A winter annual with a deep taproot and blue-green foliage, this weed is an import from southern Spain. It can grow from a height of several inches to 4 feet tall and has rigid winged stems that are covered in short, cottony hairs. Basal leaves are deeply lobed while upper leaves are narrow and small. Flowers are bright yellow and are located at the ends of branches. Beneath the flower is a bract with sharp 3/4 inch long spines. This plant usually germinates in the fall and overwinters as a seedling.
It has the unique ability, however, to germinate at any time during the summer if the conditions are right. Preferred habitats include rangeland, pastures, waste areas and roadsides. Horses may develop “chewing disease” when forced to eat yellow starthistle. Currently there are no known infestations in Garfield County, but there have been reports in the past of single plants.