Dangerous exotic tree attacks HCM City
Update: 20/01/2009
Mai duong (mimosa pigra), listed among the 100 most invasive species in the world by the Conservation of Nature and Natural Resources (IUCN), is growing in 44 ha of land in 11 districts of HCM City. On Thursday, Jan. 15, 2009 the city conducted a campaign to destroy mai duong in District 9.
The Chief of the HCM City Plant Protection Agency, Nguyen Van Duc Tien, said this tree is developing in the districts of 2, 9, 7, 12, Thu Duc, Nha Be, Can Gio, Binh Chanh, Cu Chi, Hoc Mon, and Binh Tay, totalling around 44 ha of land.
The agency recently sent a report warning about the danger caused by the spread of mai duong to the HCM City Department of Agriculture and Rural Development. It is lucky that this tree has just been developing in unused areas, not cultivated land yet.
The local Department of Agriculture and Rural Development said it would carry out a survey about the encroachment of mai duong in the city and define areas where mai duong will be destroyed first.
Mimosa pigra is a leguminous shrub, which can reach up to 6 m 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 7 mm 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 seedpods, 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 in sandy soil, but seed viability decreases more rapidly in 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.
Tien said there are several thousands hectares of this invasive tree in the southern province of Dong Nai and the seeds may have been brought to HCM City by the Dong Nai River. If not destroyed timely, this tree will spread quickly in HCM City because the city has many rivers and canals.
In late 2008, Mimosa pigra covered 2,000 ha of the Tram Chim National Park, “casting out” red-headed cranes from this area.
In late 2007, more than 1,000 ha of land around the Tri An Lake was attacked by Mimosa pigra. The lake’s water was seriously polluted while the plants around the lake died. The reason is Mimosa pigra has mimosin, a chemical that poisons many kinds of plants. Plants died and disintegrated, polluting the lake.
People in the Central Highlands are fed up of this kind of tree. The Kon Tum Provincial Department of Agriculture and Rural Development reported nearly 170 ha of land was uncultivable because of the attack of Mimosa pigra. This tree is quickly invading the area along the Se Re Poc River surrounding the Yok Don National Park and the Don Village tourist site.
The Cat Tien National Park conducted projects to kill Mimosa pigra, worth VND 50-100 million a year, but they were ineffective because Mimosa pigra seeds follow the Dong Nai River and come to the park after every rainy season.
People in some locations have asked for the National Assembly to help them kill Mimosa pigra. The Ministry of Natural Resources and the Environment has introduced a four-step method to destroy Mimosa pigra but it seems to be ineffective.