This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Native. Biennial. Propagates by seed.
Time of bloom: June to October.
Seed-time: July to November.
Range: United States and Canadian Provinces east of the Rocky Mountains.
Habitat: Prairies; meadows and pastures, waste places.
Fig. 322. - Clotbur (Xanthium canadense). X 1/4.
A plant once known only on western prairies, but now common in eastern fields, the wide and rapid distribution having been accomplished by impure commercial seeds and baled hay, of which the refuse is spread on the fields.
Stems one to three feet tall, simple or branching near the base, rather stout, bristly hairy. Leaves alternate, oblong, two to six inches in length, thick, hairy, pointed at both ends, entire, or slightly wavy-toothed, the lower ones spatulate, three-nerved and with grooved petioles, those on the stalks sessile and clasping. Heads two to four inches broad, solitary on long hairy peduncles, the rounded disk brownish purple, its florets perfect and fertile, the long sterile rays brilliant orange. Bracts of the involucre in two or three rows, spreading, rough and hairy. Achenes black or very dark brown, about an eighth of an inch long, narrow, four-angled and without a pappus. (Fig. 323.)
Being biennial, this weed is readily suppressed by pulling or close cutting before its seeds develop. But care must be taken that it is not continually reintroduced in poorly cleaned seed.