Eupatorium rotundifolium Linnaeus var. cordigerum Fernald, Rhodora 45: 477. 1943

Perennials, 40–100+ cm. Stems (from short rhizomes) single, branched distally, densely pilose. Leaves usually opposite (distal sometimes alternate); sessile; blades ± 3-nerved or pinnately nerved, ovate, 3–9.8 × 1.6–5.7 cm, bases clasping or narrowly perfoliate, margins serrate, apices acute, faces (rugose) sparsely pilose (adaxial) or densely pilose (abaxial) and densely gland-dotted. Heads in corymbiform arrays. Phyllaries 7–10 in 2–3 series, 4–12 × 1–4 mm, apices (usually white) acuminate to acute, abaxial faces pilose, gland-dotted. Florets 5–8; corollas 2.5–3 mm. Cypselae 2–3 mm; pappi of 30–40 bristles 3.5–4.5 mm. 2n = 30, 40.

Flowering Jul–Aug. Margins of woods, moist areas; 10–100+ m; Ark., Miss., N.J., N.C., S.C., Va.

Eupatorium ×cordigerum is an apomictic, polyploid hybrid of E. rotundifolium and E. perfoliatum that occurs in disturbed areas where those two species occur together. It is not uncommonly encountered; it is unclear how long it persists.