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Russian knapweed (Acroptilon repens)
ODA rating: B and T
USDA Symbol: ACRE3
Oregon Russian knapweed distribution
Other common names
Turkistan thistle, creeping knapweed, mountain bluet, Russian cornflower, hardheads.
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Image courtesy of Eric Coombs, Oregon Dept. of Agriculture.
  Image courtesy of Lesley Richman, Burns BLM.

Image courtesy of Lesley Richman, Burns BLM.
If images are downloaded and used from the ODA web site please be sure to credit the photographer.
Perennial; blooms summer to fall. Grows up to 4 feet tall. Forms dense colonies with stems that are erect and branched. Lower leaves deeply lobed, 2 to 4 inches long; upper leaves entire or serrate. Cone-shaped flowering heads 1/4 to 1/2 inch wide with one flower, pink to lavender, growing at each branchlet tip. Bracts below the flower rounded with papery margins.
Once Russian knapweed is established, it can overrun native grasslands as well as irrigated crops. It has dense growth and spreads entirely by fragments of its creeping rootstocks or by seed. Russian knapweed can be successfully controlled with combinations of grazing and herbicides but control programs must persist for several years.
This plant is an aggressive native of Eurasia that was introduced in North America in 1898. It can be found in every western state. Russian knapweed infests both native range and irrigated crop land. It is believed initial infestations were from contaminated alfalfa seed. 
Distribution in Oregon

Biological controls
One approved biocontrol agent, a gall nematode, is established in Oregon.
Subanguina picridis