1. Eupatorium altissimum L. (tall thoroughwort)
Pl. 266 a–d; Map
1109
Stems 50–200 cm
long, not hollow, moderately to densely short-hairy above the nearly glabrous
basal portion, green to yellowish green, sometimes purplish-tinged or
brownish-mottled, often somewhat glaucous, some nodes often with small
fascicles of axillary leaves less than 1/2 as long as the main stem leaves.
Leaves mostly opposite, those of the uppermost nodes sometimes alternate,
sessile or with poorly differentiated petioles to 15 mm long. Leaf blades 2–15
cm long, 3–25 mm wide, narrowly lanceolate to lanceolate or less commonly
narrowly elliptic, tapered at the base, tapered to a sharply pointed tip, the
margins entire or more commonly sharply toothed above the midpoint, the
surfaces moderately to densely short-hairy, also densely gland-dotted, with 3 main
veins from the base. Inflorescences terminal panicles, flat-topped or broadly
dome-shaped. Involucre 4.5–7.0 mm long, more or less cup-shaped, the bracts
ovate to narrowly oblong, rounded to bluntly pointed at the tip, the margins
thin and pale, mostly faintly 3-nerved, densely short-hairy, green. Disc
florets 5. Corollas 4–5 mm long, the surface often somewhat glandular, white.
Fruits 1.5–2.5 mm long. 2n=20, 30, 40. August–October.
Scattered nearly
throughout the state (eastern U.S. west to Minnesota, Nebraska, and Texas;
Canada). Upland prairies, loess hill prairies, glades, savannas, openings of
mesic to dry upland forests, banks of streams and rivers, and ledges and tops
of bluffs; also old fields, fallow fields, pastures, ditches, quarries, railroads,
roadsides, and open, disturbed areas.
For a discussion
of presumed hybrids with E. hyssopifolium, see the treatment of that
species.