This distinct species (W) occurs mostly in southeastern states east of the Mississippi Rv., but its distribution has been confused with hyssopifolium sensu stricto (W). It differs from hyssopifolium as follows (W): leaves lanceolate (versus linear to narrowly lanceolate), the principal cauline leaves 5-12 x 0.5-1.2 cm (versus 2-7 x 0.1-0.5 mm), with L/W 6-15 (versus 10-40), conspicuously and divergently toothed (versus entire to obscurely toothed), the leaves mostly opposite or in whorls of 3 (versus 3 or 4). E. torreyanum includes old records of "hyssopifolium" from se. Ohio (F), but it may exclude most records of "hyssopifolium" from southern Ind., Ill. and Mo (SERNEC; Y); it has just recently been reported from se. Mo. (Ladd & Thomas 2015). E. torreyanum is probably derived from hybridization of hyssopifolium with another species, perhaps semiserratum (Cr, FNA 21). It is mostly tetraploid (2n = 40), whereas hyssopifolium (sensu stricto) includes just triploids (2n = 30) and diploids (2n = 20). The type coll. was made by C.W. Short in 1835 (NY), from "Knobs among the barrens, near the Mammoth Cave".