Olive hymenachne

Hymenachne amplexicaulis, the West Indian marsh grass or olive hymenachne, is a robust, semi-aquatic perennial grass native to tropical and subtropical wetlands of South and Central America. It has become a serious invasive weed of seasonally flooded wetlands in northern Australia and parts of Southeast Asia. Plants produce stout, hollow, spongy culms 1-3 m long that root freely at the lower nodes and can float on water. The bright-green leaves are flat, lanceolate, 20-45 cm long and 1-3 cm wide, with a markedly cordate, stem-clasping base, the feature reflected in the species epithet amplexicaulis. The inflorescence is a dense, cylindrical, spike-like contracted panicle 20-40 cm long and only 1-2 cm thick, bearing crowded, narrowly lanceolate one-flowered spikelets 4-5 mm long. It thrives in deep, fertile swamp soils that are permanently or seasonally inundated.