Yellow star thistle, a member of the Aster or Sunflower family, Thistle tribe, and is an introduced annual, which reproduces by seeds. The stem is white-woolly and branches from the base and above to form a bushy plant, one to two feet tall.
The leaves are white-woolly, and are deeply lobed, two to three inches long. The upper leaves are narrow, pointed, and much smaller. The yellow flowers, located slightly on the ends of branches, have outwardly pointed, stiff, yellow spines up to one inch long. The plant may have two types of seeds: one is light colored with a tuft of soft bristles; the other is dark with no bristles.
Yellow star thistle may grow in many different soil types. It frequents roadsides and neglected areas. The plant, while not yet common in Colorado, has been discovered on the Montrose and Ouray County line(100 A), and on top of Grand Mesa (4 A). It is considered a serious rangeland threat in our neighboring states. It has infested 40 million plus acres in California. In comparison, that is 37 times the size of La Plata County.