Xanthium strumarium

Native

Moist to moderate moisture. Croplands, abandoned fields, beaches, sand dunes, disturbed sites. Full or partial sun.

August to September

Green

8″ to 32″

This is an annual forb that rises from a stout, somewhat woody taproot. It can be from 8″ to 32″ tall at maturity, but in Minnesota is usually no more than 18″ in height.

The stems are erect or ascending and unbranched or sparsely branched. They are almost hairless or are rough to the touch and sparsely to moderately covered with short, ascending, appressed hairs. They are often covered with resinous dots (glands). They do not have spines at the nodes.

The lower leaves are alternate. The upper 2 to 6 leaves are usually alternate, sometimes opposite. They are on ¾″ to 5″ long leaf stalks, the stalks about as long as the leaf blade. The blades are broadly egg-shaped to kidney-shaped or nearly circular, ¾″ to 7″ long, and ¾″ to 6″ wide. They are usually heart-shaped at the base and are sometimes 3 or 5. The upper and lower surfaces are rough to the touch due to short, stiff hairs. The underside is not whitened. The margins are coarsely and irregularly toothed.

Male and female flowers are borne on the same plant in spikes rising from the leaf axils and at the end of the stem. The lower spikes often have all female (pistillate) flower heads, the upper ones often all male (staminate) flower heads. Most spikes have a cluster of several staminate heads at the top and sometimes 1, usually 2 to 4, pistillate heads at the base. The spikes are 1″ to 4″ long, shorter than the stalk of the subtending leaf.

The male flower heads are about ¼″ in diameter. They have 20 to 60 disk florets and no ray florets.

The female flower head is oblong ellipse-shaped, with all but the outermost bracts of the involucre fused into a bur. The burs are yellow-green, yellowish-brown, or brown, ⅝″ to 13 ⁄16″ long, and completely enclose the 2 female florets in separate chambers. They have 2 narrow, prolonged tips (beaks) that are hooked or curled at the top. They are covered with numerous, sharp spines that are hooked or curled at the tip. At the base of the spines there may be fine, short hairs and/or stalked or unstalked glands.

The fruit is two achenes, one in each chamber of the bur. One seed germinates the first year, the other germinates the following year.