This occurs mostly to the northeast and in cooler Appalachian regions (K). In Ky. it is known only from three sites in ROWA (MDKY). It is can easily be confused with atrovirens; in addition to differences in spikelets, it has long reddish/purplish rhizomes and lower sheaths (Cr, F, FNA 23). S. expansus is closely related to the Eurasian S. sylvaticus L. (especially) and to microcarpus, which can form hybrids; 2n = 62-66 in the complex.