Cyperus pennatiformis var. pennatiformis has short, ligneous rhizomes clothed with brown scales and fibrous remnants. Its culms are 40–120 cm tall and 3–7 mm in diameter in the lower part, acutely trigonous with slightly concave sides and smooth texture. There are 3–5 simple leaves per culm, subcoriaceous and flat, linear, as long as or somewhat longer than the culm, measuring 8–17 mm in width. The lower surfaces of the leaves are finely septate-nodulose, with entire margins. The sheaths are brown to reddish-brown, eventually splitting into fine parallel fibers, and stipules are absent. The inflorescences are umbelliform, moderately dense, 4–15 cm long, and 5–25 cm wide, with 5–18 spreading rays, each 2–15 cm long, either unbranched or once branched. The spikes are cylindrical to oblong-ellipsoid, 2–6 cm long, 1.5–2.7 cm wide, and densely bear numerous spikelets. The involucral bracts number 5–9 and are longer than the inflorescence, with the lower ones reaching up to 35 cm long and 22 mm wide. Spikelets are yellowish-brown or grayish-brown, 15–24-flowered, spreading to reflexed, flattened, slightly turgid, and narrowly oblong to oblong-lanceolate, 10–14 (–20) mm long and 1.7–3.5 mm wide. The flowers are bisexual (perfect) and lack corolla. The ovary is superior. Achenes are oblong and trigonous, measuring 2 mm long. Each plant produces a single seed with a thin coat, free from the pericarp and featuring a basal embryo surrounded by abundant endosperm. It is typically found in grasslands and open sites, including ridges and mesic forests, at elevations ranging from 0-1,200 meters.