Sp. Pl. 1: 46. 1753.
Herbs, annual, cespitose. Culms trigonous, 2–30 cm × 0.6–1.1 mm, glabrous. Leaves flat, 4–10 cm × 2–4 mm. Inflorescences: rays 1–3, 0.2–1.5 cm; 2d order rays present in robust plants, to 5 mm; bracts 2–3, longest erect, others spreading, 1–20 cm × 1.5–3 mm. Spikelets 3–12, narrowly ellipsoid, flattened, 3–7 × 0.9–1.2 mm; floral scales 8–12(–16), laterally purplish brown, medially greenish yellow, 3-ribbed medially, orbiculate, 0.9–1.1 × 1 mm, apex mucronate. Flowers: stamen 1; anthers ellipsoid, 0.2 mm, connective not prolonged; styles 0.3–0.4 mm; stigmas 0.3 mm. Achenes light brown, ellipsoid, 0.7–0.9 × 0.4 mm, base barely stipelike to narrowly cuneate, apex acute, surfaces glabrous.
Phenology: Fruiting summer.
Habitat: Damp, disturbed soils, emergent shorelines, puddles
Elevation: 0–1700 m
Introduced; Ont., Que., Calif., Conn., Md., Mass., Mo., Nebr., Nev., N.J., Pa., S.Dak., Va., Eurasia.
A native of temperate Eurasia, Cyperus fuscus is intermittently adventive and locally established 35°–45° N latitude. The report from New York (M. L. Fernald 1950) is based on a misidentification of C. diandrus (R. S. Mitchell and G. C. Tucker 1997).