This is one of South Africa's most beautiful and useful trees. It is integrally part of our country's history having been used for everything from raft-making to sewing needles and fencing for the houses of the royal Zulu women. The thorns were even used by early naturalists to pin the insects they collected! It is very widespread throughout southern Africa and there are different forms in some places, which can be confusing. Acacia karroo may be found from the Western Cape through to Zambia and Angola. In tropical Africa it is replaced by Acacia seyal. The name Acacia is derived from Greek "akis" a point or barb. Karroo is one of the old spellings of karoo which cannot be corrected because of the laws governing botanical nomenclature (giving of names).

It is found in a variety of habitats from low lying areas to highveld, although not usually found in mist belt and montane areas. It is an indicator of sweet veld which is prized for the good grazing and fertile soils. If an area is overgrazed the sweet thorn becomes invasive.

Acacia karroo has a rounded crown, branching fairly low down on the trunk. It is variable in shape and size, reaching a maximum of about 12m where there is good water. The bark is red on young branches, darkening and becoming rough with age. Sometimes an attractive reddish colour can be seen in the deep bark fissures The leaves are finely textured and dark green. The flowers appear in early summer in a mass of yellow pompons. Many insects visit and pollinate these flowers. The seed pods are flat and crescent shaped, sometimes with constrictions between the seeds. They are green when young becoming brown and dry. The pods split open allowing the seeds to fall to the ground. The thorns are paired, greyish to white and are long and straight. On mature trees, the thorns may be quite short. They may be held at 90° to the stem or raked forward slightly. Technically the thorns are called "spines" and are developed from modified stipules (small, leaf-like scales, seen at the base of the leaf-stalk). In some other thorny acacia species, the thorns are not stipular in origin and are called "prickles". These originate in the epidermis ("skin") and are always short and curved, a bit like rose thorns. Thorns on African acacias are important for identification, they are divided into 5 main groups according the size, shape and position of the thorns.

The sweet thorn gets its common name from the gum which is exuded from wounds in the bark. This pleasant tasting gum is eaten by people and animals, including the Lesser Bushbaby which feeds exclusively on insects and gum from trees, particularly acacia trees. It also had commercial value in the past when the gum was exported as "Cape Gum" for making confectionary. This is apparently similar to gum arabic which is used as a water soluble glue.

It is a particularly good fodder tree, stock and game feed on the leaves, flowers and pods. Seed dispersal takes place this way. There is no danger of hydrocyanic poisoning which is a self-protection mechanism used by many trees. The bark contains tannin which is used to tan leather to a reddish colour. (This unfortunately gives the leather an unpleasant odour). The heartwood is heavy and hard but susceptible to attack from borer. This apparently may be prevented by seasoning the wood in water for six months before use. The "Dune Forest" form found along the coast of Kwazulu-Natal northwards of the Tugela river has soft wood so would not be suitable for woodworking. A strong rope can be made from the inner bark which is pliable enough for rope-making when it is wet. The flowers produce lots of nectar and pollen for bee-farming and the honey has a pleasant flavour. In arid areas the sweet thorn is an indicator of water, both underground and surface. It was a very welcome sight to early travelers and nomads.

Acacia karroo has a life span of 30-40 years and is an adaptable pioneer, able to establishing itself without shade, shelter or protection from grass fires. Once over a year old, seedlings can resprout after fire. Several fungi are associated with this tree and the crown of mature trees may be parasitized by various mistletoes, leading to the tree's decline. This tree has a long taproot which enables it to use water and nutrients from deep underground, this and its ability to fix nitrogen, lead to grasses and other plants thriving in its shade.

The sweet thorn has many medicinal uses ranging from wound poultices to eye treatments and cold remedies. The bark, leaves and gum are usually used. It is also used to treat cattle which have tulp poisoning (Homeria spp - bulbous plants which are poisonous to stock).

Growing Acacia karroo

The sweet thorn makes a beautiful garden specimen. The bright yellow flowers look very striking against the dark green foliage. The rough, dark brown bark is also most attractive. The flowers are sweetly scented and are renowned for attracting insects which are essential to any bird garden. Birds also like to make nests in thorn trees as the thorns offer them some protection from predators. Caterpillars of 10 species of butterflies are dependant on the tree for survival. These include, the club-tailed charaxes (Charaxes zoolina zoolina) and the topaz-spotted blue (Azanus jesous).

In cold and dry areas the tree will be deciduous. The roots are invasive, so avoid planting near paving or buildings. It is a most useful tree for small holdings and farms where it can be planted for shade and as a windbreak. The sweet thorn is very adaptable to soil types and is frost and drought hardy. However, for best performance, water well and deeply (shallow, frequent sprinklings only encourage shallow root growth) until established. Plant with plenty of compost, bonemeal or superphosphates (commercial tree tablets also work well). The growth rate is fast, up to 1m per year.

It may be grown from seed which should be soaked in hot water and left overnight. You will see if this has been effective as the seed will swell up. Sow the following morning. Seedling trays with seedling mix can be used, or the seeds could be sown directly into black bags. Cover lightly with sand and do not allow to dry out. Germination usually takes 3 - 12 days. The seedling will transplant well in spite of the long tap root. Wait until they unfurl their second leaves before transplanting.

References

• Barnes, R D et al : 1996 Acacia karroo. Tropical Forestry Papers 32. Oxford Forestry Institute. Oxford.
• Joffe, P. 2001. Creative Gardening with Indigenous Plants: A South African Guide. Briza. Pretoria
• Palmer, E and Pitman, N. 1972. Trees of Southern Africa: Volume 2. A.A.Balkema. Cape Town.
• Pooley, E. 1993. The Complete Field Guide to Trees of Natal, Zululand & Transkei Natal Flora Publications Trust. Durban
• Skinner, J.D, Smithers, R.H.N. 1990. The Mammals of the Southern African Subregion. University of Pretoria.
• Thomas, V and Grant, R. 1998. Sappi Tree Spotting, Highveld and the Drakensberg. Jacana Education. Johannesburg.
• Venter, F and J-A. 1996. Making the most of Indigenous Trees. Briza Publications. Pretoria.
• Van Wyk, B and van Wyk, P. 1997. Field Guide to Trees of Southern Africa. Struik Publishers. Cape Town.

Alice Aubrey with additions by Yvonne Reynolds
Witwatersrand National Botanical Garden
January 2002