Yellow Starthistle

Yellow starthistle is an annual, occasionally biennial, forb in the Asteraceae (daisy) family. It grows from a deep taproot (up to 6 feet deep) to form a basal rosette followed by growth of a main stem that may reach up to 6 feet tall. This stem is usually branched near or above the base giving the plant a bushy appearance. Leaves are pinnately lobed and are arranged alternately on stems. Leaves decrease in size up the stem. The entire plant is covered with woolly hairs giving plants a grayish appearance. Yellow starthistle flowers from early summer to fall. Flowers are small, terminal, and solitary. Florets are yellow and are combined in a dense head enclosed in bracts. Bracts are spiny with spines reaching up to ¾ inch long and give the flower a star-like appearance. Yellow starthistle produces two types of seeds, both being 2 to 3 mm long. One type, produced from ray flowers, is dark brown and does not have bristles; the other type, produced from disk flowers, is white to light brown with bristly pappus on one end.

Yellow starthistle produces a currently unidentified compound that causes chewing disease (nigropallidal encephalomalacia) in horses.

Yellow starthistle is native to Europe, Asia, and Northern Africa and was introduced to North America in the 1850s as a contaminant in alfalfa seed. It spreads solely by seed. Pappus on seeds catch on clothing, fur, and equipment and are transported to new areas. It is primarily found in disturbed sites, rangeland, sagebrush steppe and other dry areas.

Prevention and cultural control strategies should be utilized as much as possible. Use certified weed free forage, and clean recreational gear and construction equipment before entering a new site.

While there are several biological control agents approved for use on yellow starthistle, the population density of yellow starthistle in Teton County is not dense enough to sustain populations of biological control agents.

Because yellow starthistle has a taproot, it can be readily managed by hand pulling or digging out the root. Wear gloves and long sleeves when handling species in the Centaurea genus as many cause allergic reactions. Mowing will not eradicate stands of yellow starthistle, but it can be beneficial in preventing flower production. Do not mow while seeds are present. Grazing by sheep, goats, or cattle will not eradicate stands, but it can prevent seed production and dispersal. Do not graze horses in areas with yellow starthistle!

Larger infestations can be controlled with herbicides. Spring and fall treatments of rosettes are recommended, but treatment of bolting plants can be effective. Read the label before using any herbicide. Contact TCWP if you have any questions about application rates or how to use an herbicide.

• Weed Report: Yellow Starthistle. Weed Control in Natural Areas in the Western United States. (2013) DiTomaso, Kyser et al. Weed Research and Information Center. University of California.
• Yellow Starthistle. Washington State Noxious Weed Control Board
• Yellow Starthistle. Minnesota Department of Agriculture.
• Yellow Starthistle: Identification, Biology and Integrated Management. MT201101AG (2017). Parkinson, H. and Mangold, J. Montana State University Extension.