Plants loosely cespitose; rhizomes long. Culms obtusely trigonous in cross section, 30-90 cm, smooth or scabrous-angled distally. Leaves: basal sheaths reddish purple; ligules as wide as long to wider than long; blades dark green, wiry, filiform, with involute margins, 0.5-2.5 mm wide, glabrous. Inflorescences 3-20 cm; proximal bract 3-15(-25) cm, shorter than to exceeding inflorescence; proximal 1-2 spikes pistillate, erect, ovoid to cylindric; terminal 1(-2) spikes staminate. Pistillate scales ovate, 3.2-5.6 × 2.6-3.8 mm, shorter than perigynia, margins entire, apex obtuse to acute, awnless. Perigynia ascending, 7-15-veined, veins extending into beak, ovate, 4-6.7 × 2.5-3.4 mm, leathery, apex tapered; beak 0.3-0.9 mm, obscurely bidentulate, glabrous, teeth straight to somewhat outcurved, to 0.3 mm. Stigmas 3. Achenes brown, trigonous, smooth. Fruiting summer. Bogs, often forming extensive stands in Sphagnum-dominated areas, poor fens, sometimes in acidic, sandy, or peaty soils in open swamps, marshes, lakeshores, riverbanks; 0-1200 m; Man., N.B., Nfld. and Labr., N.S., Ont., Que., Sask., Yukon; Ind., Maine, Mich., Minn., N.H., N.Y., N.C., Ohio, Pa., Vt., Wis. Carex oligosperma hybridizes rarely with C. rostrata.
From Flora of Indiana (1940) by Charles C. Deam
Frequent in the dune area, otherwise quite local in northern Indiana. It prefers Chamaedaphne and tamarack bogs, but is found also in marshes and swales and on borders of ponds.
Colonial by creeping rhizomes, forming a turf; stems slender, 4- 10 dm, purplish at base; lvs pale green, stiff, elongate, involute, 1-3 mm wide; ligule much wider than long; staminate spike usually solitary; pistillate 1, or 2 or 3 and widely separated, sessile or nearly so, ovoid to short-cylindric, 1-2 cm; lowest bract lf-like but not sheathing; perigynia 3-15(-18), ovoid and somewhat inflated but also compressed, 4-7 mm, half as thick, strongly several-nerved, abruptly narrowed into an emarginate beak 1-2 mm; achene trigonous, obovoid, 2-3 mm; style persistent, usually contorted at base; 2n=76. Grassy bogs and wet meadows; Nf. to Mack., s. to Conn., Pa., Ind., and Wis.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.