Prickly acacia (Vachellia nilotica)

PROHIBITED MATTER: If you see this plant report it to the NSW Invasive Plants & Animals Enquiry Line 1800 680 244

Profile

Impact

Prickly acacia is an invasive thorny tree typically growing 4–5 m high. This weedy species can halve grassland productivity, interfere with stock mustering and restrict animal access to shade and water. Prickly acacia also impacts on the biodiversity of native grasslands, tourism and land use by indigenous people. This plant is one of the worst weeds in Australia due to its invasiveness, potential for spread, and economic and environmental impacts.

Distribution

Prickly acacia is a native tree of the tropics and subtropics. It is found in Ethiopia, Somalia, Pakistan, India and Burma.

In Australia prickly acacia was planted in the early 1900s as a shade and ornamental tree in the Bowen and Rockhampton districts of Queensland. In 1926, it was recommended to provide shade and fodder for sheep in western Queensland. The introduction of cattle into western Queensland and good wet seasons during the 1950s and 1970s saw this thorny tree invade vast areas of the Mitchell Grass Downs of central and western Queensland. Scattered infestations have been found along the Queensland coast, in the Northern Territory, north-eastern South Australia and the south-eastern Kimberley Ranges in Western Australia.

Prickly acacia is not currently known to be present in New South Wales (NSW), but has the potential to invade subtropical and grassland regions, including the North Western Plains.

Distribution map

Spread

Prickly acacia reproduces by seed (175 000 seeds per medium-sized tree each year). Pods and seeds can be moved in fast flowing water. Cattle can transport seeds long distances. They eat the ripe pods and excrete the seeds up to six days after consumption (at least 40% of 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 them. If stock, particularly cattle, from affected areas of Queensland are brought into NSW they should be held in a quarantine area before and after transportation (for at least 7 days) to ensure excretion of viable seeds. The quarantine areas need to be checked for prickly acacia seedlings for seven years.

Description

Prickly acacia is a spreading tree (4–5 m tall but sometimes reaching heights of 10 m), usually with a single-stemmed trunk that has several branches near the soil surface and a deep tap root. Young trees have bark tinged with orange and/or green. Mature trees have dark, rough bark and less thorny stems. Young stems have paired spines (1–5 cm long) at the base of each group of leaves.

Key identification features

  • Leaves (30–40 cm long) are green and fern-like. Each leaf is made up of 10–25 pairs of very small (3–6 mm) leaflets.
  • Flowers are bright yellow and wattle-like. Spherical flower heads (1–1.2 cm diameter) occur on 2 cm long stems. Groups of 2–6 flower heads are found at the base of each leaf joint.
  • Seed pods (10–20