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      Mimosa tree (Photo: Colin Wilson, Parks & Wildlife Commission, Nothern Territory, Australia) - Click for full size   Mimosa leaves (Photo: Colin Wilson, Parks & Wildlife Commission, Nothern Territory, Australia) - Click for full size   Mimosa leaves (Photo: Colin Wilson, Parks & Wildlife Commission, Nothern Territory, Australia) - Click for full size   Mimosa flowers (Photo: Colin Wilson, Parks & Wildlife Commission, Nothern Territory, Australia) - Click for full size   Mimosa fruit (Photo: Colin Wilson, Parks & Wildlife Commission, Nothern Territory, Australia) - Click for full size
    Taxonomic name: Mimosa pigra L.
    Synonyms: Mimosa pellita
    Common names: bashful plant (English), catclaw (Puerto Rico), catclaw mimosa (English), chi yop (Thai), columbi-da-lagoa (Portuguese), eomrmidera (Spanish), espino (Spanish), giant sensitive plant (English), giant sensitive tree (English), giant trembling plant (English), juquiri (Portuguese), juquiri grand (Portuguese), kembang gajah (Malay), mai yah raap yak (Thai), maiyarap ton (Thai), malicia-de-boi (Portuguese), mimosa (English), mimose (German), putri malu (Indonesian Bahasa), semalu gajah (Malay), sensitiva (Spanish), trinh nu nhon (North Vietnam), una de gato (Spanish), xao ho (South Vietnam)
    Organism type: shrub
    Mimosa pigra is invasive, especially in parts of South East Asia and Australia. It reproduces via buoyant seed pods that can be spread long distances in flood waters. Mimosa pigra has the potential to spread through natural grassland floodplain ecosystems and pastures, converting them into unproductive scrubland which are only able to sustain lower levels of biodiversity. In Thailand Mimosa pigra blocks irrigation systems that supply rice fields, reducing crop yield and harming farming livelihoods. In Vietnam it has invaded unique ecosystems in protected areas, threatening the biodiversity of seasonally inundated grasslands.
    Description
    When mature, Mimosa pigra is an erect, much branched prickly shrub reaching a height of 3m to 6m. Stems are greenish at first but become woody, are up to 3m long, and have randomly scattered, slightly recurved prickles 0.5cm tocm long. Leaves are bright green, 20cm to 25cm long and bipinnate, consisting of about 15 pairs of opposite primary segments 5cm long with sessile, narrowly lanceolate leaflets that fold together when touched or injured and at night. The flowers are pink or mauve, small, regular and grouped into globular heads 1cm to 2cm in diameter. The heads are borne on stalks 2cm to 3cm long, with two in each leaf axil, while the corolla has four lobes with eight pink stamens. The fruit is a thick hairy, 20-25 seeded, flattened pod borne in groups in the leaf axils, each 6.5cm to 7.5cm long and 0.7cm to 1cm wide. The fruit turns brown when mature, breaking into one-seeded segments. The seeds are brown or olive green, oblong, flattened, 4mm to 6mm long, and 2mm wide (Walden et al. 1999).
    Occurs in:
    agricultural areas, coastland, natural forests, planted forests, range/grasslands, riparian zones, ruderal/disturbed, scrub/shrublands, urban areas, water courses, wetlands
    Habitat description
    Mimosa favours a wet-dry tropical climate and grows in open, moist sites such as floodplains, coastal plains and river banks. For example, in the Mekong Delta, Vietnam, where it is a serious weed annual rainfall levels may reach up to 2200cm. It may not be a major problem in regions with an annual rainfall of less than 75mm or greater than 2250mm. In both Australia and Vietnam it prefers to invade seasonally inundated grassland (Walden et al. 1999; Triet et al Undated).

