Cocklebur
Xanthium strumarium

Family: Sunflower (Asteraceae)
AKA: Rough Cocklebur, Clotbur, Woolgarie Bur
Photo taken on: August 20, 2012
Location: Wet Mountains, Eastern CO
Life Zones: Plains
Habitat: Moist disturbed areas

Grows to 3ft with rough, triangular, long-stemmed leaves. Flowers are inconspicuous, greenish in color, growing from the base of the leaves and at the ends of the erect stems. Fruit is egg-shaped with tiny hooked barbs.

Flora of North America reference.