White Snakeroot

Ageratina altissima (formerly Eupatorium rugosum)

Asteraceae (daisies)


An upright, much-branched perennial with smooth (sometimes hairy) stems and fibrous roots. Flowerheads in loose, terminal, flat-topped clusters. The flowers point upward and are clear white and tuftlike. Bracts of the involucre (leafy appendages at the base of the flowerhead) acutely pointed, hairless. Blooms July–October. Leaves opposite, broadly ovate, with long petioles and large teeth.

Similar species: Late boneset, Eupatorium serotinum, has gray flowers instead of white or greenish white. Also, the leaf blades of late boneset are somewhat narrower, with more narrowly angled leaf bases. Until fairly recently, both plants were placed in the same genus. Because white snakeroot is toxic, it is good to know how to tell these plants apart.


Height: to 4 or 5 feet.

Habitat and conservation

Occurs in rich or rocky woods, bottomland forests, bases and ledges of bluffs, clearings, banks of streams and rivers, pastures, old fields, roadsides, waste places and other open, disturbed areas. The common name comes from an old and incorrect belief that this plant could help treat snakebites. Instead, this plant is toxic to mammals and can kill cattle and horses (where the malady is called "trembles") as well as humans, who can be killed by drinking milk from poisoned cattle.

image of White Snakeroot distribution map
Distribution in Missouri



Because of its toxicity to livestock, this native plant is often considered a weed. Animals may ingest the toxic compounds by eating either fresh plants or hay and are most at risk when allowed to graze in wooded habitats where white snakeroot can benefit from disturbance by livestock and form dense stands. Humans who have ingested the toxins by drinking the milk of cows that have eaten the plant display weakness, muscle spasms, vomiting, constipation, thirst, delirium and coma.

Human connections

This plant is poisonous to cattle and killed many early settlers who drank milk from cows that had consumed the plant. It is estimated that in the early 1800s in parts of Ohio and Indiana, up to half of all fatalities were caused by "milk sickness." One victim was Abraham Lincoln’s mother.

Ecosystem connections

Many kinds of insects visit the flowers for nectar. Several closely related plants are eaten by the caterpillars of some tiger moths and other moths, and apparently this species is among the food plants of these moths. The toxins are incorporated into the moths' bodies as a predator deterrent.