Perennials, 30-100+ cm. Stems (from short rhizomes) single, sparsely branched distally, densely puberulent to pilose throughout. Leaves usually opposite (distal sometimes alternate); simple, sessile or subsessile; blades ± 3-nerved (distal to bases), elliptic, lanceolate, or lance-ovate, 30-90 × 20-45 mm (lengths mostly 2-2.5 times widths), bases rounded to rounded-cuneate, margins unevenly serrate, apices acute to attenuate, faces puberulent, gland-dotted. Heads in corymbiform arrays. Phyllaries 7-10 in 2-3 series, oblong to lance-oblong, 2-7 × 1-1.5 mm, apices (sometimes whitish) acuminate to acute, mucronate, abaxial faces puberulent, gland-dotted. Florets 5; corollas 3.5-4 mm. Cypselae 3-4 mm; pappi of 30-50 bristles 4-5 mm. 2n = 20, 30, 40.
Eupatorium pilosum has been most commonly treated as a variety of E. rotundifolium; it is distinct morphologically by its leaves, which are narrower and 3-nerved distal to bases, rather than 3-nerved from bases. Distal leaves of E. pilosum tend to differ from the proximal by being alternate with entire margins.
Much like no. 19 [Eupatorium rotundifolium L.], but the lvs narrower, mostly lanceolate or lance-ovate to elliptic-ovate, mostly coarsely and unevenly toothed; upper lvs and main branches of the infl tending to be alternate; diploids and polyploids. Chiefly in wet soil; Mass. to Va. and Fla., w. to e. Ky., s. Tenn., and Miss. (E. verbenaefolium; E. rotundifolium var. saundersii)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.