August 1, 2022
Photo by Ansel Oommen, Bugwood.org.

White Snakeroot

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By Robert J. Reber

Up until the early 1900s, “milk sickness” caused by white snakeroot (Ageratina altissimo; Eupatorium rugosum) poisoning was a relatively common, ever-present danger for settlers and farmers who pastured cattle on woodlands and waste ground and drank milk or ate meat from their own cows. That potential danger still remains for families who drink milk from their own cows. However, commercial milk is safe because any milk from a suspect cow is diluted, being mixed with milk from countless “white snakeroot-free” cows before sale to consumers. Of possible concern may be the novice, “back-to-the-landers,” who buy a wooded acreage, build a home and pasture a cow or two for their own milk.

A group of cattle browse on grass on a hilltop near an edge of a woodland in autumn. To the right of the cattle is an old barn.
Photo by Carolyn Wilman, Pixabay.com

Usually, a cow had to eat a substantial amount of foliage before enough poisonous tremetol—a mixture of toxic ketones—accumulated in its body to sicken and kill it. However, enough tremetol could accumulate in the milk to kill a human or a nursing calf before it sickened the cow. Thus, a person could sicken and die from milk sickness after drinking milk from apparently healthy cows. Many midwestern county histories recorded incidents of death from “the slows” as the sickness was often called. During early times, a few Indiana county records attributed nearly half of all deaths due to milk sickness; the most well-known case being that of Nancy Hanks Lincoln during 1818 in Spencer County. She was Abraham Lincoln’s mother.

White snakeroot grows in woodlands, thickets, and waste ground. Disturbances of fire and grazing may increase its occurrence. Controlled burns of woodlands that had a history of livestock pasturing years before may bring on a renewed flush of white snakeroot. Heavy, continuous pasturing for prolonged periods of time increases disturbance and may contribute to increased white snakeroot presence. Although it is less palatable than most woodland plants, livestock may eat more of it because their plant choices become limited in degraded and disturbed woods, particularly during fall and during drought conditions.

Clusters of tiny white flowers are in full bloom on a green plant with heart shape leaves with a toothed edge.
Photo by Ansel Oommen, Bugwood.org.

If white snakeroot is present in a woodland, do not pasture cattle, sheep, goats, or horses on it. Horses are particularly sensitive to tremetol. Pasturing concerns are less early in the growing season but increase later as forage becomes limited and livestock are more likely to eat the less palatable white snakeroot.

This 1- to 3-foot-tall native perennial grows throughout most of Illinois. When it blooms, white snakeroot is most easily recognized and thus easier to assess its overall occurrence in a woodland. Often, blooms are first noted along woodland lanes because this plant prefers partial sunlight to light shade. In Illinois, the flat topped to slightly rounded clusters of small white flowers appear from late July through mid-October. Flowers have five petals, and flowerheads have a fuzzy look. The leaves are paired opposite to one another along the main stem and are somewhat heart shaped. Leaves vary from 3 to 5 inches in length and have a toothed edge. Once you become familiar with the growth habits, habitat, and appearance of white snakeroot, it is easy to identify.


Robert J. Reber is an emeritus faculty member in the Department of Food Science and Human Nutrition of the University of Illinois at Urbana–Champaign. He has been a lifelong student of many aspects of the Natural World, including archaeology. Bob has served as a managing editor and author for publications such as The Illinois Steward magazine and the Illinois Master Naturalist Curriculum Guide.

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