Deciduous shrub or small tree with robust thorns, and small bright yellow spherical flower heads.
Scientific name: Acacia karroo Hayne
Common names: sweet thorn, karroo thorn, mimosa thorn
Family: Fabaceae (Leguminosae)
Status in Portugal: invasive species (listed in the Decreto-Lei nº 92/2019, 10 july)
Risk Assessment score: 22 | Value obtained according to a protocol adapted from the Australian Weed Risk Assessment (Pheloung et al. 1999), by Morais et al. (2017), according to which values above 13 mean that the species has risk of having invasive behavior in the Portuguese territory | Updated on 30/09/2017.
Synonymy: Acacia dekindtiana A. Chev., A. eburnea sensu auct., A. horrida auct., non Willd., A. inconflagrabilis Gerstner, A. karroo Hayne, A. natalitia E. Meyer
Last update: 30/06/2021
How to recognise it
Shrub or tree up to 15 m high, with a rhytidome with longitudinal ridges.
Leaves: deciduous, bipinnate with 2-7 pairs of pinnae, which in turn have 5-15 pairs of leaflets, of 4-8 x 2-3 mm; thorny stipules, with 5-100 mm, whitish.
Flowers: bright yellow, grouped in globular flower heads com 15-18 mm diameter, groups of 4-6.
Fruits: brownish-grey pods, compressed, curved sickle-shaped, and slightly constricted between the seeds.
Flowering: June to September.
There are other species of the Acacia genus with thorny stipules that show some similarity, but have not yet been found in Portugal.
Characteristics that aid invasion
It reproduces by seed, producing many seeds. An adult tree produces up to 19000 seeds each year, which may be viable for 7 years in the ground. Germination is stimulated by fire.
Native distribution area
South Africa to Zambia and Angola.
Distribution in Portugal
Mainland Portugal (Beira Litoral, Estremadura, Baixo Alentejo, Algarve).
Geographic areas where there are records of Acacia karroo
Other places where the species is invasive
Europe (Spain), South Africa, some regions of Australia.
For ornamental purposes and to form hedges.
Preferential invasion environments
It is found dispersed along roadsides, but as yet in only a few locations in this country.
It tolerates adverse conditions like frost, drought, fire, strong winds and salt spray.
In the native distribution area it grows from 0-1800 m, from sand to silt soils, and in locations where there is some available water in the soil.
Even though it’s legally considered as being an invasive in Portugal, its distribution is still limited in our country.
Impacts on ecossystems
It can potentially form dense thickets, inhibiting the development of native vegetation.
It produces a lot of nitrogen-rich litter, which promotes soil change.
It has potential expensive control measures.
The robust thorny stipules cause injuries, inhibit the presence of animals and hamper control operations.
Controlling an invasive species demands a well-planned management, which includes the determination of the invaded area, identifying the causes of invasion, assessing the impacts, defining the intervention priorities, selecting the adequate control methodologies and their application. Afterwards it is fundamental to monitor the efficiency of the methodologies and recuperation of the intervened area as to perform, whenever necessary, the follow-up control.
The control methodologies used for Acacia karroo include:
Hand pulling: preferential methodology for seedlings and small plants. When in more compacted substrates, pulling must be made during the rainy season as to facilitate the removal of the root system.
Cutting: preferential methodology for adult plants. Cut the trunk as close to the ground as possible by using manual and/or mechanical equipment. It should be done before seed maturation.
It may be strategically used to favour germination of the seed bank, e.g., after the control of adult individuals (with the adequate management of the resulting biomass) and the subsequent elimination of seedlings. This method provides an advantageous reduction of the seed bank, both by destroying part of the seeds or by stimulating the germination of the remainders.
Visit the webpage How to Control for additional and more detailed information about the correct application of these methodologies.
Department of Environment and Heritage (2003) Weed management guide – karroo thorn (Acacia karroo). Department of Environment and Heritage, Canberra, 6pp.
Department of Primary Industries (2014) Karoo and giraffe thorn. Available: http://www.depi.vic.gov.au/agriculture-and-food/pests-diseases-and-weeds/weeds/state-prohibited-weeds/karoo-and-giraffe-thorn [Retrieved 03/03/2014].
Dufour-Dror J-M (2012) Alien invasive plants in Israel. The Middle East Nature Conservation Promotion Association, Ahva, Jerusalem, 213pp.
Marchante E, Freitas H, Marchante H (2008) Guia prático para a identificação de plantas invasoras de Portugal Continental. Imprensa da Universidade de Coimbra, Coimbra, 183pp.