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yellow starthistle (Centaurea solstitialis)
ODA rating: B
USDA Symbol: CESO3
Oregon yellow starthistle distribution
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Image courtesy of Dan Sharratt, Oregon Dept. of Agriculture. 
Image courtesy of Greg Winans, Tri-County Weed. 

Image courtesy of Greg Winans, Tri-County Weed. 
If images are downloaded and used from the ODA web site please be sure to credit the photographer.
Annual; flowers July to August. Grows 2 to 3 feet tall. stems rigid, branching, winged and covered with cottony hairs. Basal leaves deeply lobed while upper leaves entire and sharply pointed. Flower heads yellow, located singly on the ends of branches and armed with thorns up to 3/4 inch long. Some seeds have parachute hairs and some don't, resulting in a distribution that produces dense stands and rapid spreading. Germinates in spring and fall.
Yellow starthistle will grow wherever cheatgrass grows, in addition to growing in canyon grasslands, rangelands, pastures, edges of cropland, roadsides, and disturbed areas. It is an aggressive, adaptable weed that inhibits the growth of desirable plants in pasture, rangeland, and wasteland. This plant may become a problem in ground where the grass stand is weak. Yellow starthistle has been found in wheat crops where wastelands are heavily infested.
Known hazards
Yellow Starthistle is toxic to horses causing “chewing disease”.
Introduced from the Mideterranean regions. Yellow starthistle will grow wherever poorly competative environments exist, predominantly in dry slopes, grasslands, overgrazed rangelands, pastures, edges of cropland, roadsides, and disturbed areas.It has adapted to a wide range of habitats and environmental conditions, mostly in California, Oregon, Washington and Idaho.
Distribution in Oregon
The first documented site in Oregon was 1933 in Deschutes County.

Biological controls
There are 6 biocontrol agents approved for release, 5 of which are established in Oregon.
Bangasternus orientalis
Chaetorellia australis
Eustenopus villosus
Larinus curtus
Urophora solstitialis
Urophora quadrifasciata
Urophora siruneseva