Perennial, stout, aquatic with evergreen aerial parts. This species is distinguished from S. melanomphala by its leaves crowded towards the base of the culm, by its lax, copiously branched panicles, by its adaxially densely-hirsute female glumes and by its strongly-beaked achene which lacks a black apex.
Perennial. Rhizome elongate or contracted, 6-10 mm thick; scales brown or reddish-brown. Culms: 1-2 m tall, glabrous. Leaves mostly crowded towards base of culm, 5-12 mm broad, tapering smoothly towards apex or, rarely, unequally laterally praemorse; glabrescent or hispidulous; sheaths with concave mouths, the margins minutely ciliate. Inflorescence with lax, copious terminal panicle c. 150 mm long and smaller lateral panicles 4-7 per node at 2-3 nodes exserted up to 200 mm from sheaths. Bracts foliaceous. Male spikelets sessile - shortly pedicillate, c. 5 mm long; glumes reddish-black proximally, castaneous distally; midrib hispidulous, margins setulose-ciliate. Female glumes similarly coloured, shortly awned, midrib and awn hispidulous, margins setulose-ciliate and distal half of adaxial surface with dense indumentum of stiff, upward-pointing hairs. Achene broadly ovoid, laterally compressed, strongly beaked, 6 x 3.5 mm; glabrous, smooth, pale brown or pinkish-brown sometimes with violet blotches; hypogynium zoniform, brown.