Classification of the genus Acacia (in the wider sense) has been subject to considerable debate. It is generally agreed that there are valid reasons for breaking it up into several distinct genera, but there has been disagreement over the way this should be done. As of 2017, it is widely (but not completely) accepted that the section that includes the majority of the Australian species should retain the name Acacia, whilst other sections of the genus should be transferred to other genera. This species is transferred to Senegalia[
Acacia brosigii Harms
Acacia nigrescens Oliv.
Acacia pallens (Benth.) Rolfe
Acacia passargei Harms
Acacia perrottii Warb.
Acacia schliebenii Harms
Albizia lugardii N.E.Br.
Plant growing in the Pretoria National Botanical Garden, South Africa
Photograph by: JMK
Senegalia nigrescens is a spiny, deciduous tree with a conical crown when young, becoming more rounded as it gets older. It usually grows up to 20 metres tall, exceptionally to 30 metres, with a bole that is usually straight and up to 75cm in diameter. The bole usually has many prickles growing on large knobs, but these are often absent in old trees, The smaller branches have pairs of hooked, blackish prickles up to 7 mm long just below the nodes[
The tree is harvested from the wild, mainly for its timber, though it does also have minor medicinal applications. It is used locally.
Especially in times of drought, many Acacia species can concentrate high levels of the toxin Hydrogen cyanide in their foliage, making them dangerous for herbivores to eat.
East Africa - Tanzania, Zambia, Malawi, Mozambique, Botswana, Zimbabwe, S. Africa.
Woodland and bushland, commonly near rivers, usually on shallow soils on rocky hillsides and on alluvial soils in the valleys; often common and locally dominant on loamy soils. Found at elevations up to 1,200 metres, exceptionally to 1,600 metres[
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Growth is slow, with a maximum annual increment of 60cm in height, usually much less[
The tree is completely deciduous and bare for several months, frequently flowering when leafless. The flowers are an important food resource to giraffe in the late dry season, and it has been suggested that giraffe could be a pollen vector[
The tree usually survives bush fires, resprouting from the base when top-killed[
This species has a symbiotic relationship with certain soil bacteria, these bacteria form nodules on the roots and fix atmospheric nitrogen. Some of this nitrogen is utilized by the growing plant but some can also be used by other plants growing nearby[
Traditional healers of the Shona people in Zimbabwe make an ointment from burnt roots to treat convulsions[
A root decoction is used as an aphrodisiac[
The bark of all Acacia species contains greater or lesser quantities of tannins and are astringent. Astringents are often used medicinally - taken internally, for example. they are used in the treatment of diarrhoea and dysentery, and can also be helpful in cases of internal bleeding. Applied externally, often as a wash, they are used to treat wounds and other skin problems, haemorrhoids, perspiring feet, some eye problems, as a mouth wash etc[
Many Acacia trees also yield greater or lesser quantities of a gum from the trunk and stems. This is sometimes taken internally in the treatment of diarrhoea and haemorrhoids[
Honey bees are particularly fond of the nectar from this plant, and produce a good quality honey from it[
The bark is a source of tannins[
The inner bark can be used to make twine[
The heartwood is dark brown, with light and darker streaks; it is distinctly demarcated from the narrow, whitish yellow sapwood. The grain is often irregular; texture moderately coarse and even. The wood is very heavy; very hard; it has very good natural durability, being extremely resistant to fungal, borer and termite attack. It is difficult to saw, even when green, and dulls tool edges; a good finish can be obtained with waxes and oil. The wood is used for parquet flooring, carving, turnery, fence posts, railway sleepers and mine props. It is occasionally made into furniture, although it is usually considered too heavy for this purpose[
The wood is used as firewood and for charcoal production[
Seed - easy to produce in the nursery the seeds are usually sown into seedbeds, but sowing directly into pots is also possible because of the high germination rate. Seedlings suitable for transplanting have been produced in less than 9 months[
The seed of many members of this genus has a hard seedcoat and may benefit from scarification before sowing to speed up germination. This can usually be done by pouring a small amount of nearly boiling water on the seeds (being careful not to cook them!) and then soaking them for 12 - 24 hours in warm water. By this time they should have imbibed moisture and swollen - if they have not, then carefully make a nick in the seedcoat (being careful not to damage the embryo) and soak for a further 12 hours before sowing.
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