Acacia mearnsii (Black Wattle)

Scientific name

Acacia mearnsii De Wild.

Synonyms

Acacia decurrens Willd. var. mollis Lindl ; Acacia mollissima hort. ex Willd.

Common names

Blue passionflower

Family

Canna edulis Ker Gawl.; C. aurantiaca Roscoe; A. barbadica Bouche; C. ammaei Andre

Origin

Native to large parts of south-eastern Australia and Tasmania.

Naturalised distribution (global)

Locations in which Acacia mearnsii  is naturalised include Australia (outside its native range), China, Japan, Taiwan, India, Israel, southern Europe, southern Africa, Madagascar, New Zealand, south-western USA and some oceanic islands with warm climates.

Introduced, naturalised or invasive in East Africa

Acacia mearnsii is invasive in parts of Kenya (A.B.R. Witt pers. obs.), Tanzania (Henderson 2002, Tropical Biology Association 2010) and Uganda (GISD 2010). A. mearnsii is grown in plantations and has become invasive in highland parts of Kenya such as Nzoia, Kisii, Limuru, Muguga, Nyahururu,  around Eldoret and Nairobi, in the Aberdares and on Mount Elgon (on both sides of the Kenya-Uganda border). It is also widespread in other wetter parts of Uganda including the forests of the Albertine Rift Valley (G.W. Howard, pers. comm.). It has escaped cultivation in south-west Tanzania where it is invasive. In the south-western Uganda highlands, it was introduced and grown in woodlots. It regenerates quite aggressively especially after fire, leaving hardly anything else growing in the dense regeneration (D.L.N. Hafashimana pers. comm.).

Habitat

Common in moist soil types of grassland, forest edges and gaps, road sides and riparian zones (banks of watercourses) and savanna.

Description

Acacia mearnsii is a round or shapeless tree growing to 15 m in height. It is an unarmed, evergreen tree with shallowly ridged branchlets. All parts are finely hairy. The trunk often bends when trees are grown outside plantations. The bark is smooth, grey, becoming black and fissured; and splits to give a resinous gum.

Its dark dull olive-green leaflets are twice-compound (bipinnate), and each part of the compound leaf (leaflet) is extremely small (less than 4 mm long) and covered in fine hairs. These leaflets are densely packed together. Raised glands occur at and between the junctions of pinnae pairs.

The flowers are cream-coloured or pale yellow, fragrant and occur in small spherical heads. The pod is straight or twisted, dark brown when ripe, up to 10cm long with 3 to 12 joints between the seeds.

Reproduction and dispersal

Acacia mearnsii produces many  seeds that are potentially dispersed by birds or rodents, in mud on people and domestic animals, in contaminated soil and by water. It also sprouts profusely from root suckers, particularly when the roots are damaged, and readily coppices from damaged stems. Germination is stimulated by fire.

Economic and other uses

Acacia mearnsii is a fast growing but short-lived tree with hard, strong wood useful for fuelwood, poles, fencing posts and tool handles. It can also be used for wood chips - large quantities are exported from South Africa and used in the manufacture of chipboard, etc. It is suitable for bee forage and the bark is used in the tanning process and in the production of gum. It is widely cultivated in many parts of the temperate world, as an ornamental and agro-forestry tree, and readily escapes from these plantings. A. mearnsii was introduced in East Africa for its tannin-rich bark, and for use as fuel wood.

Environmental and other impacts

Acacia mearnsii is potentially a weed on farmland. It outcompetes crops for nutrients and light and is capable of invading native vegetation from grasslands to dense forests. The species suppresses undergrowth and therefore it is not suitable for use in areas which are vulnerable to erosion. In some areas of Nyandarua district (Kenya), the species is particularly notorious for taking over farms and disturbed areas, hence limiting establishment, regeneration or restoration of indigenous species and pastures. A. mearnsii is very water-demanding and poses a serious threat to water resources and has a significant impact on biodiversity.

A. mearnsii is regarded as 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 Hawaii and as a Category 2 invader in South Africa (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).

Management

The precise management measures adopted for any plant invasion will depend upon many 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. Acacia mearnsii is potentially a weed on farmland and should not be used in intercropping systems (despite its nitrogenising benefits) as it competes for nutrients and light.

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.

Seedlings and saplings can be pulled out by hand when the soil is damp but care must be taken to remove the roots as A. mearnsii can resprout from its roots. Seedlings and young trees can be sprayed with herbicide while adult trees need to be cut just above ground level and an appropriate herbicide applied immediately to the cut stump to prevent resprouting (Weber 2003). A variety of chemical treatment agents and techniques are described in PIER (2010). When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert. A. mearnsii seeds are very long-lived so decades of follow-up work is required for sustainable management.

Competitive cover crops can be planted in cleared areas to reduce regeneration (Bromilow 2001).

In the south-western Uganda highlands A. mearnsii is managed and controlled from excessive spread from woodlots through harvesting the young saplings for both firewood and trellises for climbing beans. In addition, the bark from the saplings and poles is removed and used in hut and granary construction as it is very tough while the mature poles are highly prized in hut construction as they are quite durable in the ground. It is also a high quality fuel wood both as firewood and charcoal. The heavy harvesting controls its aggressive spread while debarking the young trees to the ground kills them off without coppicing (D.L.N. Hafashimana pers. comm.).

In South Africa both seed feeding insects and a mycoherbicide are used to control A. mearnsii (Henderson 2001) - the seed weevil Melanterius maculatus and a native South African fungus Cylindrobasidium laeve that attacks damaged trees has been developed into a mycoherbicide and can be applied to cut stumps to prevent resprouting. A Cecidomyiidae gall midge that inhibits reproduction of Acacia species can prevent fruit formation (and thus reproduction) without affecting vegetative growth has recently been released in South Africa.

Legislation

Acacia mearnsii is not listed as a noxious weed by any state or government authorities in Kenya, Tanzania or Uganda.

References

Bromilow, C. (2001). Problem Plants of South Africa. Briza Publications, Pretoria, South Africa.

CABI invasive species compendium online data sheet. Acacia mearnsii (black wattle). CABI Publishing 2011. www.cabi.org/ISC. Accessed March 2011.

Dharani N. (2006). Field Guide to common trees and Shrubs of East Africa. Struik Publishers, Cape Town, RSA.

GISD (2010). Global Invasive Species Database online data sheet. Acacia mearnsii (tree, shrub). www.issg.org/database. Accessed March 2011.

Henderson L. (2001). Alien Weeds and Invasive Plants: a complete Guide to declared weeds and invaders in South Africa. Agricultural Research Council. Cape Town RSA.

Henderson L. (2002). Problem plants in Ngorongoro Conservation Area. Final Report to the NCAA.

Matthews S and Brandt K. (2004). Africa Invaded: The growing danger of invasive alien species. Global Invasive Species Programme.

Maundu P and Tegnas B. (2005). eds. Useful Trees and Shrubs for Kenya, World Agroforestry Centre, Nairobi.

PIER (2010). Pacific Island Ecosystems at Risk (PIER). Acacia mearnsii De Wild., Fabaceae: plant threats to Pacific ecosystems. www.hear.org/pier/species/acacia_mearnsii.htm. Institute of Pacific Islands Forestry, Hawaii, USA. Accessed January 2011.

Tropical Biology Association (2010). Usambara Invasive Plants - Amani Nature Reserve - www.tropical-biology.org/research/dip/species.htm.

Weber E. (2003). Invasive Plant Species of the World. A Reference Guide to Environmental Weeds. Wallingford, UK: CABI Publishing.

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