Plants annual; tufted. Culms 20–120 cm. Ligules 5–11 mm, pubescent, entire or lacerate, usually folded back; blades 4–10(20) mm wide, flat, scab-rous. Panicles 7–30 cm; branches spikelike, usually 1–2 cm. Spikelets 2–3 mm, round to ovate in side view, with 1 floret, a second undeveloped or well-developed floret occasionally present. Glumes appearing inflated, strongly keeled, 3-veined, apiculate; lemmas 2.4–3.5 mm, unawned, sometimes mucronate; paleas subequal to the lemmas, acute; anthers 0.5–1(1.5) mm, pale yellow. Caryopses shorter than 2 mm, light to medium brown. 2n = 14.
Beckmannia syzigachne grows in damp habitats such as marshes, floodplains, the edges of ponds, lakes, streams, and ditches, and in standing water. It is a good forage grass, but frequently grows in easily damaged habitats.