This largely autotetraploid species (2n = 68) is widespread in open habitats of eastern and southeastern states; diploids have also been reported (Qiu et al. 2019). H. hirsutus varies much in pubescence, leaf blade dimensions and petiole length. It appears closely related to divaricatus but differs as follows (F, GC, FNA 21, W): leaf blade bases rounded to tapering (versus subcordate to rounded), the petioles usually 2-10 mm (versus 1-2 mm), the two major lateral veins subopposite and united with midrib ca. 1-5 mm above the blade base (versus opposite and united with midrib at ca. 0.1-0.5 mm); blades broadest at ca. 20-30% above base (versus 15-20%), horizontal to ascending (versus more or less horizontally divergent), hirsute and conspicuously glandular below (versus sparsely hispid to hispidulous and indistinctly glandular), densely scabrous-hispid above (versus scabrid); stem hirsute, hispid or scabrous on most or all internodes, not glaucous or purplish (versus mostly glabrous, sometimes hispid-hirsute on upper internodes, usually somewhat glaucous or purplish). heads are generally larger (see above sources),and are in full flower mostly during Aug-Sep (versus Jul-Aug) across central Ky. or earlier to the south. Rhizomes are sometimes terminally thickened; see also notes under tuberosus (Bock et al. 2014). Plants with subsessile leaves can resemble divaricatus, especially if pubescence is also reduced; hybridization between these species is often suspected despite the reported general difference in chromosome number. Some robust plants with longer petioles resemble tuberosus, and may result from hybridization with that species (Heiser et al. 1969; FNA 21). Several colls. with less hairy stems are referable to var. trachyphyllus Torr. & Gray, a largely mid-western form that appears transitional to decapetalus (F).