Tall Thoroughwort (Eupatorium altissimum)
Our broad modern highways seem to have been the making of this plant around here. It likes the median strips of interstate highways better than any other environment, and its grey-green leaves topped with dusty white flowers make it a decorative companion to the goldenrods that often grow in the same places. The plant above was growing along the side of a highway near Rostraver; the one below in a vacant lot in West Mifflin; both were blooming in late September.
Flower heads. Rayless, white, borne in layers of flat-topped clusters.
Leaves. Dark greyish-green; lanceolate; the upper ones entire, the lower toothed past the midpoint; with three prominent parallel veins. Often there are two smaller leaves where the petiole meets the stem.
Stems. Straight and study; greyish-green, paler than the leaves, often with a brown cast toward the base; much branched.
This plant apparently hybridizes with E. serotinum, and is easily confused with it, probably even on this site.
Gray describes the genus and the species:
EUPATÒRIUM [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 6-toothed. Achenes 6-angled; pappus a single row of slender capillary barely roughish bristles. —Erect perennial herbs, often sprinkled with hitter 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.)
EUPATORIUM proper. Receptacle flat.
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, xcith 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.