Description from Flora of China
Cyperus sulcinux C. B. Clarke, J. Linn. Soc., Bot. 21: 56. 1884; Pycreus substellatus E. G. Camus.
Annuals. Roots fibrous. Culms tufted, 5-40 cm tall, slender, 3-angled, smooth, with a few leaves basally. Leaves shorter than culm; sheath purplish brown; leaf blade 0.5-2 mm wide, usually folded, apical margin scabrous. Involucral bracts 3-5, spreading, leaflike, usually basal 1 or 2 longer than inflorescence. Inflorescence a simple anthela; rays 3-8, mostly to 7 cm, slender, each with 3-15 spikelets arranged into a spike. Spikelets spreading, linear, 0.5-1.5[-3.5] cm × 1-1.5 mm, compressed, 8-20-flowered; rachilla flexuose, narrowly winged. Glumes straw-colored to brownish yellow on both surfaces, lax, ovate to oblong-ovate, ca. 1.8 mm, medially veins 3 and green, laterally membranous and veinless, margin narrowly white hyaline, apex obtuse and mucronate. Stamen 1; anther oblong, ca. 0.3 mm. Style of medium length; stigmas 2. Nutlet blackish brown, oblong, 1.2-1.3 mm, ca. 2/3 as long as subtending glume, slightly compressed, both surfaces concave sulcate and puncticulate. Fl. and fr. Jun-Aug.
Under sparse forests in valleys, along trails on mountain slopes; 100-500 m. Guangdong, Guangxi, Hainan, Taiwan, SE Xizang, Yunnan [Bangladesh, Bhutan, Cambodia, India, Indonesia, Malaysia, Myanmar, Papua New Guinea, Philippines, Thailand, Vietnam; N Australia, Pacific islands].