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diffuse knapweed (Centaurea diffusa)
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.
 
Photo by Dan Sharratt
Image courtesy of Eric Coombs, ODA
diffuse knapweed rosetteImage courtesy of Eric Coombs, ODA 
 
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