Description:
Yellow starthistle is a member of the sunflower or daisy family, Asteraceae (Compositae). As its name indicates, it is characterized by a yellow thistle-like flower with 3/8- 3/4 inch yellowish spines in star-like arrangement at the base of the flower head. The flower heads occur singly a the ends of branched stems 18-36 inches high which arise from a basal rosette of leaves. The stem and leaves are a dull green color and covered with fine woolly hairs. The basal rosette leaves are pinnately lobed with the distal portion more rounded and larger. As leaves ascend the stem they become shorter and narrower with pointed tips. Their arrangement is alternate and the leaf bases extended down the stem producing a winged effect. The yellow starthistle flower heads consist of tubular yellow florets in a discoid head about one inch in diameter. The marginal flowers produce nonplumed seeds and the central flowers plumed seeds. The root is a taproot extending deeper than the annuals it may compete with for moisture and nutrients.

• Mechanical: Mechanical removal is economically unsound for dense infestations. However, initial infestations, such as occurred from contaminated seed mix in Okanogan County, have been lessened by hand pulling, flagging, and a herbicide pellet application. Proper grazing management, including rest and deferment to allow grasses to regain vigor, will both limit yellow starthistle invasions and improve the range's condition. In California, frequent cultivation slows or inhibits the plant, preventing it from fully exploiting cultivated grassland steppes.
• Biological: Washington State initiated a yellow starthistle biological control program in 1985 with the release of a beetle Bangosternus orientalis. The beetles feed on small buds and lay eggs in medium sized buds. Larvae hatch and feed on developing seed destroying all of them within the head. Pupation occurs in the damaged heads and the emerged adults overwinter in the soil. This beetle has reduced yellow starthistle seed production by about 60 percent. Two other beetles which destroy yellow starthistle seed in affected heads are Eustenopsis villosus - released and successfully colonized at a site in Whitman County in 1990 - and Larinus curtus.
• Herbicide: Yellow starthistle in the rosette stage is not difficult to kill with herbicides such as 2-4-D. Plants in the flowering and seed production phase are resistant. Seeds are also resistant. Picloram not only kills the rosette stage but, due to residual action, is effective on seedlings developing in the next season. However, skipped or missed areas of herbicide application may occur. Seeds may survive beyond residual action of picloram. Resistance to picloram has been reported. Refer to Pacific Northwest Weed Control Handbook for detailed management. Refer to the State Noxious Weed Control Board site

For More Information:
Detailed information about Yellow Starthistle is available at the Washington State Noxious Weed Control Board Web Site.