Geography and distribution
Acacia karroo is the most widespread Acacia in southern Africa and occurs from southern Malawi, southern Zambia and southwest Angola to the southern African coast and parts of Botswana. It has been introduced to North Africa, Australia, India, Myanmar, South America (including Argentina, Bolivia, Chile and Paraguay), where it is often used as a living fence. It is found from coastal dunes to 2,600 m above sea level, though it is most common at medium altitudes (1,000-1,800 m).
Sweet thorn can grow in a range of habitats, from the arid Karoo desert to the sand dunes of the KwaZulu-Natal and Mozambican coast, but is commonly found in woodlands and bushland on clay and loam soils, often in association with other Acacia and Combretum species. It can form dense stands in alluvium along rivers and on red clay.
Fertile tree with round crown and yellow inflorescences (Photo: Phil Shaw)
Overview: Sweet thorn is a small to medium-sized tree, usually 4-8 m tall, but specimens up to 17 m tall have been found. Open-grown young trees often have a ‘skirt’ of small dead branches around the stem, which fall as the tree matures to leave a clean bole with a round crown. Trees in dense stands can be tall and spindly. The smooth bark is dark brown to black on the bole, and often has an orange tinge when young, and becomes fissured when old. The trunk is normally 20-30 cm in diameter.
Twigs: Young twigs have a shiny or sticky green appearance on the youngest growth, sometimes with a few small red glands. Older twigs are dark brown, flaking to reveal a reddish under-layer.
Thorns: The long, straight thorns are whitish with brown tips and are paired at the nodes. They are normally 2-5 cm long, and are sometimes inflated along their length. The thorns are found on the later part of the season’s growth.
Leaves: The leaves are alternate, bipinnate, 3-6 x 2-3 cm with 2-6 pairs of pinnae, each with 8-20 pairs of dark green, smooth, medium-sized leaflets (of 4-7 x 1.5-3 mm). Trees at higher altitudes have the largest number of pinnae.
Flowers: The inflorescences consist of numerous globe-shaped heads each comprising many sweet-smelling golden yellow flowers, the heads are 0.8-1.8 cm in diameter, clustered at the end of the current season’s branchlets.
Pods and seeds: The smooth, slender, sickle-shaped pods are constricted between the seeds and are reddish brown when ripe. The pods are 5-10 cm long, 5-8 mm wide and hang in bundles, becoming twisted after dehiscing (opening to release seeds) on the tree. The olive-green to brown seeds are 5-8 x 3-5 mm, oblong-elliptic and compressed. The seed areole, a distinct area bounded by a fine line, the pleurogram, is lighter in colour, and occupies a large area of the seed (4.5-5.5 x 2-3.5 mm).
The flowering season usually begins in November, after the rains arrive, and continues until January. Second and even third flushes of flowers can appear later in the season, depending on the rainfall pattern. Flowering seems to be initiated by a period of high rainfall, following on from a drier spell.
Acacia karroo shows a wide range of variation, and is sometimes difficult to distinguish from A. natalitia. This is especially true for forms of A. karroo with smaller and more numerous leaflets. A. karroo is closely related to A. seyal, which can be distinguished from it by the enlarged inflated spines, which are a pair being distinct to the base and not confluent below into a rounded more or less two-lobed structure. Without flowers or fruits it is difficult to separate Acacia karroo from Acacia nilotica subspecies adstringens.
Threats and conservation
Sweet thorn tends to be an invasive species in poorly managed rangeland, but if the trees are allowed to grow and are then thinned and pruned, and cattle have access to the grass underneath, a parkland can develop with a high potential for meat production and soil conservation. For good browse production the trees should be widely spaced and should branch low down; for cattle about 2 m high and for goats about 1.5 m. Planting about 1,600 stems per hectare of Acacia karroo and thinning them to about 400 plants per hectare is suggested as an optimum plant density for mixed cattle and goat herds.
Sweet thorn is a fast-growing pioneer and readily establishes itself in suitable disturbed areas. It is resistant to frost and drought, and once it has become established it has no major threats. Outside its native range it can be a problematic weed.
Larvae of the butterflies Azanus jesous (topaz-spotted blue), A. moriqua, A. natalensis, A. ubaldus, Crudaria leroma, Anthene amarah (black striped hair tail), and the emperor moth (Heniocha apollonia) feed on Acacia karroo leaves, but these do not threaten A. karroo populations.
