Cyperus diandrus Torrey var. castaneus S. Watson; C. niger var. castaneus (S. Watson) Kükenthal

Herbs, perennial, rhizomatous. Culms solitary or close together, trigonous, 5–40(–95) cm, glabrous. Leaves (2–)3–7, broadly V-shaped, 3–20(–30) cm × 1.5–3 mm. Inflorescences: spikes loosely to densely ovoid to hemispheric, 7–16 mm diam.; rachis 1–2 mm, or absent; rays 1–2, 3–45 mm; if rays absent, inflorescence sessile, dense, 6–20 mm diam; bracts 2–3, horizontal to reflexed downward, broadly V-shaped, 1–15 cm × 1–3 mm. Spikelets (3–)5–25(–60), linear to oblong-linear, (3–)5–9 × 1.8–2.3 mm; floral scales 4–18, closely imbricate, chestnut brown, black, or brown, medially green or greenish brown, laterally ribless, medially 2(–3)-ribbed, distinctly 2-keeled basally, ovate to orbiculate, 1.5–2.1 × 1.4–2.2 mm, apex obtuse. Flowers: stamens 2; anthers 0.6–0.8 mm, connectives not prolonged; styles 0.4–0.6 mm; stigmas 0.7–2.1 mm. Achenes brown, ellipsoid, 1.2–1.4 × 0.6–0.8 mm, network of ridges forming isodiametric or square cells, slightly stipitate, apex apiculate, surfaces minutely punctate.

Fruiting summer. Swamps, ditches, wet pastures, seeps in montane forests; 0–1200 m; Ariz., Calif., N.Mex., Okla., Tex.; Mexico; Central America; South America.

Six varieties (var. niger, var. castaneus, var. robustus, var. lorentizianus, var. intricatoramosus, and var. pseudoelegantulus) were recognized by G. Kükenthal (1935–1936) under the illegitimate name Cyperus niger Ruiz & Pavon (1798), not Salisbury (1796).