Calamagrostis hillebrandii is a slender, short-rhizomatous perennial with erect, tufted, glabrous culms 30 to 50 cm (12 to 20 in) tall. Sheaths are glabrous, longer than the internodes, with tightly overlapping margins, 10 to 15 cm (12 to 20 in) long, and 2 to 4 mm (0.08 to 0.2 in) wide. Inflorescenes are paniculate, ovate, 5 to 30 cm (2 to 12 in) long, 3 to 7 cm (1 to 3 in) wide, with branches ascending to spreading, pubescent near the base, the lower half of the branches without spikelets. The rachilla are 1.5 to 2 mm (0.06 to 0.08 in) long, silky villous, with an awn up to 4 mm (0.2 in) long.