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Yellow Starthistle  (Centaurea solstitialis)
Sunflower Family (Asteraceae)

Photo of starthistleUniversity of Idaho

 

Description:Yellow star thistle is an aggressive annual that grows from a taproot and stands 1 to 3 feet (30 to 91 cm) tall, with erect and rigid branching. Stems are rigid, winged, and covered with a cottony pubescence. Basal leaves are 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, 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 and spreads rapidly. Small populations must be eradicated!

Distribution: Much of California is heavily infested with yellow starthistle. It is becoming more widespread in northern California.
Flowering Period: May to October.