There have been a few reports of this northeastern species from Ky. (e.g. Greenwell 1935), but no verified colls. are known. P. nigra appears to intergrade with americana, and it has been combined as a variety by some authors. Also, there has been some confusion in nomenclature with P. americana var. lanata (B, M), which has generally been considered a hairy variety of americana and perhaps intergrading with mexicana; see notes under those species. However, J. Rohrer (FNA 9) has indicated that the types of var. lanata and var. mollis are closer to nigra.