    It is more likely to colonise and eventually cause problems in disturbed areas. This is due to the ability of Mimosa seeds to establish rapidly on bare soils, which lack competitive pressures imposed by other seedlings (Lonsdale and Braithwaite 1988, in Walden et al. 1999). It is common along the edges of reservoirs, canals, river banks and roadside ditches, and in agricultural lands and overgrazed floodplains (Walden et al. 1999). In Vietnam it is typically found along the edge of both natural and manmade water bodies and along roadsides (Triet et al Undated). In Australia it is known to spread very rapidly within overgrazed rangelands, and within Costa Rica (part of its native range) it is common in overgrazed areas (Walden et al. 1999; Boucher et al. 1983, in Walden et al. 1999).

    Mimosa does not appear to grow preferentially in any soil type, but is found most commonly in soils ranging from black cracking clays to sandy clays to coarse siliceous river sand. Seed production and plant life expectancy are greater on black cracking clays than on the lighter clays and silty loams (Lonsdale 1992).

    General impacts
    River floodplains and swamp forests in northern Australia are threatened by dense thickets of Mimosa pigra. The weed supports fewer numbers of birds and lizards, less herbaceous plants and fewer tree seedlings. It prevents traditional food gathering by Aborigines on otherwise resource rich wetlands.
    M. pigra has the potential to harm a wide number and variety of different types of primary production. If large infestations occur over farmland, mimosa may threaten the health of pastoral industries by reducing the area of grazing land and the carrying capacity of the land. Furthermore, if livestock are reliant on natural water sources for drinking, their access to water may be blocked. As a result, meat production and income may be reduced (Praneetvatakul 2001).
    M. pigra may reduce water flow and increase silt levels, as it commonly colonises water course edges. This may threaten the sustainability of reservoirs and canals and any livelihoods reliant on them. For example, the weed negatively impacts rice cultivation in Thailand by blocking irrigation inlets (as well as encouraging increases in the numbers of rats and crabs, which damage crops) (Praneetvatakul 2001).
    M. pigra may interference with the cultivation of other economically-important plants. For example, M. pigra is able to compete with the young palm trees in immature oil palm plantations. This may cause a decrease in the production of palm oil (Praneetvatakul 2001).
    Common along roadsides, mimosa may also increase the costs of maintaining power poles and cables used for electricity transmission. It may also decrease driver visibility, increasing the potential for traffic accidents (Praneetvatakul 2001).
    Geographical range
    Native range: Mimosa pigra is native to tropical America where it occurs in a wide belt extending from Mexico through Central America to northern Argentina (Walden et al. 1999).
    Known introduced range: It is a serious introduced weed in Africa, Asia, some Pacific islands, and in the Northern Territory, Australia (Walden et al. 1999).
    Introduction pathways to new locations
    For ornamental purposes: Mimosa has been introduced to new regions as an ornamental (Walden et al. 1999). .
    Landscape/fauna "improvement": Mimosa has been introduced and planted to reduce erosion (Walden et al. 1999).
    Nursery trade: Mimosa was probably introduced to the Northern Territory (Australia) via the Darwin Botanic Gardens. This may have been due to accidental contamination of seed samples. Alternatively it may have been introduced intentionally due to its unusual sensitive leaves (Miller and Lonsdale 1987, in Walden et al. 1999).
    Translocation of machinery/equipment: The seeds may adhere to vehicles or other machinery (Lonsdale et al. 1985, in Walden et al. 1999).


