Monographs Details: Eupatorium
Authority: Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Description:Genus Description - Heads discoid; the fls all tubular and perfect; invol bracts variously imbricate or subequal; receptacle naked; fls pink or purple to blue or white; style-branches elongate, linear or linear-clavate, obtuse, papillate, with inconspicuously ventromarginal stigmatic lines near the base (or sometimes for much of their length); achenes prismatic, mostly 5(–8)-angled and -nerved, glabrous, or inconspicuously hairy along the veins, or in most of our spp. atomiferous-glandular; pappus a single series of capillary bristles; perennial herbs (all ours) or shrubs, with entire or toothed to occasionally dissected, often glandular-punctate, mostly opposite, sometimes whorled or alternate lvs, all our spp. with the lowest lvs reduced and sometimes deciduous; heads small to large, in a mostly corymbiform infl, rarely solitary or forming a true panicle. Nearly 1000, mainly New World. Divided by some authors into numerous much smaller genera, these perhaps better regarded as sections or subgenera. Some of the segregates are indicated in parentheses in the key. In the system of King and Robinson our sp. 6 is referred to Conoclinium, 7 to Fleischmannia, and 8–10 to Ageratina; the others remain in Eupatorium.
Spp. 1–5 form a closely related, hybridizing, mainly diploid group, with x=10. Spp. 8–10 form a closely related group of diploids with x=17. Spp. 11–25 form an intricately reticulate complex of diploids, autopolyploids, established or temporary allopolyploids, and temporary hybrids, based on x=10. The diploids are mainly sexual and outcrossing, the polyploids mainly apomictic.
Some spp. here recognized are wholly diploid; others consist of both diploid and polyploid elements, the polyploids being taxonomically associated with the diploids they most resemble. Only E. godfreyanum, among our spp., appears to be wholly polyploid. In addition to the taxa recognized here, there are scattered small populations representing diverse sorts of allopolyploids that may not be permanently established. It does not seem useful to provide these with formal names.