cocklebur or clotbur, any species of the genus Xanthium, widely distributed, coarse annual plants of the family Asteraceae (aster family). They are often persistent weeds; the two-seeded oval burrs are particularly troublesome to sheep growers and the very young plants are poisonous to livestock. Cockleburs are often confused with burdock. Cockleburs are classified in the division Magnoliophyta, class Magnoliopsida, order Asterales, family Asteraceae.