Acacia karroo (sweet thorn)
The fast-growing sweet thorn, with its striking yellow pompom-like flowerheads, is perhaps the most well-used acacia in southern Africa.
Species Information
- Common name(s): sweet thorn, Karroo thorn, mimosa thorn, cockspur thorn, Cape gum (English); soetdoring, doorn boom (Afrikaans)
- Conservation Status: Rated by the IUCN as of Least Concern (LC).
- Habitat: Woodlands and bushland, on clay and loam soils. Also found in desert and on coastal sand dunes.
- Known hazards: The long thorns can cause injury to humans and animals, and can puncture vehicle tyres.
Taxonomy
- Class: Equisetopsida
- Subclass: Magnoliidae
- Superorder: Rosanae
- Order: Fabales
- Family: Leguminosae - Mimosoideae
- Genus: Acacia
About this species
Acacia karroo is one of the fastest-growing acacias, and produces high-density wood (800-890 kg/m³). It is named after the Karoo region of the former Cape Province of South Africa, where it is common, and often the only tree found. The common name sweet thorn possibly refers to the sweet smell of the flowers, or to the fact that the presence of the species often indicates sweetveld (an area of vegetation that is good for grazing). Acacia karroo grows on deep, blackish nutrient-rich clay soils, and not on sand, and because of this association it is regarded as an indicator of good agricultural soils and rangeland.
Geography & 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 metres 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.
Description
More InformationFertile tree with round crown and yellow inflorescences (Image: Phil Shaw)
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. 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. 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. 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. 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. 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 & Conservation
More InformationSweet 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.
Uses
More InformationBranch with leaves and white spines (Image: 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.
Millennium Seed Bank: Seed storage
Kew's Millennium Seed Bank partnership aims to save plant life world wide, focusing on plants under threat and those of most use in the future. Seeds are dried, packaged and stored at a sub-zero temperature in our seed bank vault.
Description of seeds: Green to brown, oblong elliptic in outline, compressed, 5-8 x 3-5 mm, the two faces of the seed with a U-shaped pleurogram surrounding the central areole. There are about 18,000 seeds per kg.
Number of seed collections stored in the Millennium Seed Bank: Four.
Seed storage behaviour: Orthodox? (the seeds of this plant survive being dried without significantly reducing their viability, and are therefore amenable to long-term frozen storage such as at the MSB).
Germination testing: 100% germination on 1% agar at 21 ºC, 12/12 (pre-sowing treatments = seed sterilised (immersed in 10 % Domestos for 5 minutes), seed scarified (chipped with scalpel)).
Cultivation
Branch with leaves and yellow "pompom" inflorescences (Image: 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 can be seen growing in the Temperate House, in beds 22 and 24.
The Economic Botany Collection, one of the behind-the-scenes areas of Kew, has samples of Acacia karroo wood, bark, gum, branches and spines. Dried specimens of A. karroo are also held in the Herbarium, and details, including images, of some of these can be seen on-line in the Herbarium Catalogue. These specimens are made available to bona fide researchers from around the world 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.
Read Kew's South Africa Landscape blog.
Watch the video and view photos of the building of the Landscape.
References & Credits
Barnes, R.D., Filer, D.L. & Milton, S.J. (1996). Acacia karroo; monograph and annotated bibliography. Tropical Forestry Papers 32. Oxford Forestry Institute, Oxford.
Bisby, F.A., Buckingham, J. & Harborne, J.B. (1994). Phytochemical Dictionary of the Leguminosae. ILDIS. Chapman & Hall, London.
Carr, J.D. (1976). The South African Acacias. Conservation Press (PTY) Ltd., Johannesburg.
Timberlake, J., Fagg, C. & Barnes, R. (1999). Field guide to the Acacias of Zimbabwe. CBC Publishing, Harare.
Kew Science Editor: Lulu Rico
Kew contributors: Andrew Luke
Copyediting: Emma Tredwell
While every effort has been taken to ensure that the information contained in these pages is reliable and complete, the notes on hazards, edibility and suchlike included here are recorded information and do not constitute recommendations. No responsibility will be taken for readers’ own actions. Full website terms and conditions.
- Print friendly
- Send to a friend
-
Send to a friend Close
- Spread the word
-
Spread the word Close
- Blog this
- Donate
Fact Box
Salix x sepulcralis
golden weeping willow
Salix x sepulcralis 'Chrysocoma' is today the most widely grown weeping willow.
Related Tags
- ancient
- ornamental
- of use
- flowering
- medicinal
- edible
- common
- weed
- tasty
- wild
- passionate
- new
- newly discovered
- extraordinary
- rare
- clever
- healing
- valuable
- scarce
- massive
- collectable
- unusual
- discovered
- around the world
- adventurous
- ancient
- historical
- interesting
- amazing
- beautiful
- inspiring
- landscapes
- mysterious
- big
- pretty
- ground breaking
- fragrant
- creative
- fun
- imaginative
- exotic
- vibrant
- essential
- english garden
- garden plants
Plant & Fungi News from Kew
The Millennium Seed Bank joins seed swap events
31 Jan 2012
Following the success of our Great Seed Swap, held at Wakehurst Place on 17 September 2011, the Millennium Seed Bank is supporting seed swap events around Sussex and further afield this year.
7 likes
0 comments
Kew is recruiting a new Director
30 Jan 2012
After six years at Kew, the current Director (CEO and Chief Scientist), Professor Stephen D. Hopper FLS will be returning to Australia to take up a Chair in Biodiversity at The University of Western Australia.
14 likes
0 comments
GIS surveys in New Guinea
13 Dec 2011
Kew botanists have helped to survey the vegetation of Biak Island, off the north coast of New Guinea.
5 likes
1 comment
Welcome to Stephen Hopper's blog
by: Prof. Stephen Hopper, 04 Dec 2009
A successful 250th anniversary year for Kew.
- 123 likes
- 4 comments
World's smallest waterlily brought back from the brink of extinction at Kew
Kew’s top propagation ‘code-breaker’, horticulturist Carlos Magdalena, has cracked the enigma of growing a rare species of African waterlily. The 'thermal’ lily (Nymphaea thermarum) is believed to be the smallest waterlily in the world, with pads that can be as little as 1 cm in diameter.
84 likes
4 comments
Director (CEO and Chief Scientist) of the Royal Botanic Gardens, Kew to return to Australia
The Royal Botanic Gardens, Kew announced today that Director (CEO and Chief Scientist), Professor Stephen Hopper FLS will step down in autumn 2012 after six years in the job.
74 likes
0 comments
