This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Native. Annual. Propagates by seeds. Time of bloom: July to September. Seed-time: September to November. Range: Nova Scotia to the Northwest Territory, southward to Texas and Mexico. Abundant on the Pacific Coast. Habitat: Rich, moist soil; cultivated crops, barnyards, roadsides, and waste places.
A huge, coarse plant, one to four feet tall, branched, and widely spreading, its rough, thick, angled stem often reddish and spotted with brown. Leaves also bristly rough on both sides, alternate, large, broadly oval to heart-shaped, with toothed edges, strongly three-nerved and often three-lobed, with long, rigid petioles, often reddish like the stem and contrasting with the dark green of the leaf surface. The sterile heads are clustered at the ends of the branches, small, greenish, and inconspicuous, resembling those of Ragweed; below, in the axils, the fertile heads are densely clustered; these are thick, green, oblong, densely hairy, and spiny involucres, from which is thrust a style with two-parted stigma; these involucres develop into burs nearly an inch long, with a pair of strong, hooked beaks at the tip and a covering of hooked spines which enable them to cling to a garment or to the coats of animals for a ride to new homes. Each bur contains two "seeds," or achenes, oblong, flat, slightly ridged, with a tough, black coat, or skin. It is believed that one of the pair germinates the first season and the other the next, thus assuring a two-years' crop for one sowing. But the entire bur is also known to lie dormant in the soil for several years. Several other species of Clotbur are common and all are about as obnoxious as this one but none ranges so widely as X. canadense. (Fig. 322.)
Hoe-cutting while the plants are small; or, if not too numerous, hand-pulling before the burs are formed. Put infested corn land to a grain crop, followed by clover or grass, the harvesting of any of which beheads the weed before it has attained to much size or developed the burs. In its tender youth (three to eight inches in height) Clotbur can be killed by a spray of Iron sulfate or Copper sulfate. Plants on waste land or roadside which have been allowed to mature their burs should be cut and burned.