Acacia karroo (sweet thorn)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Biology and Ecology
- Latitude/Altitude Ranges
- Air Temperature
- Rainfall Regime
- Soil Tolerances
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Impact Summary
- Environmental Impact
- Social Impact
- Risk and Impact Factors
- Uses List
- Wood Products
- Similarities to Other Species/Conditions
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Acacia karroo Hayne
Preferred Common Name
- sweet thorn
Other Scientific Names
- Acacia dekindtiana A. Chev.
- Acacia eburnea sensu auct.
- Acacia hirtella E. Mey.
- Acacia horrida var. transvaalensis Burtt Davy
- Acacia inconflagrabilis Gerstner
- Acacia karroo var. transvaalensis (Burtt Davy) Burtt Davy
- Acacia natalitia E. Mey.
International Common Names
- English: Cape thorn tree; karroo thorn; mimosa thorn
- French: mimosa à longues épines; mimosa hérissé
Local Common Names
- Australia: gum arabic tree; sour thorn; umbrella thorn; white thorn
- Botswana: mooka
- Germany: Akazie, Süssdorn-; Akazie, Weissdorn-
- Italy: acacia orrida; mimosa karroo
- South Africa: Cape gum; cassie; doorn boom; doringboom; mookana; piquants blancs; soetdoring; umuNga
- Zimbabwe: isinga; mubayamhondoro; muunga
- ACAKA (Acacia karroo)
Summary of InvasivenessTop of page A. karroo is able to produce seeds prolifically, from an early age, is resistant to fire 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 and Aubrey and Reynolds (2002) claim that this can occur as a consequence of overgrazing and it is described as a weed of pasture land (ILDIS, 2004). A. karroo 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. Binggeli (1999) classes it as a potentially invasive species.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Fabales
- Family: Fabaceae
- Subfamily: Mimosoideae
- Genus: Acacia
- Species: Acacia karroo
Notes on Taxonomy and NomenclatureTop of page Acacia karroo is in the tribe Acacieae of the large genus Acacia, also containing the other African acacias.
DescriptionTop of page A. karroo varies from a shrub up to 2 m tall to a tree more than 20 m in height, with distinctive white thorns and attractive yellow flowers. It is quite variable morphologically and regional variations are described by Barnes et al. (1996). Bark is rough, dark, red-brown to blackish. The leaves bipinnate, typical of many legumes, light green, up to 120 mm long and about 50 mm wide, composed of 8-20 pairs of small oblong leaflets. Thorns are paried and straight, conspicuous white in colour, often 10 cm long, sometimes up to 25 cm, occurring especially on the lower branches. Flowers yellow, in round inflorescences 10-15 mm in diameter, in clusters of between four and six, and are sweetly scented. Fruit are pods, up to 160 mm long and 10 mm wide, sickle-shaped, woody and slightly constricted between the seeds. The tree is usually evergreen but loses its leaves in droughts or in very cold or dry localities.
Plant TypeTop of page Broadleaved
DistributionTop of page A. karroo is the most widespread acacia in southern Africa. It occurs naturally in Angola, Botswana, Lesotho, Malawi, Mozambique, Namibia, South Africa, Swaziland, Zambia and Zimbabwe, between 15°S and 34°S.
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 10 Jan 2020
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Planted||Reference||Notes|
|South Africa||Present||Native||Carr (1976); CABI (Undated)|
|Zambia||Present||Native||CABI (Undated)||Original citation: Aubrey and Reynolds (2002)|
|Zimbabwe||Present||Native||Drummond (1981); CABI (Undated)|
|India||Present||CABI (Undateda)||Present based on regional distribution.|
|Italy||Present||CABI (Undateda)||Present based on regional distribution.|
|-New South Wales||Absent, Eradicated||FNCW (2001)|
|-Queensland||Absent, Eradicated||FNCW (2001)|
|-South Australia||Present||Introduced||FNCW (2001)|
|-Western Australia||Absent, Eradicated||FNCW (2001)|
History of Introduction and SpreadTop of page In it uncertain when A. karroo was introduced to Australia, but it has since spread to become an invasive weed. It has also been introduced to northern Africa and elsewhere, presumably for its noted multipurpose value. A. karroo is a notifiable weed (W1) in New South Wales, the presence of which must be notified to the Local Control Authority within three days of detection, and which must be destroyed (NWSEC, 1998). It is listed as a P1/P2 noxious weed in the Agriculture and Related Resources Protection Act in Western Australia, meaning that it is not yet widely established in the state, and should be eradicated and should not be traded, sold or moved (NWSEC, 1998). Similarly it is listed as a P1/P2 weed in Queensland where its introduction is illegal, and where any plants present in the state must be destroyed (NWSEC, 1998). It has recently been declared a State Prohibited Weed in Victoria, i.e. a species posing a serious threat to state environmental, economic and social values and not present, or occurring in sufficiently small numbers that eradication can be achieved. A. karroo is believed to have been eradicated from New South Wales and Queensland (FNCW, 2001).
