Invasive alien plants in southern Africa
Part 2
The Legumes (Fabaceae)

Thirty-six (18%) of the 198 species listed as declared plants in South Africa belong to the Fabaceae or legume family. All these plants are invasive and regarded as harmful to the natural resources of South Africa. Legislation concerning their control, cultivation, and trade are contained in the Conservation of Agricultural Resources Act, Act 43 of 1983, amended in 2001. (See Appendix for species list.)

Acacias
The acacias are the most numerous, with 13 listed species. All are from Australia and with one exception, A. paradoxa, differ from African acacias—they have no spines or thorns. They can be divided into two distinct groups, one with bipinnate leaves, and the other with phyllodes. A phyllode appears to be a simple, undivided leaf, but is actually a leaf-like petiole with no blade. All the listed alien acacias are evergreen, whereas most of the indigenous species are deciduous.

The alien acacias account for a very large proportion of all plant invasion in South Africa. They are important invaders of all the major vegetation types, except for those in the arid interior, where other leguminous invaders, mainly Prosopis species, take over.

The most widespread and abundant acacias are Acacia mearnsii, black wattle; A. cyclops, red eye/rooikrans; and A. saligna, Port Jackson. Black wattle has invaded the widest range of vegetation types in South Africa and is the most widespread riverine invader; it occurs almost continuously from Louis Trichardt in the Northern Province down the eastern seaboard to Cape Town, a distance of about 2,500 km. Rooikrans stretches along the entire coastline from Port Nolloth in the north-west to beyond East London in the east, a distance exceeding 2,000 km. Port Jackson stretches along the Cape coastline from Saldanha Bay in the west to the Kei River in the east. Port Jackson and rooikrans are important invaders of fynbos vegetation. Successful biological control of Port Jackson, using an introduced gall-forming rust fungus, has greatly reduced the densities of populations and in the long term should provide complete control of this invader.

Acacia dealbata, silver wattle, and A. decurrens, green wattle, are most abundant in the grassland regions from the Eastern Cape northwards. A. longifolia, long-leaved wattle, and A. melanoxylon, blackwood, are most abundant along the Cape coastal belt stretching from Cape Town to Port Elizabeth; they also extend northwards as far as the Northern Province. Long-leaved wattle is an important invader of fynbos, whereas blackwood is an important forest invader.

A. pycnantha, golden wattle, is an invader of fynbos, mainly in the Western Cape. Gall-forming wasps, introduced for the biocontrol of long-leaved and golden wattles, have become well established, reducing flowering and seed-production.

Species of minor importance, but becoming increasingly invasive, are A. baileyana, Bailey’s wattle, A. podalyriifolia, pearl acacia, A. implexa, screw-pod wattle, and A. elata, pepper tree wattle. None of these species have been planted on a grand scale in plantations, but only as garden ornamentals or windbreaks. Pepper tree wattle is mainly naturalised in the Western Cape where it is encroaching on fynbos and forest. Bailey’s wattle and pearl acacia are spreading wherever they have been planted. Screw-pod wattle is invading watercourses and fynbos in the Western Cape.

Acacia paradoxa, kangaroo thorn, is a thorny shrub only known from Devil’s Peak on Table Mountain. It is listed under the Australian Noxious Weeds Act, which is an indication of its weed potential in South Africa.

Other Genera
Albizia lebbeck and A. procera, lebbeck and false lebbeck, are invasive in the warm and humid coastal belt around Durban. They are of tropical Asian origin and are large, spreading trees up to 15 m tall.

Alhagi maurorum, camel thorn bush, is a thorny, almost leafless bush, invading agricultural croplands and river-banks in the dry interior of South Africa. Bauhinia purpurea and B. variegata, butterfly orchid and orchid trees, are Asian trees invading the subtropical and tropical savanna regions. They are popular ornamentals, flowering and seeding prolifically.

Caesalpinia decapetala, Mauritius or Mysore thorn (after Mysore Province in India), is a very tough and spiny, scrambling shrub or climber, forming dense thickets. It invades forest margins, plantations, and watercourses. Of tropical Asian origin, it is restricted to the subtropical and tropical eastern seaboard and adjacent interior of South Africa.

