Pure stands of these beautifully shaped trees with their perfectly flattened crowns are quite stunning. Where else but in Africa would you encounter such a sight?

Description
A magnificent, widely spreading, flat crown (12 m high, 16 m wide) of deep green, feathery foliage (deciduous) and attractive creamy-tan to yellow-brown corky bark, make this an easy tree to identify. The flaky, papery bark peels off in flattish strips, revealing a yellow underbark.

Balls of creamy to pale yellow scented flowers are borne in spring to summer (September to November) and entice insects. Paired thorns are long, strong, straight and white. Light brown, woody pods are formed from autumn (March) onwards, are cylindrical and thickened (often with velvety hairs).

Natural distribution
This tree is found in woodland, wooded grassland and along riverbanks (where it can reach 15 m) in South Africa, Swaziland, Zimbabwe, northern and eastern Botswana, northern Namibia and tropical Africa north to Ethiopia. There are about 44 species of Acacia in South Africa. Most are large, thorny trees with feathery twice-compound leaves and fluffy flowers carried either in spikes or balls. Acacias (with thorns) are mostly confined to Africa. Those found in Australia do not have thorns.

Name derivation
Acacia comes from the Greek akantha (thorn), sieberiana is named for Franz Sieber (1789-1844), a Bohemian botanist, traveller and plant collector.

Ecology
It is a favourite nesting site for many birds - in valley bushveld areas, Pied and Crested Barbets make their nesting holes in this tree. Wood-hoopoes often scratch around under the loose bark for insects. Grey Hornbills crack the pods open and eat the seeds.

The flowers lure beetles, bees, butterflies and thrips, in turn attracting insectivorous birds (e.g. Bar-throated Apalis, White-bellied, Black and Collared Sunbirds). The pods have a musty scent (like old socks!) and are eaten by cattle and game (said to taint a cow's milk). They contain hydrocyanic acid, so the quantities fed to livestock should be limited (also quantities of wilted leaves).

Uses and cultural aspects
In Central Africa, a bark/root decoction is used for inflammation of the urinary passages. Leaf, bark and resin are used as an astringent for colds/chest problems, diarrhoea, haemorrhage and eye inflammation. In Tanzania, bark is used to treat gonorrhoea. The edible gum is a good adhesive. Twine from the inner bark is used for threading beads.

Growing Acacia sieberiana var. woodii

This tree is easily propagated from seed that has been immersed in boiling water and soaked overnight. Protect young plants from frost. They are suited to medium to large gardens. Allow these magnificent trees the space to show off their wonderful shapes - don't crowd and clutter them. However, on a large property, five to six trees planted fairly close together make an impressive group.

This tree is half-hardy and very fast-growing with fertile soil and sufficient water, and tolerates temperatures ranging from about -2°C to 40°C. Plant in the sun.

References

• Coates Palgrave, K. 1988. Trees of southern Africa, Struik, Cape Town.
• Germishuizen, G. & Fabian, A. 1982. Transvaal wild flowers. Macmillan, Johannesburg.
• Joffe, P. 2001. Creative gardening with indigenous plants - a South African guide. Briza Publications, Pretoria.
• Palmer, E. & Pitman, N. 1972. Trees of southern Africa. Balkema, Cape Town.
• Pooley, E. 1993. The complete guide to trees of Natal, Zululand and Transkei. Natal Flora Publications Trust, Durban.
• Van Wyk, B. & Van Wyk, P. 1997. Field guide to the trees of southern Africa. Struik, Cape Town.
• Van Wyk, B., Van Wyk, P. & Van Wyk B-E. 2000. Photographic guide to trees of southern Africa. Briza Publications, Pretoria.
• Von Koenen, E. 1996. Medicinal, poisonous and edible plants in Namibia. Klaus Hess, Windhoek.
• Watt, J.M. & Breyer-Brandwijk, M.G. 1962. The medicinal and poisonous plants of southern and eastern Africa. Livingstone, London.

Pitta Joffe
Pretoria National Botanical Garden
October 2003