Shining flatsedge

Cyperus rivularis Kunth

Herbs, annual, densely cespitose, with fibrous roots. Culms roundly trigonous, 3–25(–30) cm × 0.3–1.4 mm, glabrous. Leaves 1–3, V-shaped, 1–8 cm × 1–2 mm. Inflorescences: spike 1, loosely ovoid, compressed, 7–14 × 9–14 mm; rays 1–4, to 2(–3) cm; bracts 2–3, horizontal to ascending at 30(–45)°, V-shaped to flat, 1–12 cm × 0.5–1.5 mm. Spikelets 3–5(–8), flattened, oblong to oblong-lanceoloid, 8–18 × 2–2.6(–3) mm; floral scales (6–)10–26(–32), closely imbricate, laterally light to dark brown, medially light brown, laterally ribless, medially 2–3-ribbed, 2-keeled, oblong to ovate, 1.9–2.7 × 1.8–2.3 mm, apex obtuse. Flowers: stamens 2–3; anthers 0.4–0.5 mm, connective not prolonged; styles 0.6–1 mm; stigmas 1–1.5 mm. Achenes black, network of ridges forming isodiametric or square cells, sessile, obovoid to ovoid, 1–1.3(–1.5) × (0.6–)0.8 mm, apex apiculate, surfaces minutely punctate.

Fruiting summer. Emergent shorelines, ditches, puddles, often in disturbed places; 0–1500 m; Ont., Que.; Ala., Ariz., Ark., Calif., Colo., Conn., Del., D.C., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis.; Mexico; Central America; South America.