Fimbristylis complanata

Slender annuals; culms 10-20 cm high, tufted, trigonous at base, flat below the inflorescence, smooth. Leaves 8-12 x 0.2 cm, linear, flat, with recurved margins; sheath rounded; ligule a row of short hairs. Umbels 2.5 x 2 cm, contracted; rays 1-3, spreading. Spikelets 5 x 1.5 mm, lanceolate, solitary or 2-3 together, 15-flowered; glumes 2.2 x 1.5 mm, lanceolate, 3-nerved, dark brown. Nut 1 x 0.6 mm, obovate, sparsely tubercled, pale brown.

Fimbristylis complanata is has a pantropical distribution. The species occurs throughout much of Africa south of the Sahara, Yemen, the Seychelles, east through the Indian sub-continent to China, Japan, Korea and Taiwan, south through South-east Asia to northern Australia, on some Pacific Ocean islands and from Mexico south throughout much of South America (The Board of Trustees of the Royal Botanic Garden, Kew 2012).
This is a perennial which grows up to 15-80 cm high (Verma 2001). It grows on muddy river banks, swampy grasslands, sea shore, wet rice fields but it is not confined to wetlands and is common on open grasslands and on cultivated or disturbed ground (Cook 1996). It is associated with Bacopa monnieri, Eleocharis geniculata and Kyllinga monoceohala (Daniel and Umamaheswari 2001). It is seen in wet open grasslands at low altitudes, and also in and around cultivated fields, especially margins of rice fields (Dassanayake and Fosberg 1985).

References:

The Plant List Ver.1.1
 WCSP  Flora of China  FOC illustration  Flora of Pakistan  Annotated checklist of Flowering plants of Nepal India Biodiversity Portal  IUCN Red List (LC)