Description: Annual 1 to 3 feet, erect and rigid branching, from taproot. Stems rigid, winged, and covered with a cottony pubescence. Basal leaves deeply lobed (as are leaves of seedlings); upper leaves entire and sharply pointed. Flowers yellow, terminal, armed with straw-colored 3/4 inch thorns. Outer seed dark brown without bristles; inner seed mottled light brown with a tuft of white bristles, about 1/8 inch long.
Habitat: Native to southern Europe. Invades various soil types on waste areas, roadsides, patures and dry rangelands. Toxic to horses as it causes "chewing disease." Once a plant invades a site it may sit without increasing for several years. It becomes genetically adapted to that site and then the population explodes; it spreads rapidly. Small populations must be eradicated!
Distribution: Much of California is heavily infested with yellow starthistle. It is becoming more widespread in northeastern California. Large infestations occur in Shasta County and northern Siskiyou County. Although it is on the move, it has yet to infest extensive areas of Lassen, Modoc, Plumas, and Sierra counties.
Flowering Period: May to October.