Scientific name

Mimosa pigraL.

Synonyms

M. pellita Humb. & Bonpl. ex Willd

Common names

Giant sensitive plant, mimosa

Family

Fabaceae (Leguminosae): sub-family Mimosoidea

Origin

Mimosa pigra is native to tropical America.

Naturalised distribution (global)

Locations within which Mimosa pigra is naturalised include tropical Asia, south-eastern USA, northern Australia, tropical Africa as well as many oceanic islands with warm climates.

Introduced, naturalised or invasive in East Africa

Mimosa pigra is invasive in parts of Kenya, Tanzania and Uganda (A.B.R. Witt pers. obs.) but has been naturalised for at least two centuries (G.W. Howard pers. comm.).

Habitat

Floodplains, swamps, shallow dams, dried river beds, riparian zones (banks of water courses). Mimosa pigra has become very abundant in floodplains and seasonal wetlands in East Africa over the last few decades having previously been a riparian species.

Description

Mimosa pigra is an upright prickly shrub or small tree.

Its young stems are greenish in colour, rounded (terete), have scattered prickles (5-12 mm long), and are covered with short stiff hairs. Older stems become woody and turn greyish to dark red in colour.

The twice-compound (bipinnate) leaves are alternately arranged along the stems and borne on stalks (petioles). These leaves (20-31 cm long) often have small prickles present along their main axis (petiole and rachis). They consist of several (6-16) pairs of branchlets (pinnae), each about 5 cm long and bearing numerous (20-45) pairs of small leaflets (pinnules). The leaflets (3-12 mm long and 0.5-2 mm wide) are elongated (lanceolate) in shape and stalkless (sessile). These leaves are sensitive and they fold together when touched and also during the night.

The flowers are pale pink or mauve and cream in colour (some merging into a cream colour with age) and arranged in fluffy, globular clusters (10-20 mm across). One to three (usually two) of these small flower clusters are produced on stalks (peduncles) 2-7 cm long, emanating from each upper leaf fork (axil). Individual flowers have four tiny sepals (0.75-1 mm long), four inconspicuous petals (2-3 mm long), and eight prominent pinkish stamens that give the flower clusters their fluffy appearance. Flowering occurs mostly during summer and early autumn.

The fruit is an elongated and flattened pod (30-120 mm long and 7-14 mm wide) that is covered in bristly hairs. The pods are borne in clusters (of 1-30), are straight or slightly curved, and are initially green in colour. As they mature they turn brown, and when fully mature they break up into about twenty (14-26) one-seeded segments. The seeds (4-6 mm long and 2-2.5 mm wide) are light brown, brown or greenish-brown in colour, elongated (oblong) in shape, and somewhat flattened (compressed).

Reproduction and dispersal

This plant reproduces by prolific seed production and can also spread vegetatively via cut stems which coppice.  The pods are coved by fine hairs and float in the water which is one of the principal means of dispersal. The one-seeded pod segments become readily become attached to animals, vehicles and clothing. They are very light and can float long distances, especially during floods.

Similar species

Mimosa pigra is similar to Mimosa diplotricha var. diplotricha (creeping sensitive plant) and Mimosa pudica (common sensitive plant), which both also produce globular pink flower clusters. These species can be distinguished by the following differences:

• M. pigra is a large shrub with an upright (erect) growth habit that has large much-branched leaves (6-16 pairs of pinnae). It produces relatively large pods (3-12 cm long) that contain many (14-26) one-seeded segments.
• M. diplotricha var. diplotricha is an upright (erect) shrub, creeper or (more commonly) a climbing plant that has much-branched leaves (4-9 pairs of pinnae) on very long sinuous stems Often in excess of 5m long). It produces relatively small pods (1-3.5 cm long) that contain a few (3-5) one-seeded segments.
• M. pudica is a smaller plant, with a more low-growing (prostrate) habit, that has few-branched leaves (1-2 pairs of pinnae). It produces relatively small pods (1- 2.5 cm long) that contain a few (1-5) one-seeded segments.

Economic and other uses

Mimosa pigra can be used as a medicinal plant, a green manure for poles, hedges and for fuelwood. However, any uses this plant has do not compensate for its negative impacts.

Environmental and other impacts

Mimosa pigra is an environmental weed in many parts of the world with often severe impacts on biodiversity. It has been nominated as among 100 of the "World's Worst" invaders by the IUCN Invasive Species Specialist Group. It is on the Federal Noxious Weeds List in the USA and has been listed as a noxious weed in all Australian states and has been listed as a Category 3 invader in South Africa (no further planting is allowed - except with special permission - nor is trade in propagative material. Existing plants must be prevented from spreading).

M. pigra can affect irrigation systems by causing the accumulation of sediments, it can encroach upon rice paddies, orchards and pasture where it reduces the grazing value of the land. It can become a threat to biodiversity, where it restricts access to water, for some mammals and inhibits the regeneration of native vegetation. and can establish an impenetrable single-species thicket (up to 5 m high) excluding large mammals, large water birds, reptiles and fish (and other plants) from invaded floodplains. Its impacts on biodiversity have been demonstrated in Zambia where M. pigra infestations have reduced bird species by about 50% on the Kafue floodplains and numbers of birds by more than 95% (A.B.R. Witt pers. comm. ). M. pigra can restrict the access of fishermen to waterways.

Management

The precise management measures adopted for any plant invasion will depend upon factors such as the terrain, the cost and availability of labour, the severity of the infestation and the presence of other invasive species. Some components of an integrated management approach are introduced below.

The best form of invasive species management is prevention. If prevention is no longer possible, it is best to treat the weed infestations when they are small to prevent them from establishing (early detection and rapid response). Controlling the weed before it seeds will reduce future problems. Control is generally best applied to the least infested areas before dense infestations are tackled. Consistent follow-up work is required for sustainable management.

For large infestations an integrated approach involving mechanical removal, herbicide application, fire and pasture management can be adopted. Mechanical control can be effective as long as the plants are cut 10 cm or more below ground level. Cutting at ground level will usually result in resprouting. Various chemicals can be used to control Mimosa pigra using a variety of methods -foliar sprays, basal bark applications methods (painting herbicide onto the bark), soil application and cut stump applications. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert. Fire can be used as a management tool, but usually in combination with other methods such as chaining. Fire alone may actually increase M. pigra densities by plant regrowth and enhanced seed germination. However all mechanical and chemical management does not prevent the germination of many seeds in the seedbank which can persist for several seasons/years.

Biological control is likely to be the only long-term, cost effective control option for M. pigra across much of its invasive range. Six released biocontrol agents have established on M. pigra in Australia for biological control of M. pigra with overall positive effects.

Legislation

Not listed as a noxious weed by the state or governments in Kenya, Tanzania and Uganda.

References

CABI Invasive Species Compendium online data sheet. Mimosa pigra (catclaw mimosa). CABI Publishing 2011. www.cabi.org/ISC. Accessed March 2011.

GISD (2006). Global Invasive Species Database online data sheet. Mimosa pigra (shrub).  www.issg.org/database. Accessed March 2011.

Editors

Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat - UK.

Acknowledgments

This fact sheet is adapted from The Environmental Weeds of Australia by Sheldon Navie and Steve Adkins, Centre for Biological Information Technology, University of Queensland. We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) - Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).

Contact

BioNET-EAFRINET Regional Coordinator: [email protected]