Ageratina altissima was first described as Ageratum altissimum by Linnaeus in Species Plantarum, Vol. 2, in 1753. In 1970, R.M. King and H. Robinson reviewed the classification and renamed it under the epithet Ageratina altissima with two varieties, var. altissima and var. roanensis.
A. altissima is a perennial herb with an upright stem and clustered, white flower heads. Var. roanensis, known as the Appalachian white snakeroot, has slightly longer phyllaries with cuspidated apices. Its growth is mostly limited to the southeastern U.S. Var. altissima grows throughout the entire eastern U.S. and southeastern Canada.