This is widespread on damp fertile soils of eastern North America, especially in thin woods with a long history of browsing by deer, cattle or hogs. It is highly variable across its range, but segregates are mostly rather indistinct; 2n = 34 in all reports (as in other Ageratina species). Some relatively pubescent colls. from Ky. have been identified as var. tomentellum (B.L. Robbins.) Blake (from BULL), or as forma villicaule Fern. (from NELS, OWEN, WHIT); but these taxa are not recognized in recent treatments (FNA 21, W). Wesdt of the Shawnee Hills, plants are often relatively pubescent and appear somewhat transitional to aromatica. A few colls. from the Cumberland Mts. may tend towards var. roanensis; see below. A. altissima is the cause of the mysterious "milk sickness" that pervaded the Ohio Valley during early settlement, but its role was not widely understood until the 1920s (Mosely 1941). Cows that eat the plant pass on toxic phenolic compounds in their milk, a primary compound being the benzofuran derivative known as tremetone. However, the exact chemical cause remains unclear; trials with tremetone alone have not reproduced the toxic efffects of whole plants, which do persist when dried and stored for up to five years, when most trematone has disappeared (Davis et al. 2018). It is possible that pyrrolizidie alkaloids are involved in the toxicity, as known in other species of. Ageratina (Sharma et al. 1998, Roeder et al. 2015 and citations). Past medicinal uses of altissima remain somewhat obscure (Moerman 1998), but the healing properties of a related Mexican species have been well-documented (Romero-Cerecero et al. 2011).