This has a northeastern range and is rare in Ky., being found mostly on seasonally wet acid soils in Appalachian regions. There has been confusion with pilosum, which is a largely midwestern species that mostly occurs on drier ground. P. pilosum deserves distinction as a species (F, Gl, Ch+, W) but it has been treated as a variety (Cr,) or combined (K). IP. verticillatum lacks the general hairiness of pilosum (except for canescent upper surfaces of bracts and leaves); leaves may tend to be narrower (W); inflorescences tend to be more open with verticillate or spicate branches (versus dense corymbs with subsessile capitula), but with individual capitula remaining tight in fruit (versus opening to expose lower branchlets). Both taxa may be tetraploids (2n = 76 or 78).