Calamagrostis expansa is a robust, short-rhizomatous perennial with erect or decumbent culms, 0.5 to 2 m (1.6 to 6.6 ft) tall and 4 to 8 mm (0.16 to 0.3 in) in diameter. Sheaths are 3.5 to 4 mm (0.14 to 0.16 in) long and overlap closely along the middle of the culm. Leaf blades are flat to involute, 15 to 20 cm (6 to 8 in) long, 1 to 3 cm (0.4 to 1.2 in) wide, with the uppermost leaf blade reduced and hard-pointed. Inflorescences are paniculate, oblong, 15 to 30 cm (6 to 11.8 in) long and devoid of spikelets on the lower half of the branch. The1.5 to 2 mm (0.06 to 0.08 in) rachilla is obscured by long, whitish yellow, silky hairs. The fruit is pale brown, ovoid, 2 to 2.5 mm (0.08 to 0.1in) long, slightly grooved ventrally, with an apiculate apex.