A drought-resistant tree with the classic, umbrella-shaped canopy associated with thorn trees. Many bird species take advantage of the protection it offers and build their nests in the canopy.
Acacia tortilis subsp. tortilis reaches heights of between 5 and 20 m in nature. It is fairly slow-growing and in cultivation reaches a final height of between 3 and 5 m with a spread of 8–13 m. In extremely arid conditions, it may occur as a small, wiry bush.
The bark on the trunk is rough and varies from medium grey to almost black, with somewhat tortuous longitudinal fissures. On main branches the covering is dark and fissured. Branches about 40 mm in diameter are a dark grey with occasional blackened fissures here and there. Branchlets about 10 mm in diameter vary from olive green to brown or purplish. There are some interesting variations in the armature.
The thorns are in pairs at nodes which may be spaced at as little as 5 mm near tips, but up to 10–20 mm further down. Straight and hooked thorn pairs usually alternate at consecutive nodes, but a pair may comprise one hooked and one straight thorn.
There are usually from 2–6 leaves per node. In any such group there may be one or two major leaves, but often all are the same size. The leaf stalk length can be up to 30 mm but is ordinarily only half this dimension or even less. Both petiole (leaf stalk) and rachis (central axis of a compound leaf) are green and pubescent. There is, in most cases, a green or brown, domed or cupped gland on the petiole immediately below the first pinna pair (pinna = primary division of a compound leaf).The rachis is grooved on top, has inconspicuous raised dark glands at 2 or 3 or even more of the distal pinna junctions and ends in a short and rather blunt mucro. Pinna pairs vary, as a rule, from 4–10 in number, the average being 6.
There are up to 4 flower heads per node. They are globose, cream to white, scented and somewhat inconspicuous unless flowering happens to be profuse. The peduncle (stalk of the inflorescence) is about 13–14 mm long, slender, pubescent and has an involucel (a group of bracts) located at approximately one-third of the length up from the base. Flowers usually appear about midsummer but have been observed as late as February. Timing is probably largely dependent on the climate.
The fruits (pods) of this species cannot be confused with those of any other South African acacia. They are rolled up into a tight circle or wound into a helix of three or four turns, like a coil spring. The width is about 8 mm and the length of a helically coiled pod of three turns was calculated at 125 mm.They are longitudinally venose (veined), somewhat woody, usually glabrous and dry out to a light khaki colour.They occur in bunches and are indehiscent (not opening when ripe). Pods contain up to 14 seeds, some of which often do not mature. The seeds are well rounded, ellipsoid in outline, with face dimensions about 5,5 x 4 mm, greenish brown with a dark brown horseshoe-shaped areole (a distinct area on the face of a seed).
Distribution and habitat
This is a very widely distributed species and occurs in Namibia, Botswana, Limpopo, North-West, Gauteng, Mpumalanga, Swaziland, the Free State, KwaZulu-Natal and Northern Cape, and in other countries north of southern Africa. The plant is known to tolerate high alkalinity, drought, high temperatures, sandy and stony soils, strongly sloped rooting surfaces, and sand blasting. Plants older than 2 years have been observed to be somewhat frost resistant. Acacia tortilis occurs in deciduous woodland, thornveld and bushveld. It is found from sand dunes and rocky scarps to alluvial valley bottoms, avoiding seasonally waterlogged sites. A very drought-resistant species, the umbrella thorn grows in areas with annual rainfall as low as 40 mm and as much as 1200 mm, with dry seasons of 1–12 months. The tree favours alkaline soils but will colonize saline and gypseous soils.
Derivation of name
Acacia is derived from the Greek word 'akis' meaning a point or a barb, tortilis = twisted, referring to the shape of the pods, and heteracantha = different thorns (some thorns are straight and some are hooked).
The pods are eaten by stock and game which disperse the seeds.
Uses and cultural aspects
The timber is used for fence posts, firewood, furniture and wagon wheels. The prolific pods make good fodder for desert grazers and the foliage is also palatable, Acacia tortilis being one of the major dry season fodder trees of the Sahara-Sahelian belt. Bark is used for string in Tanzania. Gum is used as a poor man's gum arabic, said to be edible. It is the tree most recommended for reclaiming dunes in India and Africa. The thorny branches are used to erect temporary cages and pens. Bark is said to be a good source of tannin. Africans once strung the pods into necklaces. Senegalese use the roots for spear shafts; Lake Chad natives use the stems for fish spears. African nomads often use the flexible roots for frameworks of their temporary shelters. The pods and leaves are highly nutritious. Vervet monkeys and baboons eat young green pods; other animals eat those which have fallen to the ground.
Growing Acacia tortilis subsp. heteracantha
Seeds should be immersed in boiling water and soaked until they swell. The testa (seed coat) is thick and impervious and could be nicked or filed before soaking, but that is a tedious operation unless one is only concerned with a small number of seeds. Viability seems to vary considerably, both excellent and poor results having been obtained. Germination often takes place in as little as 4 days, but 10 days is a more likely period. Seedling growth is rather slow and even after planting out, trees often progress tardily. Growth to a height of 0,8 m in four years has been observed. On the other hand, young trees frosted back to ground level in winter have subsequently attained a height of 1 m in one season. Young trees do not appear to be capable of withstanding much frost. It is drought-resistant but watering and applications of fertilizer will appreciably improve the rate of growth.
References and further reading
- Carr, J.D.1976. The South African Acacias. Conservation Press, Johannesburg.
- Smit, N. (1999). Guide to the Acacias of South Africa, edn 1. Briza Publications, Pretoria.
- Umbrella thorn (www.krugerpark.co.za/africa umbrella thorn).
- Venter, F. & Venter, J.A.1996. Making the most of indigenous trees. Briza Publications, Pretoria.
Free State National Botanical Garden