Ageratina O.Hoffm. is a large genus of perennials, subshrubs, and shrubs in the family Asteraceae (tribe Eupatorieae), comprising approximately 315–350 accepted species. Plants range from 20 to 220 cm tall, with erect stems that are sparsely to densely branched. Leaves are mostly opposite and cauline, with blades that are deltate, lanceolate, ovate, or triangular in shape, typically displaying 3–5 veins from the base. Many species produce a foul-smelling, musky scent from their foliage.
The flowerheads (capitula) are discoid — lacking the ray florets characteristic of many Asteraceae relatives — and are arranged in compact, flat-topped (corymbiform) clusters. Each involucre is bell-shaped (campanulate), 3–6 mm in diameter, with 8–30 persistent phyllaries. Individual flowerheads contain 10–60 florets with white or lavender corollas. The fruits (cypselae) are prismatic or fusiform, 5-ribbed, and bear a pappus of 5–40 barbellulate bristles. The base chromosome number is x = 17.
The genus is most diverse in the Americas, with more than 150 species native to Mexico alone. Its range extends through the West Indies and into the warmer regions of South America, with several species reaching into temperate North America, including the eastern and southwestern United States. Two species of Mexican origin — most notably Ageratina adenophora — have established themselves as invasive plants in regions including Australia, Taiwan, California, and Hawaii.
Etymology
The genus name Ageratina is a compound formed from Ageratum (a closely related genus in the same tribe Eupatorieae) and the Latin feminine diminutive suffix -ina, meaning "resembling Ageratum" or "like Ageratum." The common name "snakeroot" reflects the historical use of roots by Native Americans to treat snakebites.
Distribution
Ageratina grows mainly in the warmer regions of the Americas and the West Indies. The center of diversity is Mexico, where more than 150 species are native. The genus also extends into the Caribbean, Central America, and South America, with some species reaching into temperate North America — including the eastern United States, Arizona, and New Mexico. Ageratina altissima (white snakeroot) is native to the eastern United States, where it is often found in rich woodland edges and forest margins. Outside its native range, several species — particularly A. adenophora — have become naturalized and invasive in Australia, Taiwan, California, and Hawaii.
Ecology
Species of Ageratina occupy a range of terrestrial habitats from shrublands and woodland edges to mountain forests and coastal zones. Ageratina altissima is characteristic of rich woodland edges in eastern North America. Plants spread by both rhizomes and seeds, allowing vegetative expansion as well as long-distance dispersal. They adapt to varying soil types — clay, loam, sand, and high-organic-matter soils — and tolerate both full sun and partial shade. All plant parts of white snakeroot contain tremetone and related glycosides, which are toxic to livestock; consumption by cattle can render their milk toxic to humans, a condition historically known as "milk sickness." The invasive A. adenophora forms dense stands in disturbed areas and roadsides in introduced regions, threatening native vegetation.
Cultivation
In cultivation, Ageratina species are grown primarily as ornamental perennials and subshrubs. Ageratina altissima (white snakeroot) is suited to woodland gardens, pollinator gardens, borders, and streamside plantings. Plants tolerate full sun to partial shade and are adaptable to a range of well-drained soils. Growth rate is medium and maintenance requirements are moderate. They are valued for their showy clusters of white flowerheads, which attract pollinators.
Propagation
Ageratina species spread naturally by rhizomes and by seed. In cultivation, division of rhizomes is a practical means of propagation, as is seed sowing.
Cultural Uses
Several Ageratina species have documented ethnobotanical uses. Native Americans used the roots of snakeroot species to treat snakebites, which gave rise to the common name. Ageratina pichinchensis is a traditional Mexican remedy for superficial fungal infections of the skin, with the compound encecalin identified as responsible for antifungal activity. In India, species of the genus have been used historically to treat snakebite, epilepsy, and mental disorders. Research in 1949 identified compounds in the genus with hypotensive properties relevant to regulating high blood pressure; the alkaloid reserpine, associated with the group, was later used in the treatment of schizophrenia. Conversely, A. altissima (white snakeroot) was the cause of widespread milk sickness in 19th-century North America, where toxins in the plant — ingested by cattle — passed into milk and caused fatal illness in humans.
Conservation
Some Ageratina species are listed as significant invasive plants outside their native range. Ageratina adenophora (crofton weed, sticky snakeroot) is an established pest in parts of Australia, Taiwan, California, and Hawaii, where it invades disturbed habitats. Within its native range, A. altissima is toxic to livestock and humans and is considered a range hazard in agricultural areas of eastern North America. No broad IUCN Red List assessment was found for the genus as a whole; conservation concern is primarily focused on the invasive impacts of a small number of species.
Taxonomy Notes
Ageratina is placed in Asteraceae, tribe Eupatorieae, and is accepted as a distinct genus (GBIF key 5400194). The protologue citation is O. Hoffm. (1900), Bot. Jahrb. Syst. 28: 503. GBIF records the authorship as O.Hoffm., while ITIS attributes the genus to Spach — differing authority treatments between the two databases. The genus was historically treated under Eupatorium, from which it was segregated by King and Robinson in their revision of the Eupatorieae; many former Eupatorium species were transferred to Ageratina. The base chromosome number is x = 17.
