Description:
Eupatorium has been redefined to include about 45-50 species of herbaceous perennials found in eastern North America and eastern Asia. Formerly the genus was considered to include all of the species of Eupatorieae that have 5-ribbed cypselae and a pappus of bristles, but the extensive work of King and Robinson culminating in their 1987 summary has shown that, defined this way, the genus was an unnatural assemblage that needed to be split up. Eupatorium is now defined by its styles, which are basally pubescent and have unenlarged appendages, its relatively few-flowered heads (up to 23, but in many species 5), its corollas, which are funnelform with a constricted basal tube, and its cypselae, which have glands but no setae. In many treatments, the species of Eutrochium are included within Eupatorium, but the two genera as circumscribed here are easily distinguished from each other, and no hybrids between species of the two have been reported.
See here for a fuller description and more information regarding Eupatorium.
A key to species and distribution maps for individual species may be found in the Flora North America treatment.
Images - Eupatorium torreyanum - habit (left); close-up of heads (right)
