Centaurea solstitialis L.
Yellow starthistle is a member of the Aster family and is closely related to the knapweeds. It is an introduced annual which reproduces by seeds. The hairy, branching stem appears bushy and grows from one-to-three feet tall. The leaves are hairy, deeply lobed, and two-to-three inches long. The upper leaves are narrow, pointed, and much smaller. The yellow flowers, located singly on the ends of branches, have outward-pointing, stiff, yellow spines that are up to one inch long. The plant may have two types of seeds: one is light colored with a tuft of soft bristles; the other is dark with no bristles.
Yellow starthistle may grow in many different soil types. It frequents roadsides and neglected or disturbed areas. This plant infests approximately 10 million acres in California and can cause “chewing disease” when horses are forced to eat it.
Currently, only one plant has been found in Teton County and it was likely introduced from uncertified horse hay. However, with the out-of-state traffic we get in this county, there is a good chance it will eventually show up again.
If you find this weed on your property or around Teton County, please report it immediately at 733-8419.