White snakeroot (Ageratina altissima)

Ageratina altissima, or white snakeroot, is the only species of its genus found in the Chicagoland region, but it is widespread, even weedy. (The more than 300 species in the genus Ageratina mainly occur in the warmer regions of the Americas and West Indies.)

White snakeroot is an herbaceous perennial around 1.5-3’ tall with opposite, largely hairless, and strongly serrated leaves attached to the stem with 0.5-2.5 inch long petioles. The upper surface of the cordate to lanceolate-shaped leaves features prominent palmate veins and an elevated web of veins on the underside.

Its small flowerheads, only around half an inch across, contain 10 to 30 bright white disk flowers but no ray flowers. Ageratina altissima flowers from late summer through late fall, one of our last wildflowers in bloom late in the season.

The leaves are bitter and toxic and thus generally avoided by mammals looking to fill their bellies. Containing the toxin mixture tremetol, Ageratina altissima causes what is known as the fatal “milk sickness” when it is heavily grazed by cattle and passed along to people through cow’s milk — a fate that befell many early Midwest settlers who were unfamiliar with the plant.

Ageratina altissima grows in both degraded and high-quality habitats: often in woods, brush thickets, and other shady areas with bare ground — including city parkways.