Rhizomes 2-6 mm diam.; scales much shorter than internodes, disintegrating to fibers. Culms sharply trigonous, sides convex to flat near base, concave distally, 0.7-2.3 m × 2.5-6 mm, smooth. Leaves 3-5, basal; sheath fronts not pinnate-fibrillose; blades 2-3, proximally V-shaped, distally trigonous to asymmetrically laterally flattened in cross section, angles distally scabridulous; distal blade 1-1.5 times as long as sheath, 9-30 cm × 2-7 mm. Inflorescences capitate or rarely with 1-2 branches to 5 mm; proximal bract usually erect, resembling leaf blade, trigonous proximally, (1-)3-20 cm. Spikelets 3-35, 6-15 × 4-5 mm; scales bright orange-brown or partly purplish, ovate, 3.5-5.5 × 2-3 mm, smooth or awn sparsely spinulose, margins deciduously ciliolate, flanks of proximal scale sometimes with a few ribs, midrib prominent, apex acute, notch 0.6-1 mm deep, awn irregularly bent, 1-1.5 mm. Flowers: perianth members 3-6, orange-brown, bristlelike, very slender, some equaling achene, some often rudimentary, retrorsely spinulose; anthers 2-3 mm; styles 2-fid. Achenes brown, unequally biconvex, obovoid to obpyriform, 1.9-2.6 × 1.3-1.7 mm; beak 0.1-0.3 mm. 2n = 78.
Fruiting late spring-summer. Brackish, coastal shores, marshes, ditches, waste places; 0-100 m; Ala., Fla., Kans., La., Miss., Mo., Tex.
Robust plants of Schoenoplectus deltarum are very distinct because of their many large spikelets; less robust plants closely resemble S. pungens. As Schoenoplectus deltarum combines some characteristics of S. americanus with most characteristics of S. pungens, it may be of hybrid origin. Intermediates between these species occur along the coast from North Carolina to Florida. Schoenoplectus deltarum is often dominant in much of the marsh on mudflats formed in the Mississippi River delta, and it is a major food of geese wintering there.