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US weed ID · Pasture & rangeland weed
Russian Knapweed
Rhaponticum repens · Asteraceae (aster family)
- Perennial
- Broadleaf
- Non-native
- Toxic / hazard
If a dense, gray-green patch of knee-high stems with small pink-to-purple button flowers spreads outward every year from dark, scaly roots, you are probably dealing with Russian knapweed.
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Poisonous to horses. Eating it for roughly one to two months, until total intake reaches about 60 to 200 percent of body weight, can cause chewing disease (nigropallidal encephalomalacia), a fatal brain disorder. Most livestock avoid it because it is bitter. Even tiny amounts lower grain and flour quality.
What is Russian knapweed?
Russian knapweed is a long-lived, creeping perennial from central Asia that arrived as a contaminant in alfalfa seed early in the 1900s and is now widespread in the semiarid West. Russian knapweed identification relies on three clues: papery, spineless flower bracts, dark scaly roots, and solid colonies that can persist for decades; one stand has been documented for more than 75 years. You will also see it listed under its older scientific name, Acroptilon repens, which USDA's PLANTS database still uses.
It is a serious weed in pastures, hay and grain fields and irrigated bottomland. It releases compounds that suppress nearby plants, so stands become nearly pure knapweed over time. Livestock avoid its bitter foliage, it poisons horses, and it contaminates grain. Because it stores reserves deep underground, single treatments rarely make a lasting dent.
Russian Knapweed identification: what to look for
Check in summer when flowers are open. Pinch a head and look at the bracts: Russian knapweed bracts are smooth and papery, without the spines or dark comb-like tips of spotted and diffuse knapweed. Then dig at the base of a stem to find dark, scaly roots, and step back: evenly spaced stems in a dense patch point to spread from roots.
- Growth form: Bushy, branched stems about 18 to 36 inches tall in dense colonies.
- Leaves: Gray-green and finely hairy; lower leaves deeply lobed, upper leaves small and narrow.
- Flower heads: Single urn-shaped heads at branch tips, pink to purple, often compared to tiny pineapples.
- Bracts: Greenish to straw-colored with broad, smooth, papery tips and no spines.
- Roots: Vertical and horizontal roots, brown to black with a scaly, bark-like surface.
- Stems: Whitish and woolly when young, turning dark brown to black.
Where it grows in the United States
It grows in most western states and adjacent Canada and has been reported in the Midwest and Great Plains and as far east as Virginia. A 2000 estimate counted about 1.56 million acres, including roughly 500,000 in Washington, 425,000 in Idaho and 160,000 in Wyoming; Colorado reported more than 132,000 infested acres in 2008.
It favors heavy, often saline or alkaline bottomland, irrigated fields and ground near ditches and waterways. It handles some flooding but not severe drought and is most competitive on clay. It rarely invades healthy native plant communities; it moves into pastures, croplands, roadsides, railways, clearcuts and other disturbed or degraded ground.
Russian Knapweed look-alikes
A few plants get mixed up with it:
- Spotted knapweed (Centaurea stoebe): Also pinkish-purple, but it grows from a single stout taproot rather than the creeping black roots that knit Russian knapweed into colonies.
- Diffuse knapweed (Centaurea diffusa): A bushy biennial with mostly white to purple heads and small spines at the base of each head; Russian knapweed bracts have no spines.
How to get rid of Russian knapweed
Pulling, digging and mowing: Pulling and digging barely touch established patches because the roots run many feet deep and fragments regrow; in Washington, pulling two or three times a year only held patches in place. Washington's weed board warns against tilling because it spreads root pieces, although Colorado State notes tillage may help break down the plant's growth-suppressing residue before reseeding. Mowing helps only as part of a plan: two mowings eight weeks apart, the first at bud stage, suppressed it for one season before it rebounded, and cutting about three times a year steadily drains root reserves. Use mowing to weaken plants ahead of a fall herbicide treatment and reseeding.
Cultural control matters most, because the right conditions keep it from coming back.
- Reseed with competitive perennial grasses such as Russian wildrye or thickspike wheatgrass after weakening the knapweed; experts consider this essential.
- In irrigated fields, dense crops such as alfalfa can suppress it.
- Nitrogen fertilizer applied with an herbicide can help surviving grasses compete if moisture allows, and better grazing management makes control last longer.
- Fence horses out of infested areas; grazing is not a control method for this weed.
Herbicides: Colorado State University Extension lists picloram, aminopyralid, clopyralid (alone or with 2,4-D), chlorsulfuron and a combination of aminocyclopyrachlor and chlorsulfuron, applied at bud to flowering in spring and summer or in late fall. Russian knapweed is especially vulnerable to fall and early-winter treatment, apparently because of its root bud cycle. In Colorado and Wyoming research, glyphosate failed to control it. Fall chlorsulfuron can briefly injure cool-season grasses, and any rate that harms grass works against you. In one study a single picloram treatment still gave 85 percent control eight years later where perennial grasses were present. Use products labeled for your site (pasture, rangeland, non-crop) and follow grazing and haying limits; the label is the law. If you hire someone to spray, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.
Keeping it from coming back
Because it travels as root pieces and seed in hay, crop seed, soil and machinery, watch what you bring in. Use weed-free hay and clean seed, clean equipment after working infested fields, and do not move soil from infested ground.
Scout ditch banks, field edges and pastures and treat new patches right away; a new plant can claim about 12 square yards in two seasons. Good grazing management leaves little bare ground for it.
Questions we get about Russian knapweed
Is Russian knapweed poisonous to horses?
Yes. Horses that eat it steadily for one to two months can develop chewing disease, a fatal brain disorder. Fence horses out and keep it out of hay.
Why does it keep coming back after mowing or pulling?
Deep, creeping roots store large reserves and resprout from buds and fragments. You need repeated stress plus competitive grass.
Does it poison other plants?
Its roots and leaves release compounds that inhibit other plants, which helps it form nearly pure stands.
When is the best time to spray it?
Bud to flowering, or late fall into winter when conditions allow spraying, when it is especially vulnerable.
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Sources: Colorado State University Extension: Russian Knapweed · USDA Forest Service, Fire Effects Information System: Acroptilon repens · Washington State Noxious Weed Control Board: Russian Knapweed · Washington State Noxious Weed Control Board: Noxious Weeds That Harm Washington State, Eastern WA Field Guide · USDA PLANTS Database: Acroptilon repens (ACRE3) · USDA APHIS: Federal Noxious Weed List · Colorado Department of Agriculture: Noxious Weed Species · Oregon Department of Agriculture: Noxious Weed Policy and Classification System (Oregon noxious weed list)
Herbicides: always read and follow the product label. Pesticide applicator licensing rules vary by state.