Plants loosely cespitose, often in large clumps; rhizomes slender, long. Culms arching, weak, 20-70 cm. Leaves: sheaths pale brown abaxially, inner band tight, thin, hyaline, truncate at summit; ligules as wide as long; blades green, flat or channeled, 5-20 cm × 0.25-2 mm. Inflorescences nodding, 5-10 cm × 3-4 mm; proximal bracts bristlelike, 3-10 cm, exceeding inflorescences. Spikes 1-3, remote, 2-4 cm apart, containing 1-5 perigynia, ovoid, 4-6 × 3-4 mm. Pistillate scales whitish with green, 3-veined center, ovate, subequal to perigynia, apex acute (mucronate). Perigynia ascending, pale green or brownish in age, finely many-veined, ovate-elliptic, 2.5-3.7 × 1.5-2 mm, subcoriaceous; beak entire or with few small teeth. Achenes brown, elliptic-oblong, 1.75-2 × 1.25(-1.5) mm, glossy. 2n = 60.
Fruiting Jun-Aug. Mires, especially sphagnum bogs, wet woods, lowlands; 0-1300 m; Greenland; St. Pierre and Miquelon; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Sask.; Conn., Del., Ill., Ind., Maine, Md., Mass., Mich., Minn., N.H., N.J., N.Y., Ohio, Pa., R.I., Vt., W.Va., Wis.
Carex trisperma var. billingsii is a reduced type, distinguished by its narrow leaf-blades (0.3-0.5 mm) and fewer (1-2) perigynia per spike than typical. It is known, at least, from Ontario, Quebec, Newfoundland, New Brunswick, Nova Scotia, Prince Edward Island, Maine, Massachusetts, Michigan, New Jersey, New York, Rhode Island, and Pennsylvania.
Loosely tufted on short, slender rhizomes; stems very slender and weak, 2-7 dm; spikes (1)2(3), sessile, 1-4 cm apart in a slender, often flexuous infl, each with 1-5 perigynia and a few basal staminate fls; lowest spike subtended by a setaceous bract 2-4 cm; scales ovate, acute, hyaline with a green center, shorter than or about equaling the perigynia, these thickly planoconvex, oval, 2.6-4 mm, finely many-nerved, the slender, smooth, emarginate beak 0.5 mm; achene lenticular, oval-oblong, filling the perigynium; 2n=60. Typical plants have lvs 1-2 mm wide and usually 2-5 perigynia per spike. Plants from Nf. to Vt. and Pa., mostly near the coast, with setaceous lvs 0.5 mm wide, and with only 1 or 2 somewhat smaller perigynia, have been called var. billingsii Knight.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.