This is a distinct species, widespread in fields and open woods on damp fertile soils of east-central states (FNA 23, K), mostly from the central Great Plains (Okl. to Neb.) to mid-Atlantic states (Va. to N.J.). However, it can appear superficially intermediate between sparganioides and either gravida or muhlenbergii (especially plants that have been called var. enervis); further review of colls. is desirable. Compared to muhlenbergii, aggregata has perigynia that tend to be larger (3.4-4.6 x 1.9-2.8 mm versus 2.7-4.2 x 1.8-2.6 mm), but with fewer nerves (up to 2-5 versus 5-15 on dorsal surface), with consistently ovate-lanceolate bodies (versus ovate to suborbicular) and relatively long beaks (1/3-1/2 of length versus 1/4-1/3). Also, C. aggregata has somewhat looser sheaths (as in sparganioides), the backs usually mottled and more green-striped, the front usually more deeply concave at the summit, with little or no cross-puckering. Its blades are wider on average (up to 3.5-5 mm versus 2-4 mm). C. aggregata differs from sparganioides in the fronts of its sheaths, with relatively firm yellow-brown summits (versus fragile whitish) and often reddish-dotted surfaces (as in gravida); well-developed leaves are narrower (up to 3.5-5 mm wide versus 5-10 mm). Also (as in gravida), its spikes tend to be more condensed, with generally larger scales, perigynia and anthers. As in sparganioides, the spikes of aggregata are often slightly compound at the base, which has sometimes led to confusion with species in section Vulpinae (F, Cr, FNA 23).