Alien Acacia species, wattles, mimosas, wattels
Acacia mearnsii...............black wattle / swartwattel
Acacia dealbata................ silver wattle / silwerwattel
Acacia decurrens.............. green wattle / groenwattel
Description: Like all but one of the 960-odd mis-named acacias or wattles of Australasia, these three species are without thorns (‘mis-named’ because the very word acacia is derived from the Greek akis, for a sharp point – i.e. thorns - as occur on all African acacia species, north Africa being the type location for the genus). A. mearnsii, the most important of the three, is an evergreen tree up to 15m high, with a dark trunk, branchlets shallowly ridged and all parts finely hairy. Raised glands occur at and between the junctions of pinnae pairs. Its leaves are a dark olive green and bipinnate, with short, crowded leaflets. Its flowers are pale yellow or cream balls, as large fragrant flowerheads, in July-September. Its fruits are dark brown pods, usually markedly constricted. A. decurrens is almost hairless and its leaves are a bright green in comparison with A. mearnsii, with bright yellow flower balls and hairy, brown, slightly constricted pods. A. deelbata, easily confused with A. mearnsii, has branchlets that are tinged grey or purple, but with glands only at pinnae pair junctions, with silvery-grey leaves and greyish to purple-brown pods, only slightly constricted.
NB: Take care not to confuse these alien wattle species with the indigenous tree Peltophorum africanum, the African-wattle or huilboom, which has a superficially similar appearance, especially when in bloom. The trunk of P. africanum is browner than that of the alien wattles; and, most diagnostically, its yellow flowers are produced in large, petalled, leguminous sprays, not in balls.
Origin: Of a total of 18 alien species ofacacia or wattle introduced into South Africa from Australasia / SE Asia and cultivated for timber, shelter and firewood, only three are found in significant numbers in the Waterberg and all three are declared invaders or weeds. A. mearnsii was introduced into KZN in the 19th century for the production of tannic acid used in the leather industry; and for timber. It is still planted commercially, unlike the other two species, A. dealbata (which resembles A. mearnsii) and A. decurrens, both of which were probably introduced by mistake.
Occurrence: All three wattles – but in our area, A. mearnsii, the black wattle in particular - are aggressive invaders of grassland, indigenous bush, roadsides and in the Waterberg, especially, watercourses, although A. decurrens is less common. They seriously threaten local biodiversity.
Why they are a problem: These alien invasive species cause the crowding out and eventual elimination of indigenous species. Like eucalypts and poplars, they lead to the choking and drying up of watercourses; and in dense stands, can represent severe fire hazards. A. mearnsii is ranked top of the list of the World’s Worst Invasive Species.
Elimination / Control Methods: Long-term control of wattles is difficult, because they coppice readily and produce large numbers of seeds that can remain dormant for decades. The seeds are efficiently distributed by water and their germination is stimulated by fire. The species are amenable to physical and herbicidal control, but their readiness to re-grow and the propensity for seedlings to germinate once exposed to sunlight necessitates repeated attention and herbicide application. Plants should not be felled or burnt without immediate folllow-up with herbicides. The most successful approach involves a combination of mechanical and chemical attack, accompanied by sustained management.
Numerous herbicides have been registered for application to Acacia sp: for A. mearnsii alone, almost 60 branded products have been listed, too many to list below. Most are based on one of the following active ingredients:
Aminopyralid/triclopyr (various formulations): A. mearnsii
*Bromacil 500g/l SC: A. mearnsii, A. dealbata
*Bromacil/tebuthiuron 250/250 g/l SC: A. mearnsii, A. dealbata
Clopyralid/triclopyr 90/270 g/l SL: A. mearnsii
Fluroxypyr (various formulations): A. mearnsii
Glyphosate (various formulations): A. mearnsii, (A. dealbata)
Imazapyr 100 g/l SL: A. mearnsii
Picloram 240 g/l SL: A. dealbata, A. mearnsii
*Tebuthiuron 500 g/l SC: A. mearnsii
Triclopyr (various formulations): A. dealbata, A. mearnsii, A. decurrens
Consult your local herbicide stockist to obtain information about the most suitable branded product registered and available for your application. See also Van Zyl (2012).
*NB: herbicides containing bromacil or tebuthiuron must be applied with extreme caution, preferably only under the supervision of a herbicide specialist. Both agents are soil sterilants and can damage surrounding woody vegetation for many years after application.
Al herbicides should be applied early in the growing season. Note that all herbicides should be used when freshly mixed (do not leave the solution overnight).
NB: Follow carefully the instructions provided on herbicide label. Many herbicides can be toxic to other plants and or game and livestock if used inappropriately. (Mis-use of herbicides is also a criminal offence in terms of Act No. 36 of 1947).
ARC-LNR Weeds & Invasive Plants website: www.agis.agric.za/wip
ARC-LNR SAPIA News No.8, July 2008; No.28, April 2013: ARC – Plant Protection Research Institute, Pretoria. www.arc.agric.za
Bromilow, Clive (2010): Problem Plants and Alien Weeds of South Africa. Briza. Pretoria.
Schmidt, Ernst, Mervyn Lotter & Warren McCleland (2007): Trees and Shrubs of Mpumalanga and Kruger National Park (2nd Edition). Jacana, Johannesburg.
Henderson, Lesley (2001): Alien Weeds & Invasive Plants. Agricultural Research Council (ARC), Pretoria.
Van Zyl, Kathy, (compiler) (2005): Control of Unwanted Plants. Xact Information, Pretoria.
Special thanks to Dr Gerhard Verdoorn of Griffon Poison Information Centre, to Mr Ferdie Jordaan of Arysta Lifescience and to Ms Lesley Henderson of ARC for their invaluable advice and guidance. Their enthusiastic support for this voluntary project is greatly appreciated.