Enum. Pl. 2: 294. 1805.
Plants perennial, densely cespitose, 80–150(–200) cm, bases deep set, stout; rhizomes absent. Leaves erect or ascending, 1/2–2/3 plant height, bases of leaves hard, leathery, usually dark brown or castaneous; sheaths distally bristly-ciliate, backs chestnut brown, glabrous; ligule absent; blades narrowly linear, 1–2(–3) mm wide or thick, mostly strongly involute or adaxially deeply sulcate, margins scabridulous, surfaces glabrous. Inflorescences: anthelae mostly compound, ascending-branched, longer than broad; scapes wandlike, narrowly linear, 1.5–2(–3) mm thick, distally round or slightly compressed; proximalmost leafy involucral bract mostly shorter than anthela or equaling it, rarely slightly longer. Spikelets chestnut brown to dull brown, ellipsoid, ovoid, or cylindric, 5–20 mm; fertile scales broadly ovate to nearly orbiculate, 3.5–4.5 mm, apex rounded, sometimes ciliolate, midrib reaching tip or excurrent as short mucro. Flowers: stamens 2–3; styles 2-fid, flat, fimbriate. Achenes lustrous brown, lenticular-obovoid or obpyriform, 1.5–2 mm, appearing striate, with many fine, vertical lines of isodiametric pits. 2n = 20.
Phenology: Fruiting summer–fall, all year southward.
Habitat: Salt marsh and brackish marsh inland
Elevation: 0–50 m
Ala., Del., D.C., Fla., Ga., La., Md., Miss., N.J., N.Y., N.C., S.C., Tex., Va., Mexico, West Indies (Antilles), Central America.
Fimbristylis castanea, commonly placed in synonymy of F. spadicea (Linnaeus) Vahl, a widespread salt marsh perennial of tropical America, is distinguishable by its relatively shorter spikelets, usually lower habit, and by its proportionately shorter involucral bracts. Fimbristylis spadicea is hardy with us only as a greenhouse plant.