This highly variable northern, circumboreal species is known to hybridize with several other tetraploids (2n = 52) as well as the octoploid illinoensis, which has been confused. P. gramineus was mapped in Ky. by FNA 22, but no verified colls. have been seen. It was reported from Ky. by Short et al. (1833) and Kellerman (1965); a potential coll. of C.W. Short (PH) needs checking. Another coll. of C.W. Short (dated 1837 at GH) was previously filed under gramineus (and originally labelled "heterophyllum") but appears to be tennesseensis instead.