Mimosa pigra

Mimosa pigra is a leguminous shrub, which can reach up to 6m in height. The stem is greenish in young plants but becomes woody as the plant matures. It is armed with broad-based prickles up to 7mm long. The leaves are bright green and bipinnate, consisting of a central prickly rachis 20 to 25 cm long with up to 16 pairs of pinnae 5 cm long, each divided into pairs of leaflets 3 to 8 mm long. Leaves are sensitive and fold up when touched and at nightfall. Flowers are mauve or pink, born in tight, subglobose pedunculate heads 1 cm in diameter, each containing approximately 100 flowers. Each flower head produces a cluster of 10 to 20 seapods, which then mature and break into segments, each containing an oblong shaped seed. Hairs on the segments allow them to float on water and stick to hair or clothing, hence aiding in dispersal. Ripe seeds are light brown to brown or olive green. Mimosa is hard seeded. Seeds can survive at least 23 years on sandy soils, but seed viability decreases more rapidly on clay soils. Mimosa pigra can germinate year round if the soil is moist but not flooded. However, most germination takes place at the start and end of the wet season. Growth in seedling is rapid, and flowering occurs between 4 and 12 months after germination. The process from flower bud to ripe seed takes about five weeks.