Mém. Acad. Imp. Sci. St. Pétersbourg Divers Savans 2: 168. 1832.
Plants cespitose. Culms 30–80 cm, distally scabrous. Leaves basal and cauline, 4–8 mm wide; proximal leaves reduced to sheaths. Inflorescences: proximal bracts usually exceeding, occasionally shorter than, inflorescences; spikes separate, the proximal often distant, pendent, long-pendunculate, elongate, cylindric or clavate, 10–40 × 7–9 mm; lateral 4–6 (–9) spikes pistillate; terminal spike gynecandrous. Pistillate scales dark-brown or black to margins, ovate or lanceolate, conspicuously shorter and narrower than perigynia, midvein lighter colored than body, conspicuous, often raised, prominent, mucronate. Perigynia ascending, green becoming pale-yellow or brown, faintly veined, ovate, 4–5 × 2.5–3.5 mm, apex gradually beaked, smooth; beak 0.3–0.4 mm, truncate or obscurely bidendate, smooth. Achenes filling proximal 1/2 or less of perigynia. 2n = 62.
Phenology: Fruiting Jun–Aug.
Habitat: Open forests, meadows, stream banks
Elevation: 0–2000 m
Alta., B.C., Alaska, Calif., Idaho, Mont., Oreg., Wash., Asia
Carex mertensii is represented in Japan and the Russian Far East by the vicariant C. urostachys Franchet [C. mertensii J. D. Prescott var. urostachys (Franchet) Kükenthal].