knapweeds and instead may increase cover and density. Also, cultivation alone will only increase the rate of spread and establishment of Acroptilon repens. Cultivation in combination with re-seeding of native competitive grasses, can reduce the success of a knapweed re-invasion.
Herbicide treatment does not inhibit the growth of Russian knapweed if used by itself. For this knapweed, a single herbicide treatment followed by reseeding with rhizomatous grasses (such as streambank wheatgrass) can provide long term control and avoid annual reapplication of herbicide. Overall, for the management of Russian knapweed a combined strategy using chemical, cultural and mechanical methods is necessary.
Benz L.J., K.G. Beck, T.D. Whitson, and D.W. Koch. 1999. Reclaiming Russian knapweed infested rangeland. Journal of Range Management. 52(4):351-356.
Mangold, J.M., C.L. Poulsen and M.F. Carpinelli. 2007. Revegetating Russian knapweed (Acroptilon repens) infestations using morphologically diverse species and seedbed preparation. Rangeland Ecology and Management. 60(4):378-385.
Panter, K.E. 1991. Neurotoxicity of the knapweeds (Centaurea spp.) in horses. In: Noxious Range Weeds. L.F. James, J.O. Evans, M.H. Ralphs, R.D. Child (eds.). Westview Press, Boulder, Colorado. pp. 316-324.
Watson, A.K. 1986. Host range of, and plant reaction to, Subanguina picridis. Journal of Nematology 18(1):112-120.
Winston, R., M. Schwarzlander, C. Randall, and R. Reardon. 2010. Biology and Biological Control of Knapweeds. USDA Forest Service Publication FHTET-2010-1. Morgantown, West Virginia. 149p.