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 Vachellia[].

Common Name: Cape Thorn Tree

General Information

Vachellia karroo is a very variable, thorny evergreen shrub or a tree growing from 2 metres to more than 20 metres tall[]. The tree can lose its leaves in times of drought or in cold weather[].
The plant has a range of uses, being valued particularly for its wood, which makes an excellent fuel, its gum and tannins, and its use in soil stabilization and reclamation projects. It is the most widespread acacia in southern Africa and has been planted experimentally in Australia, India, Israel, South Africa and Zimbabwe[]. The plant is grown as an ornamental[].

Known Hazards

None known

Botanical References

Range

Southern Africa, from the Cape as far north as Angola and Kenya.

Habitat

Woodland, wooded grassland, coastal scrub, often by rivers and in valleys, from near sea-level to 1,520 metres[]. Found in riverine communities and even in arid environments, where it can do well provided there is an assured supply of groundwater[].

Properties

Weed Potential	Yes
Edibility Rating
Medicinal Rating
Other Uses Rating
Habit	Evergreen Tree
Height	10.00 m
Pollinators	Insects
Self-fertile	No
Cultivation Status	Cultivated, Ornamental, Wild

Cultivation Details

Vachellia karroo succeeds in subtropical to tropical areas where it is found at elevations from sea level to 1,000 metres[]. It grows best where the mean annual temperature is in the range 12 - 40°c and the mean annual rainfall is between 200 - 1,500mm[]. The plant is surprisingly hardy - mature plants can tolerate some frost[], and plants have been grown successfully as far north as the British Isles. It is cultivated in Mediterranean climates, including California
Prefers a sunny position. Prefers heavy black, hydromorphic cracking vertisols with high pH, or deep alluvial clay-loam soils in river valleys[]. It sometimes does well on shale and even on acid soils[]. Established plants are drought tolerant303].
Vachellia karroo is able to produce seeds prolifically from an early age; is resistant to fire; regenerates rapidly by means of sucker and forms dense thorny thickets that out compete native flora and are a source of potential injury to people and animals. The species can behave invasively in its native range, competing for space, water and nutrients with pasture grasses and thus replacing them - it is described as a weed of pasture land. It has been declared a noxious weed in several states of Australia and was previously a problem in New South Wales where it was the subject of an eradication programme[, ]. It is also included in the national weed list in South Africa[].
The tree can tolerate fires - top growth can survive moderate fires and seeds will germinate freely after a fire to quickly colonize the newly-bare soil[].
The flowers are fragrant[].
The species has a mixed mating system. It exhibits a tendency towards out-crossing, as evidenced by the existence of trees that are entirely male. It is zoomophilus, principally insect pollinated because the strong colour of inflorescence and the heavy pollen grains attract insects[]. Isolated plants bear no fruits[].
The plant has an invasive root system, making it unsuitable for planting near buildings or paved pathways[].
Considered to be a good indicator of fertile soils for crops and an indicator of surface or underground water[].
Acacia karroo shows a wide range of variation, but not in such a way as to allow any infraspecific taxa to be recognized[].
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

The gum is eaten as a confection[].

The roasted seeds are used as a substitute for coffee[].

Children chew the sweet thorns[].

Medicinal

A root infusion is taken by local people to treat pain in the alimentary canal, rheumatism, convulsions, gonorrhoea and as an aphrodisiac[].

Root powder is applied to penile sores for treating syphilis[].

A bark decoction is an emetic for treating diarrhoea[].

Agroforestry Uses:

Universally accepted for use as a rehabilitation plant in degraded sites and dunes[]. It produces a new flush of leaves when the temperatures are highest and before the rains, when there is a great need for shade to reduce soil temperatures[]. It stimulates the development of an understorey of perennials, palatable and nutritious grasses (such as Cenchrus ciliaris, Panicum maximum) through providing them shade, fixing nitrogen and improving soil structure and infiltration[].
A pioneer species with a climax of 40 years, it occupies a successional position between the tropical forest and the bushveld[].
The long flowering season makes this plant a useful tree for apiculture. Bees collect both pollen and nectar from the flowers[].
Large specimens are an indicator of underground water[].

Other Uses

A gum is obtained from the stems[]. It resembles gum arabic and is used regionally in southern Africa as a substitute for that gum[, ]. It cannot be exported to Europe or the USA because it has not been cleared for toxins[]. Annual production is 25-30 t/ha[].

The bark and seedpods are sources of tannins[]. They impart a reddish colour[, ]. The bark contains up to 19% tannins[].
The Nama people of southern Africa extract a red dye from the bark[].

The root bark is used for twine and rope (like in the traditional Nama mat house)[].

The wood is hard, tough and moderately durable, but few trees reach a commercial size, thereby limiting its commercial use[, ]. It saws easily; planes to a smooth finish; glues and varnishes well - it is used for furniture and the interior finish of buildings[, ]. It is, however, likely to twist in seasoning and is susceptible to attack by borers and fungi[]. Kraft properties have been tested, and it pulps quite easily under standard kraft macro-pulping conditions[].
The wood burns brightly, with very little smoke and no odour[]. It splits easily and once dried does not absorb moisture from the atmosphere. It has calcium oxalate crystals that give its embers high temperatures and make them long lasting. These clean burning traits, ideal for cooking and heating, make it excellent firewood wherever it grows[]. In the coastal dunes of Zululand, South Africa, it is reputed as an excellent charcoal source[].

Propagation

Seed - sow spring in a greenhouse. Prick out the seedlings into individual pots when they are large enough to handle and grow them on in the greenhouse for their first winter. Plant them out in late spring or early summer.
Division in spring. Larger divisions can be planted out direct into their permanent positions. We have found that it is best to pot up smaller divisions and grow them on in light shade in a greenhouse or cold frame until they are growing away well. Plant them out in the summer or the following spring.
Basal cuttings in late spring. Harvest the shoots with plenty of underground stem when they are about 8 - 10cm above the ground. Pot them up into individual pots and keep them in light shade in a cold frame or greenhouse until they are rooting well. Plant them out in the summer.