Quick Facts

Quick facts

• Karroo Thorn (Vachellia karroo) is a shrub or tree of up to 12m that is a native plant of southern Africa.
• Karroo Thorn is on the National Environmental Alert list as it has great potential to become a significant weed in Australia.
• Karroo Thorn is drought tolerant and forms dense, impenetrable thickets.
• Seeds are spread by animals, wind and water, as well as by people – it has been intentionally cultivated in several states.
• The widespread distribution of Karroo Thorn in southern Africa indicates that it could become established over most of subtropical and southern Australia.

What Does It Look Like?

What is it?

Karroo Thorn (Vachellia karroo) [as Acacia karroo] is a shrub or tree which grows up to 12 m high. It has straight, white, dark-tipped, paired thorns, usually up to 100 mm long although occasionally as long as 250 mm. The bark is dark red-brown to blackish and rough. Its mature leaves are bipinnate (composed of 8–20 pairs of small oblong leaflets), to 120 mm long and to 50 mm wide.

The yellow flower heads are globular, 10–15 mm in diameter (like Australian wattle flowers), and are sweetly scented.

Seedpods (fruit) are thinly woody, sickle-shaped, to 160 mm long and to 10 mm wide, hairless and slightly constricted between seeds. The tree is usually evergreen but loses its leaves in droughts or in very cold or dry localities (CRC 2003; Weeds Australia undated).

For further information and assistance with identification of Karroo Thorn contact the herbarium in your state or territory.

Flower colour

Yellow

Growth form (weed type/habit)

Tree, Shrub

Where it currently grows? Preferred habitat

Karroo Thorn occupies a diverse range of habitats including dry scrub, river valley scrub, woodland, grassland, banks of dry watercourses, rivers, coastal dunes and coastal scrub. It prefers light to medium well drained soils in an open sunny position and does well in alkaline soil. It is highly drought tolerant. In very dry areas it occurs along watercourses and in other situations where underground water is available (Victorian Resources Online 2007).

Are there similar species?

While Vachellia karroo [as Acacia karroo] is difficult to distinguish from Acacia nilotica subsp. adstringens without fruit (ILDIS 2007) this subspecies is currently not known in Australia.

Why Is It A Weed?

What are its impacts?

Karroo Thorn is on the Alert List for Environmental Weeds, a list of 28 non-native plants that threaten biodiversity and cause other environmental damage. Although only in the early stages of establishment, these weeds have the potential to seriously degrade Australia's ecosystems (CRC 2003).

Agriculture: Karroo Thorn has considerable potential to become a troublesome weed. It is a known invader of established vegetation over a wide climatic range. It has a persistent seedbank and is fire resistant, fast growing and protected from browsing by its thorns. It is weedy in South Africa, where it grows in similar habitat and climate to that found in native grasslands from central New South Wales to southern Queensland. Losses to agriculture in these areas would be significant as the dense, thorny thickets it forms suppress the growth of grasses, prevent stock movement and restrict watering, adding to the costs of mustering (CRC 2003).

How does it spread?

Karroo Thorn reproduces by seed; large trees produce up to 19 000 seeds per year. The seeds can survive for up to 7 years in the soil. Seeds are dispersed by wind and water or in the droppings of animals. Germination is improved when the hard outer casing of the seed is disturbed, for example by fire, passing through the digestive system of animals or gradual weathering over time (CRC 2003).

What is its history in Australia?

The exact origin of the plantings of Karroo Thorn in Australia is unknown. It was first recorded in metropolitan Perth in the early 1960s and may have spread from a residential planting or the Botanic Gardens. The Western Australian population appears to be controlled and extinct (CRC 2003).

How To Manage It?

Best practice management

Contact your state or territory weed management agency or local council if you find Karroo Thorn. Do not attempt control on your own. Control effort that is poorly performed or not followed up can actually help spread the weed and worsen the problem (CRC 2003).

Non-chemical control: Land management: A South African study recommended that targeting grasslands to prevent the Karroo Thorn from reaching pod-bearing age could slow down invasion rates. Management such as removal of immature trees and regular burning may also prevent the formation of seedbanks in grasslands (Walter & Milton 2003). However, heavy defoliation and debarking by goats over one season did not kill mature Karroo Thorn (Scogings & Macanda 2005).

Chemical control: Basal bark, cut stump or stem injection methods can be used to apply herbicides. The plant reshoots from the base if cut, therefore herbicide should be applied to the stump after cutting and treated plants should be checked (NSW Weedwise 2020)

Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au 

Does it have a biological control agent?

NO

When does it grow? (lifecycle/growth calendar)

Karroo Thorn seedlings grow faster during their first three months than most other African Acacia species and can reach a height of 2 m in their first year under favourable conditions. Trees develop a deep root system and usually live for 20–30 years. Plants may flower several times during the summer. The flowers are followed by development of seed pods, which split open on the tree to release small, shiny, brown seeds. The seeds often stay attached to the pods and hang by a thread-like membrane. Germination, which can take place at temperatures from 10 to 40 °C, occurs mainly throughout summer but is possible year round under suitable conditions. Optimum temperatures for growth are between 25 and 30 °C (CRC 2003).

Where Is It Found?

Which states and territories is it found?

NSW, QLD, SA, WA

What areas within states and territories is it found?

Karroo Thorn is well suited to much of Australia's rangelands. It has been widely planted in botanic gardens and zoos in South Australia, Victoria and New South Wales as it is particularly evocative of the African landscape and is said to be the favourite food of the Black Rhinoceros (CRC 2003).

Where does it originate?

In southern Africa Karroo Thorn is the most widespread Acacia. It is very adaptable, growing under many different soil, climate and altitude conditions. Its limiting factors appear to be intense cold and lack of moisture. Eastern Cape Province (with 400-900 mm annual rainfall) is the area mainly affected by infestations of Karroo Thorn, where it forms a major part of the vegetation and has invaded grasslands and large areas of rangelands. Plant densities of 1000–2000 trees/ha have been recorded. In Natal (with 750–900 mm annual rainfall) Karroo Thorn forms part of the coastal dune forest. It is common in the watercourses of the Karroo region of central Cape Province. It is also present in southern Angola, Zambia, Zimbabwe and Mozambique. In the early days of colonisation in southern Africa, Karroo Thorn was used for fuel, fodder and shade, and for the construction of wheels, poles and rural implements. The thorny branches were also used for protection against wild animals (CRC 2003).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

NSW, QLD, VIC, WA

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

YES

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Vachellia karroo

Other scientific names (synonyms)?

Acacia karroo Hayne

Does it have other known common name(s)?

Karoo Thorn, Sweet Thorn, Mimosa Thorn, Cape Thorn Tree, Cape Gum, Gum Arabic Tree, Sour Thorn, White Thorn, Umbrella Thorn