Aquatic or subaquatic perennial 0.3–1.5 m., prostrate to straggling or erect, usually much-branched, considerably rooting at the lower nodes with dense tufts of whitish rhizoids, stem near the base up to 2 cm. thick, spongy, hollow. Upper stem and branches sulcate-striate, glabrous for the most part but increasingly appressed-pilose towards the inflorescence. Leaves lanceolate to lanceolate-oblong or oblong-ovate, cuneate or usually attenuate to the base, acute to acuminate at the apex, moderately appressed-pilose on both surfaces (densely so when young), lamina (2.5) 7–15 × (0.8) 2–5 cm.; petiole 0.4–4 cm. long. Spike c. 4–12 cm. long in flower, elongating to as much as 25 cm. or occasionally even more in fruit; rhachis moderately to densely appressed-pilose; peduncle short, mostly c. 1.5 cm. long. Bracts deltoid-lanceolate, 3–4 mm. long, hyaline (drying pale brownish) with a single midrib, glabrous, finally deflexed below the hard callus left by the fallen perianth; bracteoles more or less circular hyaline, c. 1.5–2 mm. long, glabrous. Perianth 6–8 mm. long, the upper (outer) tepal, 1–3 nerved slightly longer than the remainder, with a sharper, often slightly recurved apex, and a somewhat narrower pale border; remaining tepals blunter, with up to 7 nerves. Filaments stout, 2–3 mm. long. Style 1.75–2.5 mm. long. Capsule c. 4 mm. long, slightly broader at the base but rounded above. Seed smooth, chestnut-brown.