Acacia Karroo

This is what it's fruits looks like when in full bloom

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.

Growing and uses.
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.