Anthistiria arundinacea Roxburgh, Fl. Ind. 1: 256. 1820; A. subsericans Nees ex Steudel; Cymbopogon arundinaceus (Roxburgh) Schultes; Themeda gigantea (Cavanilles) Hackel subsp. arundinacea (Roxburgh) Hackel; T. gigantea var. sub-sericans (Nees ex Steudel) Hackel; T. subsericans (Nees ex Steudel) Ridley.

Perennial. Culms tufted, reedlike, stout, up to 6 m tall, 1–1.5 cm in diam. Leaf sheaths glabrous; leaf blades 50–100 × 1–1.5 cm, scabrid, gradually narrowed to the thick white midrib toward base, acuminate; ligule 1–2 mm. Compound panicle large with many drooping branches, each branch bearing spathes subtending 2–3 spatheoles; spatheoles 2–3.5 cm, glabrous; peduncle pubescent at apex. Raceme composed of 0–2 spikelet pairs and a terminal triad above the involucre of 2 homogamous pairs. Homogamous spikelets arising at slightly different levels, male or barren, 12–20 mm, linear-lanceolate, densely hispid with long, golden, tubercle-based hairs, finely acuminate. Sessile spikelet 7–9.5 mm; callus 2–3.5 mm, narrowly cuneate; lower glume oblong-lanceolate, densely strigose with golden or brown hairs, hairs usually deciduous; awn 4–9 cm. Pedicelled spikelet 13–20 mm. Fl. and fr. Sep–Apr.

Themeda arundinacea belongs to a group of giant Themeda species with stout, solid, reedlike culms and large, drooping panicles, also including T. caudata, T. intermedia, T. trichiata, and T. villosa. Most were treated in old literature at infraspecific rank under T. gigantea (Cavanilles) Hackel, which name is now applied only to a form endemic in the Philippines with awnless racemes of small, hairy spikelets. The species of this complex probably intergrade, and variation is not well understood.

Smaller forms of Themeda arundinacea with culms to 3 m tall, shorter spikelets, and shorter, weaker awns (3–4.5 cm) are sometimes separated as T. subsericans.

Mountain slopes, valley grasslands; 700–2000 m. Guangxi, Gui-zhou, Yunnan [Bangladesh, Bhutan, N India, Indonesia, Laos, Malaysia, Myanmar, Nepal, Philippines, Thailand, Vietnam].