It forms dense infestations and may produce allelo chemicals that slow the growth of competing species, allowing starthistle to take over large areas of land. It produces bright, dandelionlike yellow flowers with sharp spines surrounding the base. Yellow starthistle grows to heights varying from 6 inches to 3 feet. The stems of mature plants are rigid, spreading and branching from the base. Stems and leaves are covered with a loose, cottony wool that gives them a whitish appearance.
Stems appear winged due to leaf bases that extend beyond the nodes.
Basal leaves are 2 to 3 inches long and deeply lobed. Upper leaves are short, 0.5 to 1 inch, narrow, and sharply pointed. Biology Yellow starthistle is an annual, usually found below 5,000 feet elevation, in dry, light-intensive areas where average annual rainfall is between 10 and 40 inches. It is able to regrow after top removal from mowing or grazing. Seed output can be as high as 29,000 seeds per square meter, with about 95 percent of the seed being viable.