Plants perennial, cespitose, 80-150(-170) cm, coarse; rhizomes absent. Culms stiffly erect, leafy, triangular, multiribbed. Leaves ascending, overtopped by inflorescence; blades flat proximally, 3-10(-15) mm wide, apex attenuate, trigonous. Inflorescences terminal and axillary, narrow, clusters of corymbs, clusters dense, mostly broadly turbinate, 13-15 mm; bracteal leaves mostly exceeding subtended compounds. Spikelets brown, lanceoloid, 13-15 mm, apex acuminate; fertile scales lanceolate, 10-13 mm, apex acuminate, midrib short excurrent. Flowers: longer perianth bristles usually fully 2 times length of fruit body, antrorsely barbellate. Fruits 1-2 per spikelet, 20-25 mm; body pyriform obovoid, compressed, 5-6 × 2.6-3.6 mm; tubercle attenuate, 2-grooved, (15-)18-20(-21) mm. 2n = 18. Fruiting summer-fall. Acidic sunny wetlands, mostly pond shores, seeps, bogs, marshlands; 0-400 m; Ala., Ark., Conn., Del., D.C., Fla., Ga., Ind., Kans., Ky., La., Maine, Md., Mass., Mich., Mo., N.J., N.Y., N.C., Okla., R.I., S.C., Tenn., Tex., Va. Rhynchospora macrostachya is quickly distinguished from other species of its complex by more compact clusters, arranged on successive mid and distal nodes to present a narrow inflorescence outline. Its perianth bristles and fruit tubercles are the longest in the complex, probably in the entire genus.