Risk of IntroductionTop of page Like other Acacia spp., A. karroo has been promoted for soil improvement and as a fodder crop. As the species is known to behave invasively in its native range, and in its exotic range (e.g. Australia) introduction to other countries/regions is likely to carry a risk of the species becoming invasive, particularly where other factors (such as overgrazing) are likely to occur.
HabitatTop of page A. karroo is the most widespread acacia in southern Africa and occupies a diverse range of environments from acacia savannahs and woodlands on hills and rocky soils to the banks of dry watercourses. These habitats include scrub forest, bushland and thickets, more specifically Karroo-Namib scrub forest, the Kalahari-Highveld regional transition zone and the Tongaland-Pondoland regional transition zone (ILDIS, 2004). Aubrey and Reynolds (2002) state that it is generally found in low ground to highveld altitudinal zones. It can also dominate riverbed vegetation (DEH, 2005).
Habitat ListTop of page
|Terrestrial – Managed||Managed grasslands (grazing systems)||Present, no further details||Harmful (pest or invasive)|
|Terrestrial ‑ Natural / Semi-natural||Natural grasslands||Present, no further details||Harmful (pest or invasive)|
|Riverbanks||Present, no further details||Harmful (pest or invasive)|
Biology and EcologyTop of page Genetics
In is unknown whether any systematic selection or genetic improvement work has been undertaken on A. karroo, but noting the widespread appreciation of its many valued products in its native range, it is likely there has been some traditional selection of improved material.
Physiology and Phenology
A. karroo is a relatively short-lived African acacia, which may live for up to 40 years (Aubrey and Reynolds, 2002). In South Africa, the pods start to split and open seeds in January (Anon., 2001). New (1984) reports that young seedlings are vulnerable to temperature changes and dessication, and that colonization of South African veldt is promoted by long periods of sufficient moisture in combination with high temperatures. However, very high soil temperature may kill seedlings. A. karroo does not require shade, shelter or protection from fire, and plants more than one year old are able to regenerate after fire (Aubrey and Reynolds, 2002). Trees are able to coppice vigorously and produce root suckers.
The first flowers are produced when the plant is two or three years old (Anon., 2001). The flowers are attractive to insects and are insect pollinated (Aubrey and Reynolds, 2002). Pods split open on the tree, releasing the seeds which are dispersed by wind and can also be distributed in animal dung, namely by stock and game that feed on the pods (Aubrey and Reynolds, 2002). The seeds are able to withstand heat allowing A. karroo to re-establish after fire (CRC Weed Management, 2002). The Australian Government (DEH, 2005) cite information quantifying the prolific seeding ability of this species, and the long viability of the seeds. Seeds are orthodox.
A. karroo is generally a subtropical species in southern Africa, preferring mean annual temperatures of 12-24°C, and being replaced by A. seyal in more tropical climates. It can. however, tolerate mean maximum temperatures of the hottest month as high as 40°C and a dry season of up to 9 months, but will also tolerate frosts and an absolute minimum temperature of -12°C. Mean annual rainfall ranges are from as low as 200 mm up to 1500 mm. A. karro tolerates a wide range of soil types including shallow and saline soils. It is found from sea level to altitudes up to 1800 m.
A. karroo fixes nitrogen. In its native range, grasses and other plants utilize the higher nitrogen resources found below the shade of this tree (Aubrey and Reynolds, 2002).
Latitude/Altitude RangesTop of page
|Latitude North (°N)||Latitude South (°S)||Altitude Lower (m)||Altitude Upper (m)|
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Absolute minimum temperature (ºC)||-13|
|Mean annual temperature (ºC)||12||24|
|Mean maximum temperature of hottest month (ºC)||30||40|
|Mean minimum temperature of coldest month (ºC)||-2||12|
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Dry season duration||3||9||number of consecutive months with <40 mm rainfall|
|Mean annual rainfall||200||1500||mm; lower/upper limits|
Rainfall RegimeTop of page Summer
Soil TolerancesTop of page
- seasonally waterlogged
Special soil tolerances
Notes on Natural EnemiesTop of page Aubrey and Reynolds (2002) reports that A. karroo is affected by a number of fungi and parasites including mistletoes, but that none are considered to seriously affect tree growth.
