Yellow starthistle is a member of the Aster family. It is an annual, 2 to 3 feet tall, that was introduced from Europe. Flowers are yellow, located singly on ends of branches, and armed with sharp straw-colored thorns up to ¾ inch long. "Chewing disease" results when horses are forced to eat yellow starthistle. Currently, there are no confirmed reports of starthistle in the Garfield County, however single plants have been reported in the past. There are known infestations in neighboring counties.