Ann. Lyceum Nat. Hist. New York. 3: 431. 1836.
Herbs, perennial, rhizomatous. Culms trigonous, 40–70 (–100) cm × 2–3 mm, scabrid or hirtellate, either immediately proximal to apex or over distal 1/2 of culm. Leaves flat, 30–80 cm × 4–10 mm, scabrid on margins, ribs on abaxial surface. Inflorescences: spikes ovoid, 12–20 × 13–18 mm; rays 3–6, 4–10 (–16) cm, glabrous; bracts 3–5, ± horizontal to reflexed, 3–12 (–36) cm × 0.4–3 (–5) mm, scabrid like leaves; rachilla deciduous, essentially wingless or wings inconspicuous, hyaline, 0.1 (–0.2) mm wide. Spikelets 25–50 (–80), oblong, quadrangular-compressed, 5–10 × 2.5–3.5 mm; floral scales deciduous, 4–10 (–14), marginally clear, laterally brownish to clear distally, or light-brown or reddish-brown, blunt, laterally 3 (–4) -ribbed, ovatelanceolate, 2.5–3 × (1.2–) 1.4–2 mm, often erose apically, apex acute to obtuse, entire or emarginate. Flowers: anthers 0.4–0.6 mm; styles 0.8–1.2 mm; stigmas 2 mm. Achenes brown to reddish-brown, ± stipitate, ellipsoid, 1.2 × 0.5–0.6 mm, apex obtuse, scarcely to distinctly apiculate, surfaces puncticulate.
Phenology: Fruiting summer.
Habitat: Wet prairies
Elevation: 0–50 m
La., Okla., Tex., South America (Argentina), South America (Brazil), South America (Paraguay), South America (Uruguay)
The combination of spreading floral scales and scabrid culms separates this rather uncommon species from any others occurring in its range. The scarcity of Cyperus cephalanthus in the United States and its disjunct distribution suggest it might be naturalized rather than native. It was collected early in the nineteenth century in Louisiana, and the amphitropical distribution is not without parallel in the genus (cf. 12. C. eragrostis).
Cyperus cephalanthus has been treated as a variety of the widespread, polymorphic South American C. laetus (G. Kükenthal 1935–1936). Recognition of C. cephalanthus as a species follows recent American floristic practice.