Leucaena leucocephala (Leucaena)

Scientific name

Leucaena leucocephala (Lam.) de Wit

Synonyms

Acacia leucocephala (Lam.) Link; Leucaena glauca (L.) Benth. (misapplied); Mimosa leucocephalaLam.

Common names

Coffee bush, cow tamarind, horse tamarind, jumbie bean, leadtree, leucaena, white leadtree, wild tamarind, mlusina, lusina (Kiswahili)

Family

Fabaceae (Leguminosae) : sub-family Mimosoideae

Origin

Native to southern tropical America.

Naturalised distribution (global)

Locations within which Leucaena leucocephala is naturalised include parts of South America (outside its native range), Asia, southern USA, southern Europe, Australia, Africa and many oceanic islands with warm climates.

Introduced, naturalised or invasive in East Africa

Leucaena leucocephala is invasive in parts of Kenya, Tanzania and Uganda (Lyons and Miller 1999). It is especially abundant in the humid coastal lowlands and in other humid areas. L. leucocephala is common in many provinces of Kenya; Western, Rift Valley, Coast and Central. In Tanzania L. leucocephala was introduced on experimental basis in Morogoro where it proved a success as an agroforestry tree.

Habitat

A very troublesome weed of riparian zones (banks of watercourses) and roadsides in tropical and subtropical regions. It is also found in open woodlands, gardens, parks, waste areas, disturbed sites and on coastal foreshores and offshore islands.

Description

Leucaena leucocephala is a shrub or small tree usually growing 2-10 m tall, but occasionally reaching 15 m or more in height.

The younger stems are green and usually densely covered in fine greyish coloured hairs (finely pubescent). Older stems have a relatively smooth, greyish or greyish-brown, bark with numerous small raised spots (lenticels).

The leaves (up to 35 cm long) are twice-compound (bipinnate) and have 3-10 pairs of branchlets (pinnae). They are alternately arranged along the stems and borne on stalks (petioles) 2-5 cm long. A small raised structure (gland) is usually present on the leaf stalk (petiole), or just below where the lowest pair of branchlets (pinnae) meet. pinnae are 2-10 cm long and each bears 5-22 pairs of leaflets (pinnules). These leaflets (7-21 mm long and 1.5-5 mm wide) are elongated (narrowly-oblong to lanceolate) in shape with pointed tips (acute apices), and are either hairless (glabrous) or have hairy (ciliate) margins.

The flowers are borne in dense globular clusters (12-30 mm across), which look like a 'pompom' when the flowers open. These clusters are borne in the leaf forks (axils) on stalks (peduncles) 2-6 cm long, with one to three clusters present in each leaf fork (axil). Each of the small flowers has five tiny sepals (2-2.5 mm long), five small greenish-white coloured petals (2-4 mm long), and ten prominent pale yellow or whitish coloured stamens (6-10 mm long).

The fruit are elongated (linear), flattened, pods with a pointed tip (beaked apex). These pods (8-22.5 cm long and 10-20 mm wide) are initially green in colour, but turn brown or reddish-brown as they mature. Several pods will usually develop from each flower cluster. Each of these pods contains 10-25 hard seeds (6-10 mm long and 3-6 mm wide) that are glossy brown, flattened (compressed), and somewhat oval (elliptic-oblong) in shape.

Reproduction and dispersal

Leucaena leucocephala is a prolific seed producer and it also resprouts after its stems are cut or damaged.

The seeds are often dispersed by small animals (rodents and birds) and cattle. The light pods may also be spread short distances by wind and can float on water.

The use of L. leucocephala as an agroforestry species continues to increase its spread and it is from such plantings that it often spreads to degraded lands, livestock pastures, forest plantations and wild vegetation areas.

Similar species

There are two sub-species of Leucaena leucocephala: L. leucocephala subsp. leucocephala   and L. leucocephala subsp. glabrata. These two sub-species can be distinguished by the following differences:

  • L. leucocephala subsp. leucocephala is a relatively small and much-branched tree with younger stems that are densely covered with fine greyish-coloured hairs (they are puberulous).
  • L. leucocephala subsp. glabrata is a relatively large and sparsely-branched tree with younger stems that are hairless (glabrous).

