“Brown knapweed (Centaurea jacea,) closely resembles Russian knapweed, but its bracts are brown, not yellow-green and it is also not rhizomatous”.
Russian knapweed (Acroptilon repens, formerly Centaurea repens) is a rhizomatous, deep-rooted, long-lived perennial forb that grows about two feet tall. Stems are thin, stiff and covered with soft, short hairs. Rosette leaves are narrow at the base and widen toward the tip. Shoots emerge in the early spring, and plants bolt in early summer and flower from mid-summer to early fall. Like spotted knapweed, flower color is light pink to purple, but two characteristics distinguish Russian knapweed from spotted knapweed and other similarly colored knapweeds:
- Flower head bracts have light thin hairs, a papery, translucent tip and are green at the base
- Rhizomatous instead of tap-rooted
Origins, Current Status and Distribution
Russian knapweed is native to Mongolia, Russian Turkestan, Iran, Turkish Armenia and Asia Minor. Seeds of Russian knapweed were present in alfalfa seed imported from Russian Turkestan beginning in 1898.
Once imported, it spread widely by sale of domestically produced alfalfa seed or hay containing weed seeds. It was first reported in the Northwest in Yakima County, Washington, in 1922 and in Fergus County, Montana, in 1934.
By 1991 the weed was recorded in every Montana county and infests an estimated 51,000 acres.
Russian knapweed may occur in a wide range of habitats including open forests, rangeland, roadsides, Conservation Reserve Program (CRP) lands, pasture land and ditch banks, but its tolerance for poorly drained, saline, alkaline soils extends its range to irrigation ditches, flood plains and river corridors, too. It is likely to be a pest in crops and hay fields where its rhizomatous growth makes it difficult to control. In the north-central part of Montana, it is common in alfalfa and grain fields in the Missouri River bottomlands.
Spread and Establishment Potential
Russian knapweed seed production is highly variable, but generally lower than the other knapweeds. Plants along roadsides or on rangelands average 100 to 300 seeds per plant, but may increase to 1,200 under optimal conditions. Seed longevity is two to nine years. Russian knapweed has no appendages for dispersal, and seed is spread by the same mechanisms as for spotted knapweed. Once established, patches expand mainly by rhizomatous growth. Mature plants can spread radially from established plants’ rhizomes and can cover up to 23 feet over two growing seasons. Rhizome fragments created after plowing or other disturbances can also increase spread. Competitive ability and spread is highly dependent on the surrounding plant community. Rhizomatous grasses can suppress this plant, but if competing vegetation is sparse or highly disturbed, or droughty conditions prevail, Russian knapweed is highly competitive.
Knapweeds are associated with reductions in native plants, reduced forage yields and degraded habitats in range, grasslands and agricultural areas. Based on estimates from 1996, knapweeds cost Montana $42 million per year in direct and indirect costs.
Russian knapweed can cause “chewing disease” in horses, a neurologic disorder that causes brain lesions and mouth ulcers. Symptoms of chewing disease include drowsiness, difficulty in eating and drinking, twitching of the lips, tongue flicking and involuntary chewing movements. There is no cure and horses die of dehydration or starvation. Horses will select other forage when available.
Biology, Ecology and Management of Montana Knapweeds , Duncan, C., Story, J., Sheley, R. Revised by Parkinson, H.., and Mangold, J
EB0204 May 2011
Photo source: http://www.malag.aes.oregonstate.edu/wildflowers/images/RussianKnapweedCentaureaRepens19July08_5.JPG