This largely midwestern species is close to puniceum, but considered distinct in most recent treatments; 2n = 16 and 32 (Cr, FNA 20, W). S. firmum differs in its long creeping rhizomes (versus stout caudex), forming large patches; upper stems are pubescent in lines to glabrous (versus uniformly hairy) and usually reddish-purple (versus reddish only above each node); leaves are distinctly crowded in the inflorescence (versus sparse to moderately dense); rays are usually paler (blue/lavendar to white versus dark blue to lavendar). It is typical of more calcareous sites; see also St and Y. In Ky., the only known record is an 1840s coll. of C.W. Short from FAYE (PH): "Kentucky River banks north of Elk Lick Creek".