Eupatorium, traditionally a large and very variable genus of probably more than 1000 species, was divided in several smaller genera by King & Robinson (1987). Their viewpoint has become widely accepted and is congruent with recent molecular research (see for instance Schmidt & Schilling 2000). Eupatorium in its narrow sense now is a genus of ca. 40 species, native to North America (obviously its center of diversity), Europe and eastern Asia. One species, Eupatorium cannabinum L., is a common native species in Belgium. Some other species are cultivated as ornamentals in Europe (Galloway 2000, Jäger & al. 2008) and might occur as escapes.
Forms similar to native Eupatorium cannabinum with entire leaves and brighter corollas are sometimes cultivated as ornamentals and have been recorded as escapes, for instance on a dump in Vorselaar in 2003 (see also Whitehouse 2007a). Their genuine identity remains obscure. Whitehouse (2007b) ascribes plants with the ‘brightest coloration’ to the Asian E. fortunei Turcz. However, the actual circumscription of this taxon (see e.g. Chen & al. 2011) does not correspond at all with the plant found in cultivation in Europe. Pending further study it is here merely included in Eupatorium cannabinum but it may well be a distinct taxon.
Eutrochium is easily distinguished in having leaves in distinct whorls (usually opposite in Eupatorium and Ageratina). Eupatorium s.str. has hairy style bases whereas in Ageratina style bases are glabrous (for additional features see also under Ageratina).
1 All leaves entire, the uppermost distinctly perfoliate. Involucral bracts acute-acuminate at apex, ca. 2-4,5 mm long. Corolla usually white, rarely pink. Leaf venation reticulate, prominent. Florets 7-11 === Eupatorium perfoliatum
At least the lowermost leaves palmately 3-5-lobed, never perfoliate. Involucral bracts rounded at apex, 4,5-6 mm long. Corolla usually pink. Leaf venation not reticulate, obscure. Florets usually ca. 5-9 (native) === E. cannabinum subsp. cannabinum
References preceded by an * treat Eupatorium in a broad sense, including Ageratina and Eutrochium.
Chen Y.L., Kawahara T. & Hind D.J.N. (2011) Eupatorieae. In: Wu Z.Y., Raven P.H. & Hong D.Y. (eds.), Flora of China Volume 20–21 (Asteraceae). Science Press (Beijing)
& Missouri Botanical Garden Press (St. Louis): 879-891. [available online at: http://flora.huh.harvard.edu/china/mss/volume20/Flora_of_China_Volume_20_21_Eupatorieae.pdf]
*Galloway A.A. (2000) Eupatorium. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 6. Cambridge University Press, Cambridge: 663-666.
Hawke R. (2014) A Comparative Study of Joe-Pye Weeds (Eutrochium spp.) and Their Relatives. Plant Evaluation Notes 37: 1-9. [available online at: https://www.chicagobotanic.org/downloads/planteval_notes/no37_joepyeweed.pdf]
Hind N. (2006) Splitting Eupatorium. The Plantsman N.S. 5(3): 185-189.
*Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.
*Johnson M.F. (1974) Eupatorieae (Asteraceae) in Virginia: Eupatorium L. Castanea 39: 205-228.
King R.M. & Robinson H. (1970) Eupatorium, a Composite genus of Arcto-Tertiary distribution. Taxon 19: 769-774.
King R.M. & Robinson H. (1987) The genera of the Eupatorieae (Asteraceae). Monogr. Syst. Bot. 22: IX + 581 p.
Robinson H., Schilling E. & Panero J.L. (2009) Chapter 43: Eupatorieae. In: Funk V.A., Susanna A., Stuessy T.F. & Bayer R.J. (eds.), Systematics, evolution and biogeography of the Compositae. International Association for Plant Taxonomy, Vienna: 731-744. [available online at: http://www.bio.utk.edu/schilling/Danielweb/eup/Eupatorieae_chapter.pdf]
Schmidt G.J. & Schilling E.E. (2000) Phylogeny and biogeography of Eupatorium (Asteraceae: Eupatorieae) based on nuclear ITS sequence data. Am. J. Bot. 87: 716-726. [available online at: http://www.amjbot.org/content/87/5/716.full]
Siripun K.C. & Schilling E.E. (2006) Eupatorium. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 21. Oxford University Press, New York-Oxford: 462-474. [available online at: http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=112351]
Whitehouse C. (2007a) The cultivated forms of Eupatorium cannabinum L. (Asteraceae). Hanburyana 2: 29-34.
Whitehouse C. (2007b) Eupatorium: Joe Pye weeds and their ornamental relatives. The Plantsman N.S. 6(4): 242-247.