|Weeds in our Area (Part Sixty One)
By Bob and Ena McIntyre – Garden Route Branch
Long ago, in July 2004 (Part Six) we looked at the invasive Australian Acacia species as a group. For the sake of clarity there is much merit in examining them individually as more detailed descriptions greatly improve one’s ability to correctly identify the different species.
One of the most common species that were introduced deliberately in 1827 for the purpose of dune reclamation must certainly be the prolific Acacia longifolia. It was only in 1945 that the species started revealing its invasive tendencies. These particularly persistent invaders are now very widely spread from dense infestations in the Western Cape, eastward along the coast and all the way up into the country as far as Mpumalanga and the Northern Province. It prefers moist sites in fynbos, woodlands and watercourses. It quickly becomes dense thickets and can totally transform the landscape.
Identification: Locally the species is very common particularly in the Belvidere, Brenton and Knysna areas where roadsides and degraded forest areas are heavily infested. The plants are quite easy to identify. Many sporting mean-looking smooth spherical galls on their stems – this is caused by an introduced wasp. The gall starts out as green in colour, turns brown in time and replaces the flower and leaf buds. In stark contrast to all other invasive Australian Acacias that have globular flower heads, the flowers of A. longifolia are long cylindrical flower heads (up to 50mm and about 7 mm wide) borne in the axils of the leaves from July to September. The flowers are also bright yellow in colour. The leaves, as with many Australian Acacia are phyllodes (leaf-like petiole with no blade) bright green and up to 180mm long with 2-5 prominent longitudinal veins.
Control: Happily A. longifolia does not coppice easily which means that felling and burning (only when conditions are safe and with the required permission of the local authority) is an accepted control method. Seed germination may be controlled with the use of a herbicide. Garlon 4 at 1% should be applied to young actively growing saplings up to 2m in height. The release of a gall wasp Trichilogaster acaciaelongifoliae in 1982 and a seed feeder Melanterius ventralis in 1985 have had a significant impact on reducing the spread of this species.
Substitutes: Salix mucronata (Safsaf willow), Rhus lancea (Karee), Brachylaena discolour (Coast silver oak), Acacia caffra (Haakdoring) are some of our local indigenous shrubs and trees that are excellent choices. For sand stabilisation, bushy shrubs such as Rhus crenata and Metalasia muricata (White bristle bush) are suitable.
Illustrations: “ALIEN WEEDS AND INVASIVE PLANTS”: Lesley Henderson. Copyright © 2001 Agricultural Research Council.