30-200 cm. Stems
erect, usually not branched proximal to arrays of heads (from caudices or rhizomes). Leaves
mostly cauline; usually opposite (rarely whorled, distal sometimes alternate); petiolate or sessile; blades usually 3-nerved from or distal to bases, or pinnately nerved, mostly deltate or ovate to lanceolate or linear (and intermediate shapes, sometimes elliptic, oblong, rhombic, or suborbiculate, sometimes pinnatifid, 1-2-pinnately, ternately, or palmately lobed), ultimate margins entire or toothed, faces glabrous or puberulent, pubescent, scabrous, or setulose, usually gland-dotted. Heads
discoid, in corymbiform or diffuse to dense, paniculiform arrays. Involucres
obconic to ellipsoid, 1-3(-5+) mm diam. Phyllaries
persistent, 7-15+ in 2-3(-4+) series, (usually green) 2-3-nerved, or not notably nerved, or pinnately nerved, elliptic, lanceolate, oblong, or obovate, usually unequal, sometimes ± equal (margins scarious, hyaline, apices rounded to acute or acuminate sometimes mucronate, faces usually puberulent or villous, usually gland-dotted, rarely glabrous). Receptacles
flat or convex, epaleate. Florets
(3-)5(-15+); corollas usually white, rarely pinkish, throats funnelform to campanulate, lobes 5, triangular; styles: bases sometimes enlarged, usually puberulent (glabrous in E. capillifolium
), branches mostly filiform. Cypselae
(brownish to black) prismatic, 5-ribbed, usually glabrous, usually gland-dotted; pappi
persistent, of 20-50 (whitish) barbellulate bristles in 1 series. x
= 10. Eupatorium
is treated here in a restricted circumscription, following R. M. King and H. Robinson (1987) in excluding genera that traditionally have been included in a broad Eupatorium
(e.g., Ageratina, Chromolaena, Critonia, Conoclinium, Fleischmannia, Koanophyllon, Tamaulipa
) is also excluded here.
Species identification within Eupatorium is sometimes complicated; polyploidy and apomixis have contributed to the complications. Some species include both sexual diploid and apomictic polyploid plants or populations. V. I. Sullivan (1972) made important contributions to understanding Eupatorium in North America by showing that some fairly distinct, sexual diploid species may include apomictic polyploid plants or populations that do not differ greatly from the diploids. Other apomictic polyploids appear to be intermediate morphologically between pairs of diploid or diploid/polyploid species and were proposed by Sullivan to have originated from interspecific hybridization. Distinction and level of recognition of hybrid apomictic taxa have a large arbitrary component, in part because some apomicts appear to be ephemeral and others may be relatively stable and in part because differences in the relative genomic contributions of the progenitors through dosage effects or backcrossing may affect whether an apomict is morphologically distinctive or part of a continuous series of variation.