Robust, emergent monoecious aquatic perennial to 3 m high.
Upper leaves with sheath auriculate. Leaves with blade to 1.5 m long, to 25 mm wide. Inflorescence with male and female spikes separated by 0–0.5 cm. Mature female spike 10–20 cm long, 15–25 mm diam., usually dark brown, sometimes red-brown; floral bracts absent; stigma narrow-obovate to obovate, usually flat, somewhat fleshy and tipped with black. Male spike 6–16 cm long, 10–15 mm diam.; pollen shed as tetrads.