Karroo thorn (Vachellia karroo)

Also known as: Karoo acacia

Karroo thorn is a shrub or tree with distinctive long white thorns and ball-shaped fluffy yellow flowers. It forms dense thickets that compete with other plants and restrict movement of people and animals.

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How does this weed affect you?

Karroo thorn grows quickly and forms dense thorny thickets. It:

• can injure people, livestock, pets and wildlife
• suppresses grasses and reduces productivity in grazing land 
• restricts movement of animals and people
• makes mustering difficult
• blocks access to waterways
• suppresses native vegetation
• reduces food and habitat for native wildlife.

What does it look like?

Karroo thorn usually grow to a height of 12 m tall. It is usually evergreen, but can lose its leaves in very dry or cold conditions. 

Leaves are:

• fern-like, with 8–20 pairs of leaflets.
• up to 12 cm long and 5 cm wide.

The leaflets are:

• light green
• oblong 
• 4–9 mm long and up to 2.5 mm wide
• hairless.

Flowerheads are:

• yellow
• fluffy and ball-shaped, like a pom-pom
• 1–1.5 cm in diameter
• sweetly scented
• in clusters of 4 to 6 balls. 

Seed pods are:

• green and shiny when young, turning brown and woody with age
• up to 16 cm long and 1 cm wide
• crescent or sickle-shaped 
• flattened but slightly moulded around the seeds.

Seeds are:

• shiny brown
• 3.5–9 mm long and 2–7 mm wide
• attached to the pod by a thread.

Stems are:

• covered in rough reddish-brown to black bark, or smooth and greyish-white to greyish-brown when older
• green when young.

Thorns are:

• in pairs
• white
• straight
• usually 10 cm long, but can be up to 25 cm long
• longer, stronger and more crowded at the base of the tree. 

Similar looking plants

Karroo thorn looks similar to mimosa bush (Vachellia farnesiana), prickly acacia (Vachellia nilotica), giraffe thorn (Vachellia erioloba), umbrella thorn (Vachellia tortilis) and other non-indigenous acacias. 

Compared to karroo thorn:

• Mimosa bush has shorter spines (usually up to 2.5 cm long) and cigar-shaped seed pods (up to 8 cm long and 1.7 cm wide). 
• Prickly acacia has shorter spines (up to 50 mm long) and the seed pods (up to 25 cm long) are greyish-green and with very obvious constrictions between seeds.
• Giraffe thorn has shorter spines (up to 60 mm long) which sometimes have swollen bases, and the seed pods are half-moon shaped, a bit like a snow pea pod. 
• Umbrella thorn has a mixture of white spines and shorter, brown, curved spines and strongly coiled pods.

Call the NSW DPI Helpline if you see anything that you suspect might be karroo thorn, prickly acacia or any non-Indigenous acacia other than mimosa bush.

Where is it found?

There are no current known infestations of karroo thorn in NSW. However, its preferred habitat and climate are similar to the native grasslands of central NSW so it could invade NSW rangelands. It also has potential to invade riparian areas in many parts of NSW. 

Karroo thorn grows in Africa, Asia, South America and parts of Europe.

It is a favourite food of the black rhinoceros and was planted in Australian botanical gardens and zoos to represent African landscapes. All karroo thorn plants that were growing in NSW have been eradicated. 

What type of environment does it grow in?

Karroo thorn grows in many different soils types, climates and at many altitudes. Growth is limited by intense cold or lack of moisture. 

It grows best in areas with:

• 400-900 mm rainfall per year
• mean annual temperatures between 12°C and 24°C (although it can tolerate extremes of -12°C to 40°C).  

Maps and records

• Recorded presence of Karroo thorn during property inspections (Map: Biosecurity Information System - Weeds, 2017-2021)
  These records are made by authorised officers during property inspections under the Biosecurity Act 2015. Officers record the presence of priority weeds in their council area and provide this to the NSW Department of Primary Industries. Records reflect the presence of the weed on the date of inspection.
  
  
• Estimated distribution of Karroo thorn in NSW (Map: NSW Noxious Weed Local Control Authorities, 2010)
  Map shows weed distribution and density estimated by local council weeds officers in 2010.

How does it spread?

By seed

Karoo thorn plants start producing fruit within 2 years. Large trees can produce up to 19,000 seeds per year. Seeds can remain viable in the soil for at least 7 years. Germination is improved by fire, weathering and passing through an animal’s gut.

The seeds can spread long distances in the droppings of animals, especially livestock that eat the pods. Seeds and pods can also be spread through waterways and short distances by wind.

References

CRC for Australian Weed Management (2003). Weed management guide - Karroo thorn.

Queensland  Department of Agriculture and Fisheries (2020). Identification of prickle bushes Prickly acacia (Vachellia nilotica), Mesquite (Prosopis spp.), Parkinsonia (Parkinsonia aculeata), Mimosa bush (Vachellia farnesiana). Retrieved 25 August 2020 from: https://www.daf.qld.gov.au/plants/weeds-pest-animals-ants/weeds/a-z-listing-of-weeds/photo-guide-to-weeds/mimosa-bush?a=72195

Queensland, Department of Employment, Economic Development and Innovation. (2011).  Weed risk assessment – Karroo thorn

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Control

Please do not attempt to treat or dispose of this weed yourself. Report this plant if you see it anywhere in NSW by calling the helpline listed at the top of this page immediately.

NSW DPI will lead an initial response for the treatment and disposal of the plant to stop it from spreading.

Early detection

Since karroo thorn is not established in NSW, finding new infestations early gives us the best chance of eradicating it.

Chemical

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.

Herbicide options

See Using herbicides for more information.

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PERMIT 9907 Expires 31/03/2025
Fluroxypyr 200 g/L (Starane™)
Rate: 35mL per L diesel/kerosene
Comments: Basal bark
Withholding period: 7 days.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Fluroxypyr 333 g/L (Starane™ Advanced)
Rate: 21 mL per L diesel/kerosene
Comments: Basal bark
Withholding period: 7 days.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L (Various products)
Rate: One part product to 1.5 parts water
Comments: Cut stump, drill, frill axe or injection
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L with Metsulfuron-methyl 600 g/kg (Various products)
Rate: 1:1.5 glyphosate to water + 1 g metsulfuron to 1 L water
Comments: Stem injection
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
Resistance risk: Moderate

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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area	Duty
All of NSW	General Biosecurity Duty All plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
All of NSW	Prohibited Matter A person who deals with prohibited matter or a carrier of prohibited matter is guilty of an offence. A person who becomes aware of or suspects the presence of prohibited matter must immediately notify the Department of Primary Industries

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Reviewed 2020