White snakeroot (Ageratina altissima var. altissima) is a late bloomer, one of the last flowers to be seen in the fall in the woods of Minnesota. At that time of year it is certainly the most conspicuous. The inflorescence is a flat-topped to dome-shaped, up to 3″ in diameter cluster of many white flower heads at the end of the stem. On larger plants they also appear on long stalks from the upper leaf axils.
This plant is poisonous to livestock. It was unusually abundant in southern Minnesota in 2004. A number of horses in the New Ulm area died in the summer of that year, and it is thought that white snakeroot is the cause. If eaten by cows the cow’s milk may also cause “milk sickness” in humans. Nancy Hanks Lincoln, the mother of Abraham Lincoln, is thought to have died of milk sickness in 1818.