Published January, 2012

Common name: White Snakeroot
Scientific name: Ageratina altissima (L.) King & H.E. Robins
Synonym: Eupatorium rugosum Houttuyn

Life Cycle: Perennial
Origin: North America
Poisonous: Yes

White snakeroot is a warm-season perennial frequently found in shaded areas of pastures near streams or woods. It reaches a height of four to five feet. Leaves are opposite each other, characterized by three main veins and serrated margins. At maturity, white flowers bloom in flat-topped or domed panicles (branched clusters). White snakeroot emerges in late spring and is often not identified until flowers develop in late summer or early fall.

White snakeroot plants are toxic to horses; both fresh and dried leaves contain the toxin. Cumulative intake between 1 and 10% of body weight is toxic and can be lethal. Clinical signs of white snakeroot poisoning in horses include depression, weakness, tremors, or congestive heart failure. Signs of poisoning generally occur within three to 14 days. The toxin also is readily passed in milk and might poison nursing animals.

Look for white snakeroot in moist, shady areas in pasture margins. Removing these plants from the pasture by hand is often the best course of action, but ensure horses cannot access dried plant material after removing. Consult your localCooperative Extension Service personnel for herbicidal control in your area.

William W. Witt, PhD, a researcher in the department of Plant and Soil Science at the University of Kentucky, provided this information.