Mapping here is provisional; several records may become be transferred to loomisii, and separation from pycnanthemoides is also difficult. P. incanum is considered by some authors (W) to be a distinct tetraploid species (2n = 76) in the Ohio Valley and Appalachian regions, extending northeast to s. New England (K). P. incanum reportedly differs from the other two species in its calyx lobes, which are deltoid (versus acuminate) and relatively short, the lower ones 1-1.5 mm (versus 1-3 mm) and less than half as long as the tube (versus usually more than half). Superficially, the name "incanum" has often been applied in Ky. to plants with relatively small yellowish green leaves (versus deeper bluish-green), less open branching, smaller cymes, and generally less spreading pubescence (F). Such plants may be typical of relatively dry and open sites. The incanum-complex is well known for its diverse volatile oils, which have recently been characterized by Dein & Munafo (2019); much more research should occur into chemical variation and uses.