Pistillate scales 2-2.5 × 1.2-1.8 mm. Perigynia veinless adaxially, 2.7-3.1 × 1.8-2.3 mm; beak 0.2-0.6(-1.0) mm.

Fruiting late spring-early summer. Dry grasslands, open forests, commonly on sand; 10-300 m; Ont.; Ala., Ark., Conn., Ga., Ill., Ind., Iowa, Kans., Ky., La., Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Tex., Vt., Va., W.Va.

The distribution and ecology of Carex muehlenbergii var. enervis are unclear due to confusion with var. muehlenbergii. Plants of var. muehlenbergii that have a veinless perigynium have often been misidentified as var. enervis, but those plants are otherwise identical to var. muehlenbergii. Carex muehlenbergii var. muehlenbergii usually occurs on basic to slightly acidic soils; var. enervis often occurs on more acidic soils. It is not clear, however, whether that correlation is consistent.

Frequent on slopes, in sandy open woods, on wooded dunes, and in dry sandy fields. It is partial to somewhat less open habitats than the species [Carex muhlenbergii] and is less often on low or level ground, its favorite habitat being on or near the crests of wooded dunes, river bluffs, and oak ridges. Specimens intermediate between C. Muhlenbergii and var. enervis in some or most of their characters seem to be too frequent to warrant the treatment of the latter as a species. The ventrally fiat perigynium is a conspicuous character of typical var. enervis when fully mature or overripe but it is very inconstant and specimens with a pronounced raised border up to maturity are especially frequent.