Xanthium strumarium

Common cocklebur produces a spike-like raceme of compound flowerheads, shorter than the leaf petioles, that arises from the axils of each upper leaf and from the tip of the central stem. Each raceme produces exclusively male flowerheads in its upper parts and exclusively female flowerheads in the lower regions. The male flowerheads are about 1/4-3/4″ across, elevated on a short pedicel on top of 1-3 series of white floral bracts. Each of the approximately 20 tiny, brownish-green staminate florets (nominally disk florets) consist of a tubular 1-2 mm long corolla; a minute, non-functional ovary; and five stamens with fused filaments (like a tree trunk) but with their white anthers free, positioned in a ring like a little palm tree or, possibly, a hand. The staminate florets quickly shed their pollen and drop off. The female flowerheads (also nominally disk florets) are 3/4-1.5″ long and two-thirds as wide, stalkless, each (usually) containing two female florets nearly enclosed by floral bracts covered with curly white hairs that have a bur-like appearance from the dense, hooked prickles covering their surface. At the tip of the flowerhead are two spines, longer and much stouter than the prickles, each with a hole from which protrudes the divided style of a female floret. There is no corolla in the female florets. The florets are wind pollinated, and their styles quickly wither away after fertilization. Each female floret produces a single, oblong, dark gray to black seed 3/8-1/2″ long, tapering to a point at each end and covered by dark membranes, all buried in the bur-like bract. The female flowerhead stiffens through the autumn, turns brown, and overwinters. The seeds are highly poisonous, containing toxic diterpene glycosides. One of the contained seeds germinates the following year; the other delays germination for two years. Common cocklebur prefers wet, sandy soils.

Common cocklebur is an unlovely native annual that gets 2-4 feet tall; it is found in every state in the continental U.S. and all the Canadian provinces except the most northern ones. The main stem is little branched except for side stems that arise from the leaf axils. All the stems are green, usually spotted with purple streaks, round in section or slightly ribbed with short white hairs scattered across the surface. The leaves are alternate, up to 8″ long and 6″ across. They are heart-shaped with an indented base or oval with a rounded base; in either case the tip of the leaf is broad and blunt. The leaf margins are shallowly lobed or coarsely toothed; the upper surface of the leaves feels sandpapery. The petioles are reddish or reddish-green, covered with short white hairs, and about the same length as the leaf blades.

Common cocklebur is an unlovely native annual that gets 2-4 feet tall; it is found in every state in the continental U.S. and all the Canadian provinces except the most northern ones. The main stem is little branched except for side stems that arise from the leaf axils. All the stems are green, usually spotted with purple streaks, round in section or slightly ribbed with short white hairs scattered across the surface. The leaves are alternate, up to 8″ long and 6″ across. They are heart-shaped with an indented base or oval with a rounded base; in either case the tip of the leaf is broad and blunt. The leaf margins are shallowly lobed or coarsely toothed; the upper surface of the leaves feels sandpapery. The petioles are reddish or reddish-green, covered with short white hairs, and about the same length as the leaf blades. A spike-like raceme of compound flowerheads, shorter than the leaf petioles, arises from the axils of each upper leaf and from the tip of the central stem. Each raceme produces exclusively male flowerheads in its upper parts and exclusively female flowerheads in the lower regions. The male flowerheads are about 1/4-3/4″ across, elevated on a short pedicel on top of 1-3 series of white floral bracts. Each of the approximately 20 tiny, brownish-green staminate florets (nominally disk florets) consist of a tubular 1-2 mm long corolla; a minute, non-functional ovary; and five stamens with fused filaments (like a tree trunk) but with their white anthers free, positioned in a ring like a little palm tree or, possibly, a hand. The staminate florets quickly shed their pollen and drop off. The female flowerheads (also nominally disk florets) are 3/4-1.5″ long and two-thirds as wide, stalkless, each (usually) containing two female florets nearly enclosed by floral bracts covered with curly white hairs that have a bur-like appearance from the dense, hooked prickles covering their surface. At the tip of the flowerhead are two spines, longer and much stouter than the prickles, each with a hole from which protrudes the divided style of a female floret. There is no corolla in the female florets. The florets are wind pollinated, and their styles quickly wither away after fertilization. Each female floret produces a single, oblong, dark gray to black seed 3/8-1/2″ long, tapering to a point at each end and covered by dark membranes, all buried in the bur-like bract. The female flowerhead stiffens through the autumn, turns brown, and overwinters. The seeds are highly poisonous, containing toxic diterpene glycosides. One of the contained seeds germinates the following year; the other delays germination for two years. Common cocklebur prefers wet, sandy soils.