Description from Flora of China
Perennials. Rhizomes short, hardened. Culms 25-80 cm tall, slightly thick, triquetrous, smooth, several leaved at basal part. Leaves equaling or shorter than culm; sheath pale green and eventually reddish brown; leaf blade 0.3-2 cm wide, flat, margin scabrous. Involucral bracts 6-12, leaflike, longer than inflorescence, 0.6-1.6(-1.9) cm wide. Inflorescence a decompound anthela; rays many, mostly 12-16 cm, each with 2-8 raylets. Spikelets 1-5, sessile, sometimes more than 3 at apex of raylets or tertiary raylets, oblong to linear-oblong, 3-7(-14) × 1.5-2 mm, slightly turgid, 6-12(-22)-flowered; rachilla narrowly winged. Glumes reddish brown or rusty brown on both surfaces but middle abaxially green or grayish green, lax, broadly ovate to orbicular-ovate, ca. 2 mm, 7-11-veined, keel prominent, apex rounded and with recurved mucro. Stamens 3; anthers linear, apex with white setae. Style very short; stigmas 3. Nutlet dark brown, ellipsoid, ca. 3/4 as long as subtending glume, 3-sided. Fl. and fr. Jun-Sep.
This was referred to as Cyperus laxus Lamarck by D. A. Simpson and T. Koyama (Fl. Thailand 6(4): 366. 1998), but G. C. Tucker (Syst. Bot. Mongr. 43: 44. 1994) noted a distinction between neotropical and paleotropical material, with the latter being recognized as C. diffusus, which we accept here.
Forests in valleys, grasslands on mountain slopes, wet places by rivers, along trails, paddy fields; 100-1700 m. Guangdong, Guangxi, Hainan, Taiwan, SE Xizang, S Yunnan [Bhutan, Cambodia, India, Indonesia, Laos, Malaysia, Myanmar, Nepal, Philippines, Sri Lanka, Thailand, Vietnam; NE Australia, Indian Ocean islands, Pacific islands].