9. Eupatorium semiserratum DC.

E. cuneifolium Willd. var. semiserratum (DC.) Fernald & Griscom

Pl. 266 c, d; Map 1117

Stems 50–120 cm long, not hollow, moderately to densely short-hairy above the sometimes nearly glabrous basal portion, usually purplish-tinged or purplish brown, sometimes 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 8 mm long, twisted at the base so that the leaves appear nearly vertically oriented. Leaf blades 1–8 cm long, 2–15(–25) mm wide, narrowly oblanceolate to oblanceolate or less commonly narrowly elliptic, tapered at the base, rounded or angled to a bluntly pointed tip, the margins sharply toothed mostly above the midpoint, the surfaces moderately to densely short-hairy, also densely gland-dotted, with 3 main veins, the 2 lateral veins branching from the midvein 2–12 mm above the blade base. Inflorescences terminal panicles, more or less flat-topped. Involucre 2.5–4.0 mm long (sometimes appearing longer at fruiting), 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 2.5–3.5 mm long, the surface often somewhat glandular, white. Fruits 1.5–2.0 mm long. 2n=20. August–October.

Uncommon in the Mississippi Lowlands Division and adjacent portions of the Ozarks; also historically disjunct in the St. Louis region (southeastern U.S. west to Missouri and Texas). Edges of mesic upland forests, sand savannas, bottomland forests, and swamps; also old fields, roadsides, and open, disturbed areas.

For a discussion of possible hybrids with E. hyssopifolium, see the treatment of that species. Steyermark (1963) considered E. semiserratum (as E. cuneifolium var. semiserratum) to inhabit only moist, bottomland habitats, but most of the collections since 1990 originated from drier, upland sites.