Carex wahuensis subsp. rubiginosa is a rhizomatous perennial that forms large, dense clumps. The culms are trigonous, ranging from 15 to 100 cm tall, and bear spikes from one-third to one-half from the base upwards. The leaves are simple and 3-ranked, with up to 8 per culm. The blades are linear and flat, more than 3 mm wide, with the apex gradually tapering and long-acuminate. The upper surfaces are deep green, while the lower surfaces are scabrous on veins with coriaceous blades. The margins are entire. The sheaths loosely surround the culm bases and are reddish to dark brown, disintegrating into parallel fibers. Stipules are absent. The flowers are in inflorescences of 2 to 14 nodose, with 2–5 cm long internodes. The spikes are solitary at the nodes, with nodes occasionally having two spikes or three secondary spikes. Each spike is pedunculate, cylindrical, 5–10 cm long, and 5–15 mm wide. The terminal spike is staminate, while the lateral ones are androgynous or pistillate. The bracts are leaf-like, with the lower 1–3 slightly longer than the inflorescence, long-sheathing at the base. The pistillate glumes are brown, membranous, and ovate, measuring 2.5–5 mm long, with hyaline margins and a midnerve prolonged into a scabrous awn 3–5 mm long. The perigynium is chartaceous to subcoriaceous, reddish brown to dark red at maturity, broadly obovoid, 3–8 mm long, and 2.5–4 mm wide. It is many-veined and obtusely trigonous in cross-section, with the apex contracted gradually or abruptly to the beak and sharp apical teeth 0.2–0.9 mm long. The base is obtuse. The flowers are unisexual, and the corolla (petals) is absent. The fruit is a rhombic–ellipsoid, trigonous achene, conspicuously constricted at angles. Each fruit contains one seed, with a thin seed coat free from the pericarp. The embryo is basal, and the endosperm is abundant. It is found primarily in subalpine shrubland and dry forest or shrubland, often on rocky substrates, at elevations ranging from 240-2,500 meters.