Long stands of this dusty white thoroughwort line our highways. The narrow grey-green three-ribbed leaves (sometimes there are five ribs) are distinctive, and the mounds of white flowers are very attractive to honeybees. “All specimens in the Herbarium were collected recently,” says the 1951 Check List of the Vascular Flora of Allegheny County. “This is a Mississippi Valley species, entering the State from the southwest, particularly along the river bluffs.” Since then, the plant has become very much at home here, but it is still found mostly along the highways and railroads by which it entered.
This is a very common species here, but oddly underrepresented in photographs. As usual, Father Pitt donated these pictures to Wikimedia Commons, and these half-dozen photographs exactly doubled the collection for this species.
Gray describes the genus and the species:
EUPATORIUM [Tourn.] L. THOROUGHWORT. Heads discoid, 3-many-flowered; flowers perfect. Involucre cylindrical or bell-shaped, of more than 4 bracts. Receptacle flat or conical, naked. Corolla 5-toothed. Achenes 5-angled; pappus a single row of slender capillary barely roughish bristles. Erect perennial herbs, often sprinkled with bitter resinous dots, with generally corymbose heads of white, bluish, or purple blossoms, appearing near the close of summer. (Dedicated to Eupator Mithridates, who is said to have used a species of the genus in medicine.)
Heads 3-20-flowered; involucre of 8-15 more or less imbricated and unequal bracts, the outer ones shorter; flowers white or nearly so.
Leaves sessile or nearly so, with a narrow base, mostly opposite; heads mostly 5-flowered.
Bracts not scarious or only obscurely so, obtuse, at length shorter than the flowers.
E. altissimum L. Stem stout and tall, 1-2 m. high, downy leaves lanceolate, tapering at both ends, conspicuously 3-nerved, entire, or toothed above the middle, 0.5-1.3 dm. long, the uppermost alternate; corymbs dense; bracts of the involucre obtuse, shorter than the flowers. Dry soil, Pa. to Minn., Neb., and southw.