    Local dispersal methods
    Consumption/excretion: Seeds pass unharmed through the digestive tract of animals resulting in rapid spread.
    Hikers' clothes/boots: The seed segments are covered with bristles that enable them to adhere to animals and clothing (Walden et al. 1999).
    On animals: The seed segments are covered with bristles that enable them to adhere to animals and clothing (Walden et al. 1999).
    Road vehicles: People transport the seed on vehicles, equipment and clothing.
    Transportation of habitat material (local): The seeds are dispersed in soil and mud (Lonsdale et al. 1985, in Walden et al. 1999). For example, M. pigra seeds were probably brought into U Minh Thuong National Park, Vietnam, via contaminated construction sand (Triet et al Undated).
    Water currents: Seeds pods are buoyant, and may be dispersed by flood waters and water currents. Fast-moving water in channels is perhaps the most important seed dispersal mechanism within the Mekong Delta, Vietnam (Triet et al Undated). Water facilitated spread has also been a notable natural form of spread in northern Australia. Heavy flooding, in combination with overgrazing of rangelands, catalysed the spread of M. pigra through these Australian floodplains during the 1970s (Walden et al. 1999).
    Management information
    Preventative measures: Preventative weed control is the most cost efficient form of weed management. Comprehensive surveys should ensure isolated infestations are identified and targeted before they expand to uncontrollable levels. Educating the community is also an important tool. Restricting the movement of vehicles, stock, stock feed, soil and sand from infested areas is important to prevent the spread of mimosa seeds (Walden et al. 1999).

    A Risk Assessment of Mimosa pigra for Hawai‘i and other Pacific islands was prepared by Dr. Curtis Daehler (UH Botany) with funding from the Kaulunani Urban Forestry Program and US Forest Service. The alien plant screening system is derived from Pheloung et al. (1999) with minor modifications for use in Pacific islands (Daehler et al. 2004). The result is a score of 25 and a recommendation of: "Likely to cause significant ecological or economic harm in Hawai‘i and on other Pacific Islands as determined by a high WRA score, which is based on published sources describing species biology and behaviour in Hawai‘i and/or other parts of the world."

    A Risk assessment of Mimosa pigra for Australia was prepared by Pacific Island Ecosystems at Risk (PIER) using the Australian risk assessment system (Pheloung, 1995). The result is a score of 23 and a recommendation of: reject the plant for import (Australia) or species likely to be a pest (Pacific).

    Click here for Information about biological, physical and chemical control of Mimosa pigra.

    Reproduction
    Seeds are produced in individual segments of seed-pods that ‘burst’ apart when mature (Walden et al. 1999). Under optimal conditions annual seed production may reach up to 220,000 per plant.
    A study carried out within the Mekong Delta found that the average number of seeds in the topsoil was 100 seeds per metre squared (Triet et al Undated). In contrast, an average of 12,000 seeds per metre squared was reported for a mimosa-infested area in northern Australia (Lonsdale 1992, in Triet et al Undated).
    Lifecycle stages
    Plants mature quickly and can set seed in their first year of growth (Walden et al. 1999). Flowering may begin 6 to 8 months following germination. Flowers are bee-pollinated and possibly wind-pollinated. Plants are thought to be self-compatible. Flowers develop in about 7 to 9 days, and seed pods in about 25 days. M. pigra fruits and flowers all year round in the Mekong Delta, but its main fruiting season occurs during the dry season (December to May) (Triet et al Undated).
    Seeds are extremely hardy and can remain dormant for more than 15 years depending on the environment. For example, half of a seed population was no longer viable after 99 weeks at a depth of 10cm in a light clay soil, while a similar loss in viability was observed after only 9 weeks in heavier cracking clay (Lonsdale et al. 1988, in Walden et al. 1999). In sandy soils the lifespan of seeds may be much longer. Dormancy of seeds in the soil is broken by expansion and contraction of the hard seed-coat by temperature changes ranging from about 25–70°C. Seeds buried deeper than 10cm generally do not successfully germinate unless brought to the surface (Walden et al. 1999).
    This species has been nominated as among 100 of the "World's Worst" invaders
    Reviewed by: Colin Wilson, Parks and Wildlife Commission of the Northern Territory, Australia.
    Annie Lane, Northern Department of Primary Industry and Fisheries, Australia.
    Compiled by: Colin Wilson, Parks & Wildlife Commission of the Northern Territory & IUCN/SSC Invasive Species Specialist Group (ISSG) Palmerston, Australia.
    Annie Lane, Northern Department of Primary Industry and Fisheries, Resource Management Division, Australia.
    Last Modified: Wednesday, 19 July 2006


ISSG Landcare Research NBII IUCN University of Auckland