Plants colonial; rhizomes long. Culms terete or very bluntly trigonous in cross section, 8-90 cm, smooth distally. Leaves: basal sheaths brown, occasionally tinged with pinkish red; ligules as long as wide; blades whitish green, U-shaped, with involute margins, widest leaves 1.5-4.5(-7.5) mm wide, papillose adaxially. Inflorescences 10-30 cm; proximal bract 18-45 cm, exceeding but no more than 2.5 times longer than inflorescence; proximal (1-)2-3 spikes pistillate, erect or the proximal ascending, ca. 20-150-flowered, cylindric; terminal (1-)2-4 spikes staminate, well elevated beyond summit of separate pistillate spikes. Pistillate scales lanceolate ovate, 2.5-4.5(-8.8) × 0.8-1.6 mm, mostly shorter than perigynia, margins entire, apex acute to acuminate (rarely acuminate-awned). Perigynia spreading, often green or straw colored, 9-15-veined, veins running into beak, ovate, 3.6-5.8 × 1.7-2.8 mm, apex contracted; beak (1-)1.2-2 mm, bidentulate, smooth, teeth straight, 0.2-0.7 mm. Stigmas 3. Achenes brown, symmetric, not indented, trigonous, smooth. 2n = 60. Fruiting Jun-Aug. Fens, especially in flarks in patterned fens, bogs and bog pools, lake and stream shores, often in shallow water or on floating mats; 0-1600 m; Greenland; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., Que., Sask., Yukon; Alaska, Mich, Minn., Mont., Wash., Wis.; Eurasia. Carex rostrata is infrequent and local in large portions of its range, often forming large colonies where found. Carex rostrata hybridizes with C. oligosperma and C. saxatilis; rare sterile intermediates with C. utriculata are likely hybrids. The vast majority of records of C. rostrata from North America are C. utriculata.
Similar to no. 218 [Carex utriculata Boott], and often confused with it, but the lvs papillate- glaucous on the upper surface, mostly 1.5-4 mm wide, tending to be involute; perigynia avg smaller; 2n=60, 72, 74, 76. Wet soil or shallow water; circumboreal, s. to n. Mich. and n. Minn.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.