They look attractive both from afar and from close-up. These evergreen trees have long bright green, spear-shaped leaves and are covered with bright, finger-shaped, yellow flowering heads during the winter.
The Acacia longifolia (Long-leafed Wattle) is one of several species of wattle brought to this country from Australia well over a century ago to assist with the stabilisation of sand dunes near Cape Town. They have since spread to other parts of the country, being particularly invasive in both the Eastern and the Western Cape, Kwa-Zulu Natal and in parts of Mpumalanga. Such is the nature of introducing an alien species from one country to another, only to find that it not only flourishes to the detriment of indigenous vegetation but appears to have no natural predators in its new abode.
In some areas Acacia longifolia has also been planted as an ornamental shrub. Looking at the flowers, it is easy to see why.
These trees form dense, impenetrable thickets that threaten the existence of indigenous vegetation. Given that South Africa is a water-scarce country, it is concerning that the Acacia longifolia trees have spread so widely, both on hill slopes and along the country’s riparian zones. Seedlings grow very quickly – several others are visible behind the one in this photograph.
One of the methods employed to curb the rampant growth of the Acacia longifolia has been to release biological control agents, such as Trichilogaster acaciaelongifoliae, an Australian bud-galling wasp from the Chalcidoidea family that parasitizes these plants. This wasp species was introduced in 1982. They lay their eggs in the immature flower buds. Chemicals secreted by the young grubs induce bud galling. The larvae live and feed on the plant tissue inside these galls, which helps to reduce the reproductive potential of the wattle.
The Working for Water Project has made inroads by physically cutting down stands of wattle and applying herbicide to the stumps. Landowners, however, do not always seem to follow up on this initial clearing process and we can see the proliferation of these trees along the country roads we often drive along in this area.