The muddled nomenclatural history of this species has been recently addressed by Weakley et al. (2023); the lectotype of I. brachiatus was an illustration of Trichostema dichotomum. I. coeruleus is widely scattered across eastern and central states, but generally restricted to limestone outcrops or other base-rich rocks. There is some "equivocal" evidence that Isanthus should be included within Trichostema sect. Orthopodium, which has a more western range; n = 7 in both taxa (Huang et al. 2008). It might alternatively be treated as a monotypic section (Weakley et al. 2023). However, the plant has generally been treated as a monotypic genus distinct from Trichostema (F, Cr, W). Isanthus differs from Trichostema sensu stricto as follows (W): calyx lobes subequal (versus trongly bilabiate); stamens straight, < 10 mm long (versus strongly arched, 12-20 mm); leaves acute to slightly acuminate (versus obtuse), the two main lateral veins reconnecting to the midvein near the leaf tip (versus not reconnecting); plants annual (versus annual or perennial); 2n = 14 (versus 38). Isanthus is known as a "false pennyroyal" and it does have some pleasant aromatic character, while Trichostema in Ky. is not particularly aromatic. However, there does not appear to have been any published analysis of chemical differences.