Idaho OnePlan recommends the website of the
Idaho Weed Awareness
Campaign as the best resource for up-to-date information about
Idaho's noxious weeds, and their control.
Source for this page: Idaho's Noxious Weeds by Robert H. Callihan & Timothy W. Miller
Yellow starthistle (Centaurea solstitialis) originated in the Mediterranean area and Asia. It spreads exclusively by seed, which may lie dormant for as long as 10 years. It causes "chewing disease" and death in horses. Yellow Starthistle will grow wherever downy brome (cheatgrass) grows.
Yellow starthistle is a winter annualPlant that germinates, flowers, seeds, and dies during one growing season, maturing from 2 to 72 inches tall. A rosetteA circular, normally basal, clump of leaves of deeply lobed leaves up to 8 inches long forms after seed germination in the fall. Stem leaves up to 4 inches long develop in early spring, their blades forming fringelike extensions down the side of the stem. Yellow flower headsA group of flowers borne tightly together develop at the tips of branched stems from late spring until fall. Flower head bractsLeaflike structure at the base of flowers or leaves bear stiff, sharp thorns ¾ inch long. Seeds are tan with white and brown mottling, 1/8 inch long; both plumedA hairlike or featherlike structure, often on a seed and unplumed seeds are borne in each flower head. Plumed seeds are not highly windborne; unplumed seeds not at all.
Yellow starthistle is widely scattered throughout the U.S., but is a severe problem only in the West.
Three seed head weevils and 2 seed head flies have been good to excellent biological control agents in Idaho.