Leaves bipinnate, sensitive to touch. Straight, erect or forward pointing, prickle at the junction of each of the 6-14 pairs of pinnae and sometimes with stouter spreading or deflexed prickles between the pairs. Seeds light brown to brown or alive green, oblong, light, dispersed by water and floating for an indefinite period. Wet places in the humid and subhumid tropics.
Favors climates with a dry season.
Seeds may remain dormant for many years. A prolific seed producer. On exclusion lists for Hawai'i, US, and French Polynesia. A problem species in Australia and Thailand. Because there is little grassy understorey in thickets of M. pigra , it is difficult to destroy infestations with fire without applying fuel such as gelled gasoline from aircraft.