Yellow Starthistle (Centaurea solstitialis)
Sunflower Family (Asteraceae)
University of Idaho
Description: Annual 1 to 3 feet (3 to 9 dm), 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 (2 cm) thorns. Outer seed dark brown without bristles; inner seed mottled light brown with a tuft of white bristles, about 1/8 inch (.32 cm) long.
Habitat: Native to southern Europe. Invades various soil types on waste areas, roadsides, pastures and dry rangelands at elevations below 4,265 feet (1300 meters). 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 and spreads rapidly. Small populations must be eradicated!
Distribution: California is heavily infested with yellow starthistle statewide.
Flowering Period: June through September.