11. Eupatorium sessilifolium L. (upland boneset)

Eupatorium sessilifolium var. brittonianum Porter

Pl. 267 h, i; Map 1119

Stems 30–120 cm long, not hollow, glabrous below the inflorescence, tan or less commonly purplish brown, rarely somewhat glaucous, not producing small fascicles of axillary leaves much shorter than the main stem leaves. Leaves mostly opposite, those of the uppermost nodes rarely alternate, sessile or less commonly with petioles to 2 mm long. Leaf blades 3–18 cm long, 8–60 mm wide, lanceolate to narrowly ovate, rounded, truncate or shallowly cordate at the base (the bases of the pair at all but the uppermost nodes often somewhat overlapping), tapered to a sharply pointed tip, the margins sharply toothed, the surfaces glabrous, also sparsely to moderately gland-dotted, with 1 main vein. Inflorescences terminal panicles, more or less flat-topped. Involucre 4.5–6.5 mm long, cup-shaped, the bracts ovate to lanceolate or narrowly oblong, rounded to bluntly or less commonly sharply pointed at the tip, the margins thin and pale or transparent, mostly faintly 3-nerved, densely short-hairy, green. Disc florets 5(–7). Corollas 3–4 mm long, the surface often somewhat glandular, white. Fruits 2–3 mm long. 2n=20, 30. July–September.

Scattered in the Ozark and Ozark Border Divisions and in the eastern half of the Glaciated Plains (northeastern U.S. south to North Carolina and Missouri). Bottomland forests, mesic to dry upland forests, bases and ledges of bluffs, and banks of streams and rivers.

Missouri plants represent a phase with relatively long, slender leaves that has been called var. brittonianum. Elsewhere to the east, var. sessilifolium tends to have shorter, broader leaves and is sometimes similar in leaf shape to long-leaved variants of E. rotundifolium. However, as noted by Steyermark (1963), there is a lot of intergradation between the leaf variants, which argues against formal taxonomic recognition of varieties.