Prickly acacia (Vachellia nilotica)

Prickly acacia is a thorny tree with yellow flowers. It invades grasslands, forms dense thickets and reduces livestock productivity.

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How does this weed affect you?

Prickly acacia is a small tree that can form dense prickly thickets. It:

• can halve livestock productivity
• makes livestock mustering difficult
• restricts animal access to water and shade
• reduces habitat and food for native animals
• reduces biodiversity in grasslands
• has negative impacts on tourism and land use by indigenous people.

What does it look like?

Prickly acacia is a thorny, spreading tree. It usually grows to 4–5 m but occasionally is up to 10 m tall.

Leaves:

• are fern-like with 3–10 pairs of primary leaf segments which are further divided into 10–25 pairs of leaflets
• usually have a gland just below or between the two primary leaf segments closest to the stem.

The leaflets are

• green
• oblong
• 3–6 mm long and 0.5–1.5 mm wide.

Thorns are:

• in pairs at the base of the leaves
• light grey
• straight
• 1-5 cm long
• some spines drop off as the plant matures.

Flowers are:

• bright yellow fluffy balls that look like wattle flowers
• 10–12 mm in diameter
• in groups of 2–6 flower heads at the base of each leaf joint.

Seeds pods are:

• grey-green, white, or grey
• brown when dry
• 6–25 cm long and 1.4–1.7 mm wide 
• flat with up to 16 seeds
• constricted between each seed with the size of the gap between seeds varying down the pod
• covered in fine hairs. 

Seeds are:

• 7 mm long
• hard with a brown coat.

Stems are:

• covered in orange and/or green tinged bark when young and dark, rough bark when mature
• branched near the soil surface, with the branches rising like single trunks.

Roots:

Prickly acacia has a deep taproot. 

Similar looking species

Prickly acacia looks like:

• Karroo thorn (Vachellia karroo) which has much larger spines (up to 25 cm long). 
• Mesquite (Prosopis spp.) which has cylindrical greenish-cream flowers up to 8 cm long.
• Mimosa bush (Vachellia farnesiana) which has shorter spines (usually up to 2.5 cm long) and cigar-shaped seed pods.
• Parkinsonia (Parkinsonia aculeata) which has flowers with 5 petals, short curved spines (up to 1 cm) and does not have fern-like leaves.

Call the NSW DPI Biosecurity Helpline if you see a tree that you suspect might be prickly acacia or any other introduced acacia species.

Where is it found?

There are no known infestations in NSW.  Prickly acacia could invade grasslands such as the North Western Plains.

Prickly acacia is native to arid and semi-arid regions of Africa and Asia.

It was brought to QLD in the early 1900s as a shade, fodder, and ornamental tree. The tree invaded vast areas of the Mitchell Grass Downs in Queensland during the 1950s and 1970s. 

What type of environment does it grow in?

Prickly acacia grows in arid and semi arid regions with warm temperate or subtropical climates. It grows well in cracking clay soils with high water holding capacity, but it can also grow on sandy soils when water is abundant. Prickly acacia often grows along waterways, around bore drains, and on seasonal floodplains with an annual rainfall of 350–1500 mm.

Maps and records

• Recorded presence of Prickly acacia during property inspections (Map: Biosecurity Information System - Weeds, 2017-2023)
  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.
  
  

How does it spread?

By seed

A medium sized prickly acacia tree produces about 175,000 seeds each year. The seeds can survive in the soil for at least 6 years before germinating.  

The seeds are spread long distances by water and cattle. Cattle eat the ripe pods and excrete the seeds up to six days later. At least 40% of these seeds remain viable. The manure provides extra moisture and nutrients for seed germination and seedling growth. Goats and sheep chew the seeds and are less likely to spread the weed.

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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.

Prevention

Livestock, particularly cattle, from affected areas in QLD should be held in a clean paddock for at least seven days before being moved to areas that are not infested with prickly acacia. Check quarantine paddocks regularly and remove any prickly acacia. The quarantine areas need to be checked for prickly acacia seedlings for seven years.

Physical

Prickly acacia can regrow from cut stumps or roots so ensure that these are all removed. 

Chemical

Foliar spraying, basal barking, cut stump and stem injection methods may be used.

Herbicide options

See Using herbicides for more information.

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Fluroxypyr 333 g/L (Starane™ Advanced)
Rate: 450 mL in 100 L of water
Comments: Spot spray, seedlings and young plants up to 2 m tall
Withholding period: Do not graze failed crops and treated pastures or cut for stock food for 7 days after application. See label for more information.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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Fluroxypyr 333 g/L (Starane™ Advanced)
Rate: 900 mL per 100 L of diesel
Comments: Basal bark cut stump application
Withholding period: Do not graze failed crops and treated pastures or cut for stock food for 7 days after application. See label for more information.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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Picloram 44.7 g/L + Aminopyralid 4.47 g/L (Vigilant II ®)
Rate: Undiluted
Comments: Cut stump/stem injection application. Apply a 3–5 mm layer of gel for stems less than 20 mm. Apply 5 mm layer on stems above 20 mm .
Withholding period: Nil.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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Triclopyr 240 g/L + Picloram 120 g/L (Access™ )
Rate: 1.0 L in 60 L of diesel (or biodiesel such as Biosafe).
Comments: Basal bark application for plants with stems up to 10 cm diameter at the base. Cut stump application for plants with stems greater than 10 cm diameter at the base. See label for information about biodiesel.
Withholding period: Nil
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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Triclopyr 600 g/L (Garlon® 600)
Rate: 500 mL in 60 L of diesel
Comments: Basal bark application for plants with stems up to 10 cm diameter at the base. Cut stump application for plants with stems greater than 10 cm diameter at the base.
Withholding period: Nil.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
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 pest 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 2022