Sp. Pl. 4(1): 279. 1805.
Plants densely cespitose; rhizome internodes 0.2–6 (–8) mm, 1.6–2.2 mm thick. Culms strongly purple-red to 3.2–8.8 cm high at base, 10–55 cm; longest vegetative shoots 0.88–1.4 times as long as longest culms. Leaves: sheaths glabrous; blades green, widest blades (1.8–) 2.4–4 (–4.6) mm wide, smooth abaxially. Inflorescences 0.46–0.94 (–0.99) of culm height; peduncles of lateral spikes barely scaberulous or smooth; peduncles of terminal spikes 2.6–113 mm, barely to much exceeding lateral spikes; proximal bract with sheath tight, abaxially glabrous, sheath front convex, elongated (0.8–) 1.1–4.0 mm beyond apex; ligules 0.8–4.1 (–6.4) mm; blade uniformly green; distal bract usually much exceeding terminal spike. Spikes 3–5, widely separate or distal 2–3 spikes overlapping; lateral spikes pistillate with 2–8 (–10) perigynia, (4–) 9–19 × 3.8–7.5 (–9.2) mm, ratio of spike length (in mm) to flower number = 1.5–3.8; terminal spikes (7–) 14–32 (–45) × 1.1–2.3 (–3) mm. Pistillate scales 2.8–6.8 × 1.5–2.4 mm, margins whitish, entire, apex with awn 0.7–4.1 mm. Staminate scales 4–5 × 1.3–1.6 mm. Anthers 2.8–3.5 mm. Perigynia distichously imbricate, 51–67-veined, unwrinkled, narrowly obovoidellipsoid, obtusely triangular in cross-section, 3.7–4.7 × 1.6–1.9 (–2) mm, (1.8–) 2.2–2.6 times as long as wide, dull, base gradually tapered, abruptly contracted; beak straight, 0.4–1.2 mm. Achenes obovoid, 2.8–3.4 × 1.5–1.8 mm, tightly enveloped by perigynia; stipe straight, 0.3–0.6 mm; beak straight, (0.05–) 0.2–0.4 (–0.5) mm.
Phenology: Fruiting spring–early summer.
Habitat: Mesic or dry-mesic deciduous forests, usually in calcium-rich loams on rocky slopes above streams
Elevation: 40–800 m
Ont., Que., Ala., Ark., Conn., Del., D.C., Ga., Ill., Ind., Iowa, Kans., Ky., Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.J., N.Y., N.C., Ohio, Okla., Pa., Tenn., Vt., Va., W.Va., Wis.
Carex oligocarpa is frequent, particularly in the Midwest. When C. oligocarpa grows with C. planispicata, it usually grows farther upslope than C. planispicata, apparently in drier soils. The two species do grow intermingled in some localities.