Amer. J. Sci. Arts 9: 63, plate C, fig. 12. 1825.
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.
Phenology: Fruiting Jun–Aug.
Habitat: Mires, especially sphagnum bogs, wet woods, lowlands
Elevation: 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.