Family: Asteraceae, Aster view all from this family

Description An aggressive weed with a spiny, thistle-like yellow flower on a rounded, bushy, dull green plant.
Habit: Introduced winter annual or biennial herb; winged stems; taprooted; forms dense colonies.
Height: 4-40 in (0.1-1 m)
Leaf: gray-wooly; in basal rosette, lyrate, pinnately lobed, to 20 cm long, to 5 cm wide; on stem, smaller, becoming linear, pointed, less lobed.
Flower: thistle-like, rayless, yellow or yellow-orange, about 1 in (25 mm) wide; perched atop vase-shaped involucre, about 0.5 in (12 mm) high, radiating treacherous, grooveless yellow spines, 0.5-1 in (11-22 mm) long.
Fruit: small dry seed, mottled brown, to 1/8 in (3 mm) long; with no bristles, or tipped with white bristles.

Warning This plant grows in dense stands that are painful to walk through because of the piercing spines. If continually eaten, it is poisonous to horses. Both horses and cattle have been known to develop infections of the lips and tongue from eating hay contaminated with the spines.

Flower June to October, or year-round in frost-free coastal areas.

Habitat Dry areas: Roadsides, fields, pastures, rangeland, woodlands, railways, disturbed sites; to 6500 ft (2000 m).

Range Native to the Mediterranean Basin; introduced in ballast to the West Coast in the mid-19th century; now naturalized throughout North America, in Canada from Alberta to Ontario, and throughout the continental U.S.; not reported in Alaska, Vermont, Maine, Arkansas, Louisiana, Mississippi, Alabama, or Georgia.

Discussion Also known as St. Barnaby's thistle, yellow cockspur. This plant is considered weedy and invasive in most locations; it is listed as a noxious weed in twelve states and two provinces. It is a serious problem especially in western rangeland, since the long sharp spines can injure livestock, and the plant is poisonous to horses when ingested over a prolonged period.