Branch with leaves and white spines (Photo: Phil Shaw)
A red-gold gum is collected from the tree and is sold commercially as a gum arabic substitute. This edible gum is often chewed by children, monkeys and bush-babies, and is used in the food, pharmaceutical, glue, detergent, ink, paint and agrochemical industries, as well as for glazing pottery. Zimbabwe is the largest producer of Acacia karroo gum, but nowadays gum from Zimbabwe cannot be sold in Europe or the USA.
Sweet thorn is also used in traditional medicine. An infusion of the roots is used by the Ndebele against general body pains, by the Shona against dizziness, convulsions and gonorrhoea, and sometimes as an aphrodisiac. Roots are placed in chicken runs to reduce parasites. A decoction of the bark is used as an astringent, emetic and as an antidote to ‘tulip’ (Moraea language) poisoning in cattle. The mucilage of the gum is used to relieve thrush in the mouth. A substance has been found in the heartwood which is said to control high blood pressure.
The wood is dense, hard and durable and is used for furniture, wagon wheels, yokes, rural implements, turnery, fence posts, coffins and wood wool. Individual trees can produce over a cubic metre of wood in 25 years under conditions where few other species, including exotics, could do as well. The wood has a high calorific value, making it a valuable as a source of fuelwood and charcoal. The bark contains 19% tannins and yields a dye which gives a yellow-brown colour to leather.
Like many other legumes, sweet thorn can fix nitrogen from the air with the aid of symbiotic bacteria living in root nodules, and this nitrogen then enriches the soil. In drylands, it is widely used to rehabilitate degraded land (such as mine spoil) and to stabilise dunes.
Sweet thorn is very effective when used for live fencing and forms one of the best brushwood fences. Acacia karroo leaves and pods provide good forage for cattle and goats. The sweet inflorescences are good bee forage; the long flowering season makes this tree a particularly valuable source of pollen and nectar for honey production. The roots are used to drive away evil spirits (as are Acacia nilotica roots). Sweet thorn is also used for fibres and resins and the seeds are sometimes roasted to make a coffee substitute.
Branch with leaves and yellow 'pompom' inflorescences (Photo: Phil Shaw)
Sweet thorn seeds often have a high rate of bruchid (beetle) infestation, but once a seed collection has been cleaned a germination rate of up to 70% can be achieved. The seed coat is not very hard, and nicking or filing can easily break seed dormancy, although this can be hard to carry out as the seed is so small. An alternative method is to carefully pound the seed in a mortar, or to pour boiling water over the seeds, followed by cold water and then allow the seeds to soak. The seedlings emerge after 5-13 days, and are ready to be planted out when they are 50 cm tall. Sweet thorn seedlings are fast-growing and readily establish themselves on degraded sites. Trees normally live to about 25 years old, and few reach 30-40 years.
Acacia karroo has been successfully propagated from seed at Kew, where it is grown in the Temperate House. The temperature is kept above 2˚C, as sweet thorn is not hardy in the UK. The roots can be invasive, so it is important to site trees carefully to prevent damage to nearby structures. A. karroo will grow in most soil types, as long as they are free-draining. Fertiliser such as bonemeal should be added to the planting hole. In the Temperate House, watering is reduced in the winter. During the rest of the year sweet thorn should be watered thoroughly and deeply from time to time until established. A karroo is prone to attack by mealy bugs, and at Kew pest populations are kept in check by spraying the affected areas with a forceful jet of water.
Sweet thorn at Kew
Sweet thorn was previously grown in Kew's Temperate House (now closed for restoration until 2018).
Kew's Economic Botany Collection has samples of Acacia karroo wood, bark, gum, branches and spines. Dried specimens of A. karroo are also held Kew's Herbarium, and details, including images, of some of these can be seen on-line in the Herbarium Catalogue. These specimens are available to researchers by appointment.
South Africa Landscape - Kew at the British Museum
Between April and October 2010, Kew and the British Museum brought a small corner of South Africa to the heart of London.
The South Africa Landscape celebrated a shared vision to strengthen cultural understanding and support biodiversity conservation across the world.
Acacia karroo (sweet thorn) was one of the star plants featured in the Landscape.