Plants perennial, tall, and reedlike, with stout, knotty rhizomes, forming large colonies. Flowering stems 200–600 cm long, stout, unbranched or few‑branched, glabrous. Leaves all on the flowering stems. Leaf sheaths glabrous, the ligules membranous with a minutely hairy margin. Leaf blades 40–75 cm long, 30–70 mm wide, flat, somewhat clasping at the base, roughened along the margin, but otherwise glabrous, the midvein noticeably thickened beneath. Inflorescences large, dense panicles with numerous, ascending branches. Spikelets 10–15 mm long, with 2–4 florets, obovate to obtriangular in outline, somewhat flattened, the rachilla glabrous between the florets. Glumes 8–13 mm long, similar in size and shape, nearly as long as the spikelet, narrowly lanceolate, tapered to a sharply pointed tip, 3(5)‑nerved, glabrous. Lemmas 5–10 mm long, lanceolate, wrapped around the palea and fruit, tapered to a sharply pointed or more commonly short‑awned tip, 3‑ or 5‑nerved, densely long‑hairy in the basal half. Paleas shorter than the lemmas. Fruits not produced (narrowly oblong elsewhere). 2n=110. September–November.

Introduced, uncommon, and widely scattered in Missouri (native of southern Europe, southern Asia, and northern Africa; escaped from cultivation in the southern U.S.). Open, disturbed areas.

Giant reed is cultivated as an ornamental and sometimes escapes when rhizome pieces are dispersed by water or discarded by humans. Once established, it can form large, dense colonies but does not appear to be invasive in natural communities in Missouri. In its native range, this species is used for building materials (both stems and leaves), woven mats, and pulp for paper. The reeds of woodwind instruments are cut from its stems. For discussion of differences with the superficially similar giant cane (Arundinaria gigantea (Walter) Muhl., tribe Bambuseae), see the treatment of that species.