Plants cespitose; rhizomes short, tough, fibrous. Culms: fertile ones erect or reclining; nodes sometimes with axillary bulblets. Leaves 14–22(–26) per culm; sheaths of proximal leaves green or brownish; proximal sheaths and blades with septa usually few, inconspicuous; blades 16–31 cm × 5–8 mm. Inflorescences terminal, rarely also with 1 lateral inflorescence from distal leaf axil; rays divaricate, smooth throughout or scabrous distally, rays usually with axillary bulblets; bases of involucral bracts green, not glutinous. Spikelets in dense clusters of 3–9 (largest cluster with 6 or more), spikelets sessile, broadly ovoid, 2.5–4 × 1.6–3 mm; scales reddish brown with green midribs, circular or nearly so, 1–1.5 mm, apex mucronate or occasionally short-awned, mucro or awn 0.2–0.3 mm. Flowers: perianth bristles persistent, 6, stout, contorted, much longer than achene and projecting beyond it, retrorsely barbed in distal 1/2, exserted from scales at maturity; styles 3-fid. Achenes pale brown, obovate or nearly obtriangular in outline, plumply trigonous or plano-convex, 1.1–1.4(–1.8) × 0.8–1 mm. 2n = 58.

Fruiting summer (Jul–Aug). Along wooded streams and other swampy places, usually shaded by trees; 0–1000 m; Ala., Ark., Conn., D.C., Ga., Ill., Ind., Ky., Md., Mass., Miss., Mo., N.H., N.J., N.Y., N.C., Ohio, Pa., S.C., Tenn., Vt., Va., W.Va.

Scirpus polyphyllus occasionally hybridizes with S. atrovirens.