Perennials, 30-100+ cm. Stems (from tuberous rhizomes) multiple, densely branched distally, puberulent throughout. Leaves usually opposite (distal sometimes alternate); sessile or subsessile; blades 3-nerved distal to bases, oblanceolate, 20-80 × 5-10(-20) mm, bases cuneate, margins serrate proximally, entire distally, apices acute, faces puberulent (abaxial), glabrous or glabrate (adaxial), gland-dotted. Heads in corymbiform arrays. Phyllaries 7-10 in 1-2 series, oblanceolate, 1-3 × 0.2-0.5 mm, apices rounded, abaxial faces puberulent, gland-dotted. Florets 5; corollas 2-4 mm. Cypselae 1-2 mm; pappi of 20-30 bristles 2.5-3 mm. 2n = 20, 30, 40. Flowering Jul-Sep. Moist, low ground, margins of ponds, sandy soils; 10-100+ m; Ala., Fla., Ga., La., Miss., N.C., S.C., Tex., Va. As treated here, Eupatorium mohrii includes both sexual diploid (sometimes segregated as E. recurvans) and apomictic polyploid populations that are suggested by molecular data to be autoploids. Eupatorium saltuense refers to hybrids shown by molecular data to involve E. mohrii and E. serotinum. Molecular data also document frequent hybridization with E. rotundifolium.
Plants with short, conspicuously tuberous- thickened rhizomes, the erect branches therefrom often branching near ground-level into two or more aerial stems, these 3-10(-12) dm, puberulent or strigose-puberulent; lvs opposite or the upper alternate, rarely ternate, mostly lance-elliptic, tapering to the sessile or subsessile base, bluntly few-toothed, tending to be recurved or deflexed, small, mostly 1.5-6 cm נ3-10(-12) mm, glandular-punctate, strigose or subglabrous; invol 3-5 mm, its bracts imbricate, broadly obtuse to acutish, atomiferous-glandular and slightly to evidently villous-puberulent, the inner inconspicuously scarious-margined and often somewhat pale distally; fls 5(6), white; 2n=20, 30, 40. Pond-margins, ditches, shores, and moist low ground, often in sandy or peaty soil; coastal plain from se. Va. to Fla. and La. July-Sept. (E. recurvans, the sexual diploid phase)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.