22. Eupatorium semiserratum de Candolle in A. P. de Candolle and A. L. P. P. de Candolle, Prodr. 5: 177. 1836.
Eupatorium cuneifolium Willdenow var. semiserratum (de Candolle) Fernald & Griscom
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.