Ischaemum rugosum Salisb.
Family Poaceae, Tribe Andropogoneae
Spikelets heteromorphic. Sessile spikelets dorsally compressed, 3–6 mm long, of 1 fertile floret and one basal sterile floret. Lower glume chartaceous to cartilaginous below with 4–6 pronounced transverse ridges, membranous above, 2-keeled, margins ciliate toward apex; upper glume membranous or chartaceous, with single keel, boat-shaped. Sterile floret hyaline, as long as spikelet. Fertile lemma with bifid apex, apical awn; awn twisted, geniculate, 15–20 mm long. Caryopsis 2–2.5 mm long. Internode 3-angled, inflated, widening toward concave apex, margin ciliate; pedicel similar, but shorter and narrower. Pedicellate spikelets sterile, smaller than sessile spikelets, with reduced awn, often separately deciduous.
Distinct transverse ridges on lower half of lower glume are highly diagnostic for Ischaemum rugosum. Other distinguishing characters: upper glume equal to or slightly longer than lower glume, spikelets less than 6 mm long. Note that hairs on margins of pedicel and internode may be rubbed off.
western coastal Africa and Madagascar; Asia, from the Indian subcontinent through tropical Asia to Australia and the Pacific Islands; tropical South America, Central America, Mexico, West Indies, United States
native to Southeast Asia
warm, humid regions near the equator, both wet and dry habitats; swamps, paddy fields, wet grasslands, plains, low hills
Ischaemum rugosum is a tufted, usually annual grass, to 130 cm tall, that is one of the most serious weeds in rice throughout its natural and introduced range. It is difficult to distinguish from rice prior to flowering, making it difficult to remove during hand-weeding. The grains may contaminate ricestocks.