Time of bloom: May to October.
Seed-time: June to November.
Range: Pacific Coast from California to Washington.
Habitat: Fields, roadsides, and waste places.
A most unpleasant weed, covered with a viscid, ill-scented excretion which injures everything that it touches, from the crops among which it is harvested to the clothing of passers-by. None of the native Tarweeds are so offensive as this, which is an immigrant from Chile. A remarkably sweet and limpid oil is expressed from the seeds, good for table use and particularly valuable for a lubricant, as it does not readily congeal; in order to obtain this oil the plant is extensively cultivated in South America and in Europe. Stem stout, one to four feet tall, finely hairy, beset with viscid, pedicellate glands. Leaves alternate, entire, varying from broad lance-shape below to linear above, all sticky and strong-scented. Heads numerous, sessile or on short peduncles at the ends of the short branches and in the upper axils; they are about three-fourths of an inch broad, with eight to twelve pale yellow rays and darker disk.
Fig. 332. -Western Tickseed Sunflower (Bidens aristosa). X 1/4.
Both ray and disk florets are fertile; bracts of the involucre in a single series, strongly keeled, and hairy. Disk achenes are oblong, wedge-shaped, and four-angled, those of the rays longer and curved. As soon as ripe they fall readily from the receptacle, and nearly matured plants should never be left on the ground when cut as the seeds ripen on the stalks.
Prevent seed production by close and repeated cutting throughout the growing season.