Ann. Lyceum Nat. Hist. New York 1: 71. 1824.
Plants extensively colonial; rhizomes long. Culms trigonous in cross-section, 18–65 cm, smooth distally. Leaves: basal sheaths pale-brown; ligules as wide to wider than long; blades pale to mid green, flat to W-shaped, 4–11 mm wide, glabrous. Inflorescences 8–24 cm; proximal bract 11–37 cm, exceeding inflorescence; proximal 2–4 spikes pistillate, erect or often the proximal ascending to spreading; terminal 1 (–2) spikes staminate. Pistillate scales lanceolate-acuminate, 2.4–6.9 × 0.3–0.9 mm, shorter than or the proximal longer than perigynia, margins serrulate-ciliate, apex with prominent scabrous awn. Staminate scales acute to acuminate, smooth except at tip. Perigynia ascending, 7–11-veined, narrowly elliptic, 4.2–7 × 1.3–1.8 mm, apex tapered; beak 1.4–2.4 mm, bidentulate, smooth, teeth straight, 0.2–0.5 mm. Stigmas 3. Achenes pale-brown, trigonous, smooth.
Phenology: Fruiting Jun–Jul.
Habitat: Shallow cold streams, springheads, seeps, margins of fens, and seepy stream, pond, and lakeshores in open or lightly shaded sites, in highly calcareous soils
Elevation: 100–600 m
Ont., Conn., Mass., Mich., Mo., N.Y., Pa., Vt., Va., Wis.
Carex schweinitzii is very local and uncommon but often forms large colonies where present. It hybridizes occasionally with C. hystericina. The single, old Missouri collection (G. Yatskievych 1999+) seems so far disjunct as to suggest that there may have been a label mixup.