Perennials, 40-100+ cm. Stems (from short caudices or stout rhizomes) single, sparsely branched distally, pubescent throughout. Leaves usually opposite (distal sometimes alternate); sessile; blades ± 3-nerved or pinnately nerved, elliptic to oblanceolate, 30-120 × 10-40 mm (lengths mostly 3-4 times widths), bases narrowly cuneate (sometimes oblique), margins subentire, serrate, or serrulate, apices rounded to acute, faces setulose (denser on midribs and veinlets) to glabrate, usually gland-dotted. Heads in corymbiform arrays. Phyllaries 9-15 in 2-4 series, linear, 1.5-9 × 0.6-1 mm, apices acuminate to attenuate, sometimes mucronate, abaxial faces pubescent throughout, gland-dotted. Florets (4-)5; corollas 4-4.5 mm. Cypselae 2.5-3.5 mm; pappi of 40-50 bristles 3.5-4.5 mm.
Eupatorium album is represented by widespread diploid populations and by polyploid, apomictic populations, segregated as varieties, that may have arisen by hybridization with other species. Molecular data suggest that var. vaseyi arose via hybridization between var. album and E. sessilifolium. They also suggest that E. album var. subvenosum arose from hybridization between E. album var. album and E. serotinum.
Stems mostly solitary from a crown or very short, stout rhizome, 4-10 dm, conspicuously spreading-villous at least below (except often in var. vaseyi), often merely villous-puberulent above; lvs opposite, elliptic to elliptic-ovate, lance-elliptic, or elliptic-oblanceolate, sessile or nearly so, 4-13 נ1-4 cm, the larger ones seldom less than 1.5 cm wide except sometimes in var. subvenosum, glandular-punctate, evidently hairy to sometimes subglabrous; infl dense, corymbiform, ±flat- topped; invol 8-11 mm, often with dark sessile glands, otherwise generally glabrous or only slightly hairy, its bracts imbricate, conspicuously white-scarious upward (especially the inner), all narrow and long-acuminate, or the inner with broader, more rounded, mucronate tip; fls 5, the cor white, 4-5.5 mm; 2n=20, 30, 40. Dry, open woods, especially in sandy pinelands; coastal states from s. Conn. to c. Fla. and w. to Miss., also inland in the mt. regions to s. O. and e. Ky. and Tenn., and in Ark. The sp. consists of a widespread, common diploid phase and two more sporadic or local polyploid phases that reflect hybridization with other spp.:
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.