Azadirachta indica (Neem)

Scientific name

Azadirachta indica A. Juss

Synonyms

Antelaea azadirachta (L.) Adelb.; A. javanica Gaertn.; Melia azadirachta L.; M. indica (A. Juss) Brandin

Common names

Neem, neem tree, mkilifi, mwarubaini kamili (Kiswahili)

Family

Meliaceae

Origin

The exact native range of this species is obscure, but it is thought to be native to the Indian Sub-continent (India and Bangladesh) and South-east Asia.

Naturalised distribution (global)

Locations within which Azadirachta indica is naturalised include northern Australia, tropical Asia, Africa, Fiji, Mauritius, Puerto Rico, the Caribbean and many countries in South and Central America.

Introduced, naturalised or invasive in East Africa

Azadirachta indica is invasive in parts of Kenya, Tanzania and Uganda (A.B.R. Witt pers. obs.). It is a serious problem along parts of the Kenyan and Tanzanian coasts and it is spreading rapidly.

Habitat

Azadirachta indica can invade shrublands, open woodlands, grasslands, floodplains, riparian zones (banks of watercourses), coastal sites and other disturbed natural vegetation.

Description

Azadirachta indica is a fast-growing tree that can reach a height of 15-20 m, though it occasionally reaches 35-40 m. It is evergreen, but in severe drought it may shed most or nearly all of its leaves. The branches are spread wide. The fairly dense crown is roundish or oval and may reach the diameter of 15-20 m in old, free-standing specimens. At low altitude and hot/ humid or hot dry environments in Uganda it grows slowly, hardly attaining 10m height (D.L.N. Hafashimana pers. comm.).

The opposite, simple pinnate (once-divided) leaves are 20-40 cm long, with 20-31 medium to dark green leaflets about 3-8 cm long. The leaf stalks (petioles) are short. Very young leaves are reddish to purplish in colour. The shape of mature leaflets is more or less asymmetric and their margins are toothed (dentate).

The white and fragrant flowers arise from the junction of the stem and petiole (are arranged axillary), normally in more-or-less drooping flower clusters (panicles) which are up to 25 cm long. These branching inflorescences, bear from 150 to 250 flowers. An individual flower is 5-6 mm long and 8-11 mm wide.

The fruit is a smooth (glabrous) olive-like drupe which varies in shape from elongate oval to nearly roundish, and when ripe are 1.4-2.8 x 1.0-1.5 cm. The fruit skin is thin and turns yellow when ripe. The bitter-sweet pulp is yellowish-white and very fibrous. The pulp is 0.3-0.5 cm thick. The white, hard inner shell of the fruit encloses one, rarely two or three, elongated seeds (kernels) having a brown seed coat. Birds are known to gorge themselves on the fruits but they are extremely poisonous to mammals.

Reproduction and dispersal

Azadirachta indica reproduces mainly from seeds which are dispersed by birds and bats that eat its fruit. Its seedlings can germinate and grow in dense shade. This species has been widely distributed as a very beneficial tree to many drier parts of the world and is still being spread and planted - even in places where it has become invasive.

Similar species

Azadirachta indica is sometimes confused with closely related species of Melia, but the two genera can be distinguished on leaf and ovary morphology. Azadirachta spp. have simple pinnate (once-divided) leaves with a pair of circular (orbicular) glands and a pair of elongated glands at the base and an ovary with three cells (3-locular ovary), while Melia spp. possess 2- to 3-pinnate (twice-divided or three times divided) leaves with one pair of orbicular glands and a an ovary with four-eight cells (4- to 8-locular ovary). A common tree planted for shade in East Africa is Melia azedarach (syringa, or Australian white cedar) which is sometimes invasive and can be confused with A. indica.

Economic and other uses

This drought-tolerant species is grown to prevent soil erosion and to help in soil conservation and improvement. Azadirachta indica improves soil fertility and water holding capacity, and can neutralise acidic soils, and so is used for the reclamation of degraded land. However, using A. indica for this purpose may result in long term problems because of its tendency to dominate to the exclusion of other species.

The wood is hard and resistant to termites, borers and fungi. A. indica is considered suitable for general purpose plywood, fire retardant plywood and plywood for blockboard.

In Tanzania and other Indian Ocean states it is known as 'the panacea' in Kiswahili, literally 'the tree that cures forty [diseases]', A. indica is deemed very effective in the treatment of scabies, although only preliminary scientific proof, which still has to be corroborated, exists, and is recommended for those who are sensitive to permethrin, a known insecticide which might be an irritant. Also, the scabies mite has yet to become resistant to A. indica, so in persistent cases A. indica has been shown to be very effective. There is also anecdotal evidence of its effectiveness in treating infestations of head lice in human. The oil is also used in sprays against fleas for cats and dogs as well as in some beauty products. However, prolonged internal use (ingestion) may cause irritation of the liver and kidneys.

Environmental and other impacts

Azadirachta indica is a weed in many areas, including some parts of the Middle East, northern Australia and much of sub-Saharan Africa. Large infestations have also been observed in Ghana (Accra plains) and elsewhere in West Africa.  It is also invasive along the south coast of Kenya and is naturalised in parts of Ethiopia (A.B.R. Witt, pers. obs.). A. indica is drought resistant and thrives in sub-arid to sub-humid areas, with an annual rainfall between 400 and 1200 mm.  It grows in many soil types, but thrives in well drained deep and sandy soils (PIER). It is a serious weed in northern Australia where its seeds are spread down watercourses, and by fruit-eating birds. 

A. indica can readily spread from cultivated populations into natural environments, mainly as a result of dispersal by birds and bats that eat its fruit. Its seedlings can germinate and grow in dense shade. Riparian zones (banks of watercourses) in drier tropical regions seem to be most at risk from invasion, but coastal bushland is also under threat from this species. There is also concern over the potential impact of this plant on native insect populations, and there is evidence that extracts from A. indica can affect certain freshwater wildlife including fish and tadpoles. In Ghana, this species has interfered with agriculture as it rapidly establishes in fallowed fields and makes land unavailable for farming. It also negatively impacts upon biodiversity by excluding native species.

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.

Seedlings can be removed by hand but this is difficult for larger plants as Azadirachta indica coppices readily so roots and shoots must be completely removed. Mature trees can be controlled through the application of a suitable herbicide either as an application to cut stumps or as a basal bark treatment (herbicide painted onto the bark). Seedlings can be treated with foliar sprays. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert.

Azadirachta indica seedlings are susceptible to fire but mature trees withstand fires.

The editors are not aware of a biological control programme for this species. Any biological control programme for a species such as this which is grown on a large scale for its fruits and other plant parts is likely to engender conflicts of interest.

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. Azadirachta indica (neem tree). CABI Publishing 2011. www.cabi.org/ISC. Accessed March 2011.

Ganguli, S. (2002). Neem: A therapeutic for all seasons. Current Science. 82(11):1304.

Pacific Island Ecosystems at Risk (PIER). Azadirachta indica A. Juss., Meliaceae: plant threats to Pacific ecosystems. www.hear.org/pier/species/azadirachta_indica.htm. Institute of Pacific Islands Forestry, Hawaii, USA. 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: eafrinet@africaonline.co.ke