This "B" rated weed native to Europe is troublesome but not widespread in Contra Costa County. Like the other noxious weeds, it threatens native plants and degrades range land.
Purple Star Thistle is typically an annual but can be biannual or a short-lived perennial. The flowers appear in June and the seeds mature from August to October. Prolific seed production allows this plant to establish and spread quickly. Controlling this weed is one of the top priorities of the Department of Agriculture.