Cytisus monspessulanus (Genista monspessulana), Montpellier broom, is a Mediterranean shrub, invading fynbos on Table Mountain. Cytisus scoparius (Genista scoparia), Scotch broom, is an almost leafless shrub with bright yellow flowers, mainly invading grassland and forest margins on the lower slopes of the Drakensberg in KwaZulu-Natal. Gleditsia triacanthos, honey locust, is a large, spreading tree, armed with formidable three-branched spines. It is of North American origin. It has been cultivated for its edible pods, honey production, shade, and for donga reclamation. Now it is invading river-banks, drainage lines, and other moist sites in the interior of South Africa.

Leucaena leucocephala, leucaena or giant wattle, possibly of Mexican origin, invades mainly the subtropical coastal belt of KwaZulu-Natal and lowveld of Mpumalanga. It is also a highly valued plant for fodder and woodlots.

Mimosa pigra, giant sensitive plant, is believed to be of tropical American origin, but is now pantropical. It has long been known in the Ndumu area of KwaZulu-Natal, where it appears not to be a problem; it is, however, invasive and has caused much concern further north at Tzaneen along the Letaba River. This species has invaded extensive tracts of floodplains in Australia.

Paraserianthes lophantha, stink bean, is an Australian tree that has invaded moist sites in forest and fynbos in the Western and Eastern Cape. It resembles a large-leaved black wattle.

Prosopis spp., mesquite, introduced from the southwestern USA and Mexico, have invaded the arid interior of South Africa. Their preferred habitat is drainage lines and river-banks, which are the most fertile sites in these regions. They are cultivated for shade, firewood, and for their pods, which are fed to livestock. However, dense thickets not only produce fewer pods, but also prevent access to livestock. Biocontrol using seed-feeding bruchid beetles has the potential to greatly reduce the development of further infestations.

Pueraria lobata, Kudzu vine, is a vigorous, long-running vine that can reach heights of 18 m. It invades forest margins and river-banks. So far, it is only known from a few sites in Mpumalanga and Northern Province.

Robinia pseudoacacia, black locust, is a North American thorny tree that forms dense suckering thickets, particularly along watercourses. Senna bicapsularis (Cassia bicapsularis) and Senna pendula var. glabrata (Cassia coluteoides) are rambling ornamental shrubs or climbers from South America. They are invasive in the subtropical and tropical savanna regions, particularly along watercourses. Senna didymobotrya (Cassia didymobotrya), peanut butter cassia, is indigenous to central Africa. It has been widely planted in southern Africa as an ornamental and hedge plant, particularly around animal kraals. It is poisonous.

Sesbania punicea, red sesbania, is an ornamental South American shrub or small tree that has invaded watercourses throughout South Africa, except in the arid interior. A very successful biocontrol programme, using three species of introduced beetles, is effectively maintaining this species at population levels that are no longer problematic.

Spartium junceum, Spanish broom, is invading urban open space, wasteland, and fynbos in the Western Cape. It is an almost leafless shrub or small tree with bright yellow flowers. A native plant of the Mediterranean, it is now invading a similar climatic region at the tip of Africa.

Tipuana tipu, tipuana, is a large, spreading South American tree that is invading watercourses in the savanna regions.

Next Instalment
The next article in this series will deal with the Asteraceae (daisy family). There are some very important invaders in this group, including Chromolaena odorata (triffid weed), a very serious conservation weed in moist savanna, and a real threat to biodiversity; a close relative, pom-pom weed, Campuloclinium macrocephalum, invades grassland; and Parthenium, a potentially disastrous agricultural weed, which causes severe allergic dermatitis and asthma.

—by Lesley Henderson

SABONET News 6.3: 192

HENDERSON, L. 2001. Alien weeds and invasive plants. Plant Protection Research Institute Handbook No. 12. Agricultural Research Council, Pretoria.

OLCKERS, T & HILL, M.P. 1999. Biological control of weeds in South Africa (1990–1998). African Entomology Memoir No. 1.