Monographs Details: Eupatorium
Authority: Lamont, E. E. 1995. Taxonomy of section (Asteraceae). Mem. New York Bot. Gard. 72: 1-68.
Scientific Name:Eupatorium
Description:Genus Description - Stout perennial herbs, erect, rhizomatous, fibrous-rooted, often forming clonal colonies. Stems 0.33.5+ m, unbranched, terete, fistulose or solid, purple-speckled or dark purple only at nodes to uniformly purple or greenish purple throughout, sometimes glaucous, viscid-puberulent near summit, glabrous to densely puberulent or glandular-pubescent below. Leaves verticillate in 3’s to 7’s, rarely merely opposite; blade simple, mostly 6-30 cm long, 1.5-18 cm wide, lance-elliptic or lance-ovate to deltate-ovate, acuminate at apex, gradually or sometimes abruptly tapered to the short-petiolate base, pinnately veined or triplinerved, lateral veins branching and anastomosing before reaching margin, margins serrate with a large gland at apex of each marginal tooth, adaxial surface glabrescent to scabrous-hirsute, abaxial surface atomiferous or conspicuously glandular pubescent to loosely hirsute on the midrib and main veins or densely pubescent throughout; petiole 0.5-2(-7) cm long, sometimes obscured by decurrent blade, glabrescent to ciliate. Inflorescence a flat-topped to convex compound corymbiform cyme of many dense heads with foliaceous bracts alternately scattered throughout; subtending whorl of leaves reduced to size of bracts or large and foliaceous; peduncles 1-8 mm long, glandular pubescent to glabrescent; flowering sequence within heads indeterminate (when numerous flowers present) but flowering sequence among heads determinate. Involucre cylindrical, 5.5-9 mm high, 2.5-7 mm wide, pale pink to purple, rarely white; bracts 10-22 in 5 or 6 series (outer 2 or 3 series glabrescent to densely pubescent and often resinous-glandular, the innermost ones glabrous to glabrescent), persistent or innermost slightly deciduous, lanceolate, imbricate, tightly appressed in several lengths (the outermost series half or less than half as long as the innermost series), the outer ones obtuse, the inner acutish, few-striate or at least with prominent midvein; receptacle naked, flat or weakly convex. Flowers tubular, perfect, 4-22 in a head, scarcely exserted at anthesis; pedicels less than 1 mm long; corollas regular, 5-toothed, purplish or very pale pinkish to rarely white, 3-7.5 mm long, outer surface of tube usually minutely atomiferous-glandular, sometimes with a few scattered hairs, lobes ovate to deltate, less than 1 mm long, usually slightly longer than wide; stamens as many as the corolla lobes, anthers with a small, hyaline, apical appendage, minutely rounded-auriculate at the base; style to 12.5 mm long, style branches about 2-6 mm long, equalling or exceeding the undivided part, slightly broadened or flattened distally, papillate; stigmatic lines obscure (only below middle of style branch), style base puberulous, bulbous. Fruit a prismatic, 5-ribbed achene, 3-5.5 mm long atomiferous-glandular, sometimes with a few setulae along the nerves, usually dark brown to black, sometimes yellowish-brown; pappus a single series of 25-40 slender, capillary, antrorsely barbed, persistent bristles, 4-7 mm long, cream colored to pinkish purple. Pollen grains echinate, tricolporate, approximately 40 µm in diam. 2n = 20.

Discussion:Type species: E. verticillatum Muhl. ex Willd. (= Eupatorium purpureum L.). Eutrochium Raf., New Fl. 4: 78. 1838. Type species: none. Eupatoriadelphus R. King & H. Robinson, Phytologia 19: 431. 1970. Type species: Eupatorium purpureum L. A section of five species restricted to North America, from Newfoundland to northern Florida, west to Saskatchewan, Wyoming, Utah, and Arizona, with outlying populations in southwestern British Columbia and adjacent Washington (Fig. 7).