Means of Movement and DispersalTop of page The pods and seeds may be blown by the wind, but it more likely that spread is facilitated by being eaten by mammals and distributed in dung (CRC Weed Management, 2002). Concerning long-distance dispersal, A. karroo has been deliberately introduced to countries in Asia, Europe, Australasia and Indian Ocean islands for use as a fodder plant, an ornamental and for soil stabilization and improvement.
Impact SummaryTop of page
|Fisheries / aquaculture||None|
Environmental ImpactTop of page Lubke and Bredenkamp (1998) report that overgrazing can lead to invasion of A. karroo into grassland, forming dense thickets at the expense of grass species. The species' ability to fix nitrogen is likely to lead to changed patterns of nutrient cycling where it occurs as an alien invasive species.
Social ImpactTop of page It is difficult to handle due to its thorns which may cause injury to people or animals.
Risk and Impact FactorsTop of page Invasiveness
- Invasive in its native range
- Proved invasive outside its native range
- Highly adaptable to different environments
- Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
- Highly mobile locally
- Has high reproductive potential
- Has propagules that can remain viable for more than one year
- Damaged ecosystem services
- Ecosystem change/ habitat alteration
- Negatively impacts agriculture
- Negatively impacts tourism
- Reduced amenity values
- Reduced native biodiversity
- Competition - monopolizing resources
- Produces spines, thorns or burrs
- Difficult to identify/detect in the field
- Difficult/costly to control
UsesTop of page A. karroo is of great socio-economic importance in the seasonally dry to arid regions of southern Africa. It fixes nitrogen, provides shade, stabilizes sand dunes and disturbed areas and is resistant to fire, drought, frost, termites and salinity. When pruned or after losing peripheral thorny branches, A. karroo only has a moderately dense crown that provides a suitable environment for sustained production of nutritious perennial grasses such as Panicum maximum and Cenchrus ciliaris (Barnes et al., 1996). Due to the presence of thorns, it is used and promoted as a living hedge. However, its use as an urban ornamental is limited as its invasive root system means that it is unsuitable for planting near buildings or paved pathways.
The wood is reddish brown, hard, moderately heavy and makes excellent fuel as it burns brightly and evenly with little smoke or odour, producing good coals and little ash. It is however, noted for its multipurpose nature, and that in southern Africa it is highly regarded, with almost every tree part being used in some way.
The leaves, flowers and pods are good fodder, although it has a high tannin content that may impair accessibility of protein in livestock rumens. It yields clear gum of high quality, although the only country in which it is collected and used as a substitute for gum arabic (gum from A. senegal) is Zimbabwe (Barnes et al., 1996). It is a good tree for bees (Carr, 1976; Timberlake, 1980), providing large quantities of pollen and nectar and can flower three or four times a year.
The bark can be used for tanning leather and the inner bark makes good cord, twine and rope. A. karroo roots are prescribed as an aphrodisiac in Zimbabwe and for treating pain in the digestive tract, rheumatism, convulsions and gonorrhoea (Gelfand et al., 1993). Thorns were used as needles for stitching cloth, and even by insect collectors to 'fix' their specimens.
Uses ListTop of page
Animal feed, fodder, forage
- Fodder/animal feed
- Boundary, barrier or support
- Soil improvement
Human food and beverage
- Honey/honey flora
- Carved material
- Miscellaneous materials
- Source of medicine/pharmaceutical
Wood ProductsTop of page
- Building poles
- Roundwood structures
- Industrial and domestic woodware
- Tool handles
Similarities to Other Species/ConditionsTop of page ILDIS (2004) reports similarity of A. karroo to A. nilotica subsp. adstringens when not in fruit, noting that records from Mauritius included Acacia nilotica subsp. adstringens.
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.No information on the control of A. karroo specifically is available from the literature, though it may be assumed that similar treatments as used on other invasive acacia species may be appropriate.
ReferencesTop of page
Anon, 2001. Acacia karroo, No. 120. Pretoria, South Africa: University of Pretoria, Manie van der Schijff Botanical Garden. http://www.up.ac.za/academic/botany/garden/species/120.html.
