Eupatorium is a genus of flowering plants in the daisy family (Asteraceae), placed in the subfamily Asteroideae and the tribe Eupatorieae. Carl Linnaeus formally described the genus in Species Plantarum in 1753, and Eupatorium remains the accepted name today. The number of recognized species varies between roughly 36 and 60 depending on how strictly the genus is circumscribed, a reflection of more than two centuries of taxonomic reshuffling that has moved hundreds of former Eupatorium species into segregate genera such as Ageratina, Chromolaena, Conoclinium, and Eutrochium.
Most species are herbaceous perennials standing between 0.3 and 3 metres tall, with a few shrubby exceptions. Stems are erect and usually unbranched below the inflorescence, and the leaves are mostly cauline and oppositely arranged, with blades that are deltate to lanceolate or linear and typically three-nerved or pinnately nerved. The flower heads are discoid — that is, they lack ray florets — and bear five to twenty-odd tubular florets, usually white but occasionally pale pink, each with a five-lobed corolla. Heads are gathered into dense, flat-topped or rounded clusters at the tops of the stems. The involucre is obconic to ellipsoid with 7 to 15 or more phyllaries arranged in two or three series, and the receptacle is flat or convex and lacks chaffy bracts (paleae). The fruits are prismatic, five-ribbed cypselae topped by a persistent pappus of barbellulate bristles, an arrangement well suited to wind dispersal.
The genus is native to the temperate Northern Hemisphere, with most diversity concentrated in eastern North America and a much smaller showing in Eurasia, where Eupatorium cannabinum — the type species — is the sole European representative. North American species are widely known by the common names boneset, thoroughwort, and snakeroot, while the European species is called hemp-agrimony. Most species inhabit moist or wet ground: stream banks, marsh edges, wet meadows, low woodland clearings, and roadside swales. They flower late in the season, from midsummer into early autumn, and the abundant nectar of their dense corymbs makes them magnets for butterflies, bees, and other pollinators. Several species are valued ornamentals in temperate gardens and are widely grown in Asia as well as in their native ranges.
Etymology
The genus name Eupatorium commemorates Mithridates VI Eupator, the first-century BCE king of Pontus on the southern shore of the Black Sea. Mithridates was famously preoccupied with toxicology and is traditionally credited with discovering medicinal uses for a plant of this lineage, which Linnaeus honoured when he formalized the genus in 1753.
Distribution
Eupatorium in its modern, narrower sense is a genus of the temperate Northern Hemisphere. Diversity is centred in North America, where species range from Atlantic Canada south to Florida and west across the Midwest into the southern Great Plains, with additional representation in the desert Southwest. Eupatorium cannabinum is the only species native to Europe and extends across temperate Eurasia, including Switzerland, where it is the genus's sole representative. Several species are documented from Arizona and New Mexico in SEINet herbarium records, and others — particularly those of the West Indies and warmer parts of the Americas in the broader pre-segregation sense — overlap with the modern genera Ageratina and Conoclinium.
Ecology
Most Eupatorium species are plants of moist to wet, sunny or lightly shaded ground. Typical habitats include swamps, bogs, marshes, wet pastures, stream banks, and the rich edges of open woodland. Their late-summer-to-autumn bloom period coincides with a peak demand for nectar among migratory and overwintering pollinators, and the dense, flat-topped flower clusters are heavily worked by bees and butterflies. Several species are recognized larval or adult food plants for butterflies and moths, and the genus is widely promoted as a high-value addition to pollinator plantings.
Cultivation
Garden-worthy Eupatorium species are easygoing perennials that thrive in full sun to partial shade and tolerate a wide range of soils — clay, loam, sand, and rich, high-organic-matter substrates — provided drainage is acceptable. They handle moist conditions and occasional drought once established, and most species fall somewhere within USDA hardiness zones 3 through 10 depending on origin. In the landscape they are at home in woodland gardens, pollinator gardens, mixed borders, and wet sites along ponds or streams, where their tall stature and late-season corymbs of white flowers provide structure and abundant nectar. They are generally deer resistant and free of significant pest or disease problems. Plants are cultivated as ornamentals in their native ranges and are also widely grown in Asia.
Propagation
Eupatorium spreads by both rhizomes and seeds. Vigorous clonal expansion from underground rhizomes makes division of established clumps a reliable propagation method, while seed set is heavy and wind-dispersed by the pappus-tipped cypselae. Pollination is performed by insects.
Cultural Uses
Several Eupatorium species have a long history in folk and herbal medicine. Eupatorium perfoliatum — boneset — was used by Native American and, later, African-American populations across eastern North America to treat fevers, colds, and influenza-like illness; the common name reflects its application to dengue fever, sometimes called "breakbone fever," not to mending broken bones. Roots of several species were used as diaphoretics, diuretics, febrifuges, stimulants, and tonics for urinary complaints, diarrhea, and toothache. Modern assessments are far more cautious: plants in the genus contain pyrrolizidine alkaloids and other toxic compounds capable of causing liver damage, and E. perfoliatum is listed on the U.S. FDA's Poisonous Plants Database. There are no known edible Eupatorium species, and the genus is recognized as poisonous to humans and livestock.
History
When Linnaeus established Eupatorium in 1753, the genus was understood very broadly, and over the next two centuries it grew to include as many as 800 named species spread across the New World tropics, North America, and Eurasia. Late twentieth-century taxonomic work — most notably Robert M. King and Harold Robinson's 1987 revisionary treatments of the tribe Eupatorieae — split this assemblage along morphological and phylogenetic lines, transferring large numbers of species into segregate genera including Ageratina, Chromolaena, Conoclinium, and Eutrochium. The modern, much narrower Eupatorium retains Eupatorium cannabinum as its type and contains roughly 36 to 60 species, depending on author.
Taxonomy Notes
Eupatorium L. is the type genus of the tribe Eupatorieae within the subfamily Asteroideae of the Asteraceae. GBIF records the genus as accepted with thousands of descendant taxonomic records, reflecting the long history of broader circumscription. The current narrow concept dates from the late twentieth-century revisions of King and Robinson and is followed by Flora of North America (volume 21) and most subsequent regional treatments. Earlier popular and horticultural sources frequently still apply the name Eupatorium to species now placed in Ageratina (white snakeroot, A. altissima), Conoclinium (mistflowers), and Eutrochium (Joe-Pye weeds), so context and authority matter when interpreting older literature.
