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diffuse knapweed (Centaurea diffusa)
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Article Content
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ODA rating: B
USDA Symbol: CEDI3
Oregon diffuse knapweed distribution
Other common names
white knapweed, spreading knapweed, tumble knapweed
Click on photo below to view larger image.
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Photo by Dan Sharratt
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 Image courtesy of Eric Coombs, ODA
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 Image courtesy of Eric Coombs, ODA
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If images are downloaded and used from the ODA web site please be sure to credit the photographer.
Description
Biennial; flowers midsummer to fall. Grows to 3 ft tall. Single-stemmed plant with numerous lateral branches. Flowers white to rose, sometimes purplish. Flower heads slender with pointed, fringed bracts. Spreads by seed, aided by the tumbling of windblown mature plants.
Impacts
Diffuse knapweed will form dense stands on any open ground, excluding more desirable forage species. Once established, the necessary extensive control measures are often more expensive than the income potential of the land. Grows under a wide range of conditions, such as riparian areas, sandy river shores, gravel banks, rock outcrops, rangelands and roadsides.
Known hazards There are possible health hazards from absorbing plant juice through bare hand pulling of plants. It is recommended that gloves are warn while handling plants.
Introduction
Diffuse is a member of a large genus of over 400 species, most originating in the Mediterranean region. Diffuse knapweed was first introduced to the Pacific Northwest at the turn of the century as a contaminant in alfalfa seed imported from Turkestan, Turkmenistan or hybrid alfalfa seed from Germany.
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Distribution in Oregon
 
Biological controls
Agapeta zoegana root boring moth
Bangasternus fausti seed head weevil
Chaetorellia acrolophi seed head fly
Cyphocleonus achates root weevil
Larinus minutus seed head weevil
Metzneria paucipunctella seed head moth
Pterolonche inspersa root boring moth
Sphenoptera jugoslavica root boring beetle
Terellia virens seed head fly
Urophora affinis seed head gall fly
Urophora quadrifasciata seed head gall fly
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