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[].

Common Name: Hookthorn

General Information

Acacia mellifera is a low branched, very spiny shrub or tree with a more or less spherical crown; it rarely grows more than 5 - 6 metres tall, though can occasionally reach 9 metres[, , ]. The tree is heavily armed with pairs of thorns that are small, black, sharp and hooked - the tree having the reputation of being one of the most vicious of all the thorny acacias[, ].
The plant has a number of local uses, but is valued mainly for its gum, which is edible and has a number of other applications. The plant can be grown as a wind shelter and soil stabilizer.

Known Hazards

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.

Botanical References

Range

Northeast Africa from Egypt to Tanzania east to Arabia. Southern Africa - Namibia and Botswana.

Habitat

A commonly occurring shrub on rangelands throughout the savannah in western, eastern and southern Africa[]. The terrain preference is rocky hillsides with rainfall along seasonal watercourses, mixed with other trees[].

Properties

Edibility Rating
Medicinal Rating
Other Uses Rating
Habit	Deciduous Shrub
Height	5.00 m
Growth Rate	Medium
Pollinators	Bees, Lepidoptera
Cultivation Status	Cultivated, Wild

Cultivation Details

A plant of the semi-arid grasslands of tropical and subtropical Africa, it is found at elevations from sea level to 1,500 metres[]. It grows in areas where the mean annual rainfall is in the range 250 - 700mm[].
Prefers loamy soils, though it grows well in black cotton soils and can succeed in a variety of soils from sandy to heavy clays[, ]. It grows on brackish soils in Botswana[]. Established plants are drought-tolerant[].
The plant has a shallow but extensive root system radiating from the crown, allowing the plant to exploit soil moisture and nutrients from a large volume of soil[]. The roots rarely penetrate more than 1 metre into the soil[].
Trees have a moderate growth rate of up to 500mm per year[].
Trees do not coppice well[], but often sucker very freely[].
Flowering and fruiting start 3 years after planting and generally occur twice a year. Flowers are borne on shoots produced the previous year, that is, old wood. The flowers open and shed before the leaves appear[]. The cream/white flowers produce excellent quality honey[].
Due to its high rate of seeding, long viability of seed and ability to overcome arid climates, profuse natural regeneration is generally common[].
If left unattended, especially if grazing is heavy and no fires check its spread, this species may form dense, impenetrable thickets, 2 - 3 metres high and sometimes hundreds of metres across, slowly taking over grazing land[].
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[].

Edible Uses

Gum collected from injured stems is edible and relished by children[]. The tree is a lesser source of gum arabic, more commonly obtained from A. senegal[]. Gum arabic is used as follows:-
The resin obtained from the trunk is important in the food industry, where it is used as an emulsifier, stabilizer and flavour fixative[]. It is also used as an additive (E414) that retards the crystallization of sugar[]. It is found especially in products such as chewing gum and confectionery[].
The resin is harvested after the rainy season by scraping it off the trunk and branches from which it oozes[]. Unhealthy trees tend to give higher yields and incisions are sometimes made into the bark in order to increase yields[].

Medicinal

A decoction of the bark is used in the treatment of stomach-ache, sterility, pneumonia, malaria and syphilis[].

A decoction of the roots is a medicine for stomach pain[].

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[].

Agroforestry Uses:

The plant makes a good live fence and hedge[]. It has a shallow and aggressive root system, which limits its use in farms with crops[]. This root system, however, makes it potentially useful as a soil stabilizer and it has been used experimentally for stabilizing dunes[].
The flowering tree is an excellent bee fodder and is recommended for honey production in dry tropical areas. The flowers smell most strongly at night, when they attract moths for pollination[].

Other Uses

A gum obtained from the trunk is a minor source of gum arabic, more commonly obtained from Senegalia senegal[]. It has a variety of uses, including adding lustre to crape and silk, thickening colours, calico printing, manufacturing ink and as a mucilage[]. When added to clay, the gum forms a hard substance that can be used as a flooring material[].

Baskets made of the roots serve for collecting gum arabic from the trees[]. The bast fibres are used for wickerwork[].

The twigs (with thorns removed!) are used as toothbrushes[].

The heartwood is a very dark brown, almost black; it is clearly demarcated from the yellowish sapwood. The wood is very tough, slightly elastic, does not split easily, and is very durable, being termite and borer-proof. It takes a high polish. Highly valued for axe and other tool handles, it also makes good fence posts[]. It is usually only available in fairly small sizes and so is only taken for small projects such as building huts and the branches for fencing[, ].
The wood makes an excellent firewood and can be used to make charcoal[].

Propagation

Seed - pre-soak overnight in hot water that is allowed to cool[]. Commercially the seeds are scarified by soaking them in concentrated sulphuric acid for 5-15 min prior to sowing[]. Direct sowing of seeds is the common method of artificial propagation - the seeds usually germinating from the 5th day onwards[].
Seed storage behaviour is orthodox; viability can be maintained for several years in hermetic storage at 10°c with 4.5-9% mc[].

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