Perennials, 40-100+ cm. Stems (from short rhizomes) single, densely branched distally, puber-ulent throughout. Leaves usually opposite (distal sometimes alternate, lateral buds dormant or producing 1 pair of leaves); simple, sessile or subsessile; blades ± 3-nerved distal to bases, elliptic to lance-elliptic, (30-)50-70 × 5-25 mm (lengths mostly 2-5 times widths), bases narrowly cuneate, margins usually serrate, apices acute, faces puberulent or villous, densely gland-dotted. Heads in corymbiform arrays. Phyllaries 7-10 in 2-3 series, elliptic, 1-3 × 0.5-1 mm, apices rounded to acute (not mucronate), abaxial faces puberulent, gland-dotted. Florets 5; corollas 2.5-3 mm. Cypselae 1.5-2 mm; pappi of 30-40 bristles 2.5-3 mm. 2n = 20. Flowering Aug-Sep. Moist to boggy, sandy, peaty soils, margins of pine flatwoods, gum swamps, bayheads, disturbed sites, roadsides; 10-100+ m; Ala., Ark., Fla., Ga., La., Miss., N.C., S.C., Tenn. Eupatorium semiserratum has been included within E. glaucescens (E. cuneifolium); it is distinguished by its consistently smaller heads and stems that branch only within the capitulescences. It differs from the similar and sometimes sympatric E. lancifolium by its smaller heads, leaves 3-nerved distal to bases (rather than at bases), as well as preference for wetter habitats. It has been proposed that E. rotundifolium var. scabridum (E. pubescens) represents hybrids between E. semiserratum and E. rotundifolium; it also apparently hybridizes with E. hyssopifolium.
Stems 5-12 dm, mostly solitary from a very short, stout rhizome or crown, densely villous-puberulent, sometimes also atomiferous-glandular, often with loose axillary fascicles of a few reduced lvs; lvs opposite or the uppermost scattered, firm, twisted at the base to bring the blade into a vertical plane, elliptic or elliptic-oblanceolate, mostly 4-8 cm נ8-30 mm, 2.5-6 times as long as wide, gradually narrowed to the sessile or shortly petiolar base, usually serrate or crenate- serrate, especially above the middle, finely and densely puberulent (and often also atomiferous-glandular), sometimes more shortly so above than beneath, triplinerved, the principal pair of lateral veins arising as branches from the midrib; invol 2.5-4 mm, softly short-hairy like the peduncles, its bracts imbricate, broadly rounded or obtuse to sometimes submucronately acute, the inner obscurely scarious-margined; fls 5; cor white, 2.5-3.5 mm; 2n=20. Low woods, clearings, and swampy places; Va. to s. Fla., w. to Tex., and n. into Ark., se. Mo., and s. Tenn. (E. cuneifolium var. s.)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.