This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Introduced. Biennial. Propagates by seeds. Time of bloom: July to October. Seed-time: August to November. Range: Newfoundland and New England to western Ontario and Ohio. Habitat: Roadsides and waste places.
An escape from gardens, and usually a waste-land weed; but occasionally it gets into hay, causing cattle to reject their fodder - or, if eaten, the consequence is ruined milk and butter, for the extreme bitterness of the plant is proverbial. The herb is used medicinally as a vermifuge, and collectors receive about four cents a pound for its leaves and flowering tops, gathered when in early bloom and dried.
Stem somewhat shrubby, two to four feet high, the new leaves white with fine, silky hair, the older foliage grayish green, soft, and thin; the leaves are thrice pinnatifid, the segments finely divided and again cut and lobed; lower ones with slender petioles, but upper ones sessile or nearly so. Flowers in loose compound racemes, often more than a foot in length, the branchlets exceedingly slender and closely strung with nearly globular, nodding heads less than a sixth of an inch in diameter. These small, dried heads are called "seed," though of course they contain a number of very small achenes.
Prevent seed production by frequent close cutting during the growing season; if this is done, unless the ground is foul with dormant seed, the weed will disappear.