Semi-deciduous Tree may reach 10m high. Leaves bipinnate with non-spinescent stipules. Bisexual, regular Flowers are in spikes with many free stamens. Fruit: straight dehiscent pod.
Acacia erubescens, Senegalia erubescens, Acacia dulcis, Acacia kwebensis, Acacia longipetiolata.
RSA Tree No. 164.
Common names: Blue Thorn, Blue Acacia, Yellow bark, Soetdoring.
Family: Fabaceae or Leguminosae (Pea, bean or legume family). After the Orchidaceae and the Asteraceae, the Fabaceae is the third largest Angiosperm (flowering plants) family with 700+ genera and close to 20 000 species. Local genera include Acacia (Vauchellia, Senegalia), Albizia, Bauhinia, Bolusanthus, Burkea, Calpurnia, Colophospermum, Cyclopia, Erythrina, Erythrophleum, Faidherbia, Indigofera, Philenoptera and Schotia. The Fabaceae are recognisable by their fruit and by their pinnately compound Leaves. Leaves may also be simple and usually have stipules – some of which may be spinescent. Leaflets are usually entire. Flowers are bisexual and bracteate. Regular flowers usually have 4-5 sepals and the same number of petals. Irregular flowers have 4-5 sepals and 5 or less petals. Stamens have anthers that have 2 pollen sacs and there are usually at least twice the number of stamens as petals – often 10. The superior Ovary has one locule that may contain 1 or more ovules. The Stigma and Style are simple. The single carpel develops into the Fruit, which is usually a pod. The pod dehisces on both sides and may break into segments. Seeds vary.
Name derivation: Acacia – typically thorny. Senegalia – from Senegal. erubescens: reddening, rose – may refer to the young pods or flowers.
Conservation Status: L C (Least Concern). 2009 (Raimondo et al.).
This Tree is up to 10m high and has a flattened or spreading crown. It often has many stems. Thorns are hooked, paired, sharp and may be blue. Bark is pale blue, yellow or white, scaly, corky or papery and pealing with pale greyish-yellow papery flakes. Young branches may be orange tinged or greyish-brown and shining. The paired, hooked, recurved, dark tipped and very sharp Thorns are up to 7mm long. These stout thorns occur just below the nodes. If three thorns present then all 3 will point in the same direction.
This semi-deciduous tree has Leaves that are up to 6 x 4cm. They are bipinnate (compound: twice pinnate. The central axis (rachis) has lateral “branches” not leaflets and the leaflets are on these “side branches”). The Rachis (main axis bearing flowers or leaflets) is up to 3cm long with 3 to 7 pairs of pinna (usually 6), curving downwards. Each pinna has between 10 and 30 pairs of leaflets (usually about 14). Glands may be present between the top pair of pinnae. Leaflets are obliquely oblong and small – up to 7,5 x 2mm. They may be grey, yellowish green to blue-green. Petiole (leaf stalk) is usually 1,3 – 2,5cm long and usually has a raised gland near the base of the leaflets. Stipules (basal appendages of the petiole) are not spinescent and are soon lost.
The bisexual and actinomorphic (regular, symmetrical. Perianth, the calyx and corolla, can be divided into 2 or more identical sectors) Flowers resemble bottlebrushes. Flowers are in short, fat, white or pale-yellow axillary Spikes (simple indeterminate inflorescence with sessile flowers on a single unbranched stalk) that are up to 4,5cm long. They may be tinged with pink and usually appear before or with the new leaves. The Calyx is up to 4,5mm long and the Corolla is up to 6,5mm long and has hairs that closely and flatly pressed against it. These hairs are most noticeable near the apex of the lobes. The many Stamens have free Filaments and are distinctly exserted (sticking out; projecting beyond). There is a single Pistil (a unit of the Gynoecium, the female element of the flower, composed of the Ovary, Style and Stigma) and the superior Ovary has a single locule (compartment). The single Stigma rests on a small, thread like Style. (Aug-Oct).
The Fruit is a straight, light to reddish brown, dehiscent Pod that ends in a point. Pods are up to 13 x 2cm and are leathery or papery with many crossing veins present. The compressed Seeds are up to 1,1 x 1,1cm. (Sep-Jan).
Distribution & Ecology
These drought resistant Trees are common in hot, dry areas, on hill slopes and rocky areas below 1 000m. They can be found Gauteng, Limpopo, Mpumalanga and North West in the RSA and are present in Botswana, Angola, DRC, Namibia, Botswana Tanzania, Zimbabwe, Mozambique and Malawi. The tree has a symbiotic relationship with nitrogen fixing bacteria forming Nodules on the roots. This relationship also benefits other plant life.
A good tasting, yellow Gum (a water-soluble sugary polysaccharide that is exuded to seal wounds and prevent infection by bacteria and fungi and prevents trees from freezing) can be harvested from the tree. Dried gum can be pounded then eaten. Wood is hard, termite resistant and makes a good fence posts and fuel. Pods are nutritious and sought by animals. Trees grow best in deep sand.
Coates Palgrave, M. 2002. Keith Coates Palgrave Trees of Southern Africa, edn 3. Struik, Cape Town.
Lawrence, G. H. M, 1951. Taxonomy of Vascular Plants, The Macmillan Company, New York. Tenth Printing 1965.
Palmer, E. & Pitman, N. 1972. Trees of southern Africa, Balkema, Amsterdam, Cape Town.
Ross, J. H. A conspectus of the African Acacia Species. 1979. Botanical Research Institute.
Schmidt, S. Lotter, M. & McCleland, W. 2002. Trees and Shrubs of Mpumalanga and the Kruger National Park.
van Wyk, B. & van Wyk, P. 1997 Field guide to Trees of Southern Africa, Struik, Cape Town.