Aubrey A; Reynolds Y, 2002. Acacia karroo Hayne. South Africa National Botanical Institute (SANBI), Witwatersrand National Botanical Garden. http://www.plantzafrica.com/plantab/acaciakar.htm.
Binggeli P, 1999. Invasive woody plants. http://members.lycos.co.uk/WoodyPlantEcology/invasive/index.html.
Carr JD, 1965. The propagation of indigenous trees. Trees in South Africa, 17(2):30-40.
Chidari G; Charambaguwa F; Matsvimbo P; Mhiripiri A; Kamanya P; Muza W; Muyombo T; Chanakira J; Mutsvanga X; Mvumbe A; Nyamadzawo P; Fortmann L; Drummond RB; Nabane N, 1992. The use of indigenous trees in Mhondoro district. Occasional paper-NRM No. 5. Zimbabwe: University of Zimbabwe, Centre for Applied Social Sciences.
Coates-Palgrave K, 1996. Trees of Southern Africa. Cape Town, South Africa: C. S. Struik Publishers.
CRC Weed Management, 2002. Thorny issue at the Waite. Australia: CRC Weed Management (Cooperative Research Centre for Australian Weed Management). http://www.weeds.crc.org.au/documents/thorny_issue.pdf.
DEH, 2005. Weeds on the National Environmental Alert list. Australian Government, Department of the Environment and Heritage, Canberra, Australia. http://www.deh.gov.au/biodiversity/invasive/weeds/alert-list.html.
Drummond RB, 1981. Common trees of the central watershed woodlands of Zimbabwe. Salisbury, Zimbabwe: Natural Resources Board.
FNCW, 2001. W1 Category noxious weeds in the far north coast weeds county district. New South Wales, Australia: Far North Coast Weeds (FNCW), Far North Coast County Council. http://www.fncw.nsw.gov.au/noxious/w1.html.
Gelfland M; Mavi S; Drummond RB; Ndemera EB, 1993. The traditional medical practitioner in Zimbabwe. Gweru, Zimbabwe: Mambo Press.
ILDIS, 2004. International Legume Database and Information Service. University of Southamptom, UK. http://www.ildis.org/.
Lubke R; Bredenkamp G, 1998. Eastern Thorn Bushveld. Department of Environmental Affairs and Tourism. http://www.ngo.grida.no/soesa/nsoer/Data/vegrsa/veg16.htm.
Muller TH, 1979. Acacia species cultivated in the National Botanic Garden, Salisbury, Rhodesia. International Group for the Study of Mimosoideae Bulletin, 7:36-39.
Ncube S; Dube JS; Hove L, 1992. Value of browse, wild fruits as livestock feed. The Farmer (Zimbabwe), 60(18):7-8.
New TR, 1984. A biology of the acacias. Oxford, UK: Oxford University Press.
NWSEC, 1998. Noxious Weeds List for Australian States and Territories. National Weeds Strategy Executive Committee. http://www.weeds.org.au/docs/weednet6.pdf.
Scott JD, 1967. Bush encroachment in South Africa. South African Journal of Science, 63:311-314.
Timberlake J, 1980. Handbook of Botswana Acacias. Gaborone, Botswana: Ministry of Agriculture, Division of Land Utilisation.
Timberlake JR; Nobanda N; Mapaure; I, (1993). Vegetation survey of the communal lands north and west Zimbabwe. Kirkia, 14(2):171-270.
van Aarde RJ; Smit AM; Claassens AS, 1998. TI: Soil characteristics of rehabilitating and unmined Coastal Dunes at Richards Bay, KwaZulu-Natal, South Africa. Restoration Ecology, 6(1): 102-110.
CABI, Undated. CABI Compendium: Status as determined by CABI editor. Wallingford, UK: CABI
CABI, Undated. CABI Compendium: Status inferred from regional distribution. Wallingford, UK: CABI
CABI, Undated. Compendium record. Wallingford, UK: CABI
Drummond RB, 1981. Common trees of the central watershed woodlands of Zimbabwe., Salisbury, Zimbabwe: Natural Resources Board.
FNCW, 2001. W1 Category noxious weeds in the far north coast weeds county district., New South Wales, Australia: Far North Coast Weeds (FNCW), Far North Coast County Council. http://www.fncw.nsw.gov.au/noxious/w1.html
Distribution MapsTop of page
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