Time of bloom: July to September.
Seed-time: September to November.
Range: Ontario to Florida, westward to Illinois, Missouri, and Texas. Also abundant on the Pacific Coast.
Habitat: Warm, moist soil; invades almost any crop.
A very pernicious weed which came to us from tropical America. It is sometimes cultivated for the odd appearance of its white-veined, white-lined, dark green leaves, yellow spines, and green burs. But these last, with their hooked spines, are so easily transported on clothing and by animals that the plant should be considered an undesirable resident of any neighborhood, particularly as the fruits retain their vitality for years, biding the time when some stirring of the soil shall furnish them the needed warmth and moisture for germination. It is a worse weed than the other Cockleburs, for it spreads as freely in sod lands as elsewhere. (Fig. 321.)
Stem one to three feet tall, many-branched and hoary with whitish hairs. Leaves alternate, two to five inches long, lance-shaped, long-pointed, and narrowing to short petioles, the lower ones lobed and the upper ones entire, white-woolly underneath and on midribs and veins above. Just below each leaf is a slender, yellow, three-pronged spine about an inch long. Flowers of two kinds, the staminate ones in short terminal spikes, the heads very small and greenish, like the Ragweed. Fertile flowers in the axils below, consisting of a x 1/4.
Fig. 321. -Spiny Clot-bur (Xanthium spinosum).
pistil with its cleft style slightly exserted from a hairy and spiny involucre which later becomes a bur about a half-inch long, with two straight beaks at apex and a covering of short, smooth, hooked spines. The burs are two-celled, each cavity containing a thick-coated, dark brown, flattened seed.
In pastures and meadows the plants should be watched for and cut off in May or June with a sharp hoe or spud; some will be overlooked, to appear conspicuously later, bristling with spines and burs; these should be cut, piled to dry for a few days, and burned. In cultivated ground tillage should be continued late in order to prevent the development of seed from late-blooming flowers.