Other Leucaena species have been introduced to East Africa such as L. diversifolia, L. pallida and L. trichandra. They are all probably invasive.

Economic and other uses

Leucaena leucocephala is widely cultivated, mostly in farming situations, for forage (leaves and shoots), firewood, poles, medicine (roots), shade, soil conservation and improvement, tannin, dye. It is also planted as a windbreak, a garden ornamental and as an urban shade tree. . In Uganda, it was introduced in tea plantations and as a host for the vanilla orchid (Vanilla planifolia) and later planted among other crops as a nitrogen fixer.

Environmental and other impacts

Leucaena leucocephala grows very fast in suitable sites; pollarding and coppicing to form dense, homogenous thickets that are difficult to control once established. Invaded areas become unusable and inaccessible with most other vegetation replaced. L. leucocephala constitutes a threat to native biodiversity. Once L. leucocephala establishes itself it displaces native vegetation and can promote suitable conditions for the establishment of even more aggressive invaders. The mimosine in the leaves of L. leucocephala can cause hair loss, infertility and stomach problems in livestock, especially those that are not ruminants.

L. leucocephala is an environmental weed in many parts of the world. It has been nominated as among 100 of the "World's Worst" invaders by the IUCN Invasive Species Specialist Group and it has been listed as a noxious weed in Western Cape South Africa (prohibited plants that must be controlled. They serve no economic purpose and possess characteristics that are harmful to humans, animals or the environment) and a Category 2 invader (invaders with certain qualities, e.g. commercial use or for woodlots, animal fodder, soil stabilisation, etc. These plants are allowed in certain areas under controlled conditions). in the rest of the country.

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.

Grazing by goats has been used as an effective control method in controlled situations. Cutting and uprooting can work for younger plants at a localised scale but older plants are likely to resprout after such interventions. In such cases cutting must be followed by herbicide application to the cut stump. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert.

In Kenya, a psyllid insect pest, Heteropsylla cubana, defoliates Leucaena leucocephala, resulting in severely reduced fodder as well as wood but does not kill the plants. A parasitoid was introduced to control the psyllid so it no longer does so much damage. There are also insect seed predators that affect the seed production but do not seem to stem its spread.

In South Africa permission was recently granted for the release of the seed-feeding bruchid beetle, Acanthoscelides macrophthalmus as a biological control agent against L. leucocephala.

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. Leucaena leucocephala (leucaena). CABI Publishing 2011. www.cabi.org/ISC. Accessed March 2011.

Henderson, L. (2001).  Alien weeds and invasive plants. A complete guide to declared weeds and invaders in South Africa.  Plant Protection Research Institute Handbook No. 12, 300pp. PPR, ARC South Africa.

Germplasm Resources Information Network (GRIN). www.ars-grin.gov/npgs/index.html. National Germplasm Resources Laboratory, National Genetic Resources Program, Agricultural Research Service (ARS), United States Department of Agriculture (USDA), Beltsville, Maryland, USA. Accessed March 2011.

GISD (2010). Global Invasive Species Database online data sheet. Leucaena leucocephala (tree). www.issg.org/database. Accessed March 2011.

Lyons, E.E. and Miller, S.E. (eds) (1999). Invasive Species in Eastern Africa: Proceedings of a Workshop held at ICIPE, July 5-6, 1999.

Pacific Island Ecosystems at Risk (PIER). Leucaena leucocephala (Lam.) de Wit, Fabaceae (Leguminosae): plant threats to Pacific ecosystems. www.hear.org/pier/species/leucaena_leucocephala.htm. Institute of Pacific Islands Forestry, Hawaii, USA. Accessed March 2011.

Maundu P. and Tegnas T. (eds.) (2005). Useful trees and shrubs for Kenya. Technical handbook No. 35. Nairobi, Kenya.

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: eafrinet@africaonline.co.ke