Perennial with annual aerial parts. This species is morphologically similar to S. rehmannii C.B.Cl. and S. longispiculata Nelmes. All three species have ± horizontal, woody rhizomes; that of S. welwitschii is reddish, those of S. rehmannii and S. longispiculata pale stramineous. The inflorescences of S. welwitschii and S. rehmannii are simply glomerate-spicate or, in S. welwitschii and less often in S. rehmannii with one or a few lateral branches from the proximal glomerules, whereas that of S. longispiculata is unbranched. The inflorescence of S. welwitschii is lax and drooping, those of S. rehmannii and S. longispiculata are rigid, ±erect. Spikelets of 5. welwitschii are 5-8 mm long; of S. rehmannii 4-5 mm long; of S. longispiculata 8-9 mm long. The glumes of S. welwitschii are fulvous-stramineous or castaneous; of S. rehmannii wholly or partly blackish-red; of S. longispiculata light castaneous. Achenes of S. welwitschii and S. longispiculata are smooth; those of S. rehmannii smooth or lightly tuberculate. Achenes of S. welwitschii and S. rehmannii are grey or light brown; of S. longispiculata brown with the stipe black. Achenes of S. welwitschii and S. rehmannii are ± 2 mm long; those of S. longispiculata ± 4 mm long.
Perennial. Rhizomes 3-4 mm thick, woody, red; scales stramineous-reddish. Culms 0.30-1 m tall, villous to glabrescent, 5-7 mm distant. Leaves 2-3 mm broad, glabrous to villous; sheath with mouth produced into a triangular or rounded, villous or glabrescent tongue 1-2 mm long. Inflorescence branched or unbranched, (60)-150-250 mm long, drooping. Glomerules (1)-3-8 per branch, of 1-2-(6) sessile spikelets. Bracts shortly awned, shorter than the glomerules. Spikelets 5-7-(8) mm long. Glumes 3-5 mm long, glabrous or minutely hispidulous distally, shortly awned, pale stramineous with faint reddish streaks. Achene ellipsoid to ovoid, 1.5-1.8 x 1-1.2 mm, glabrous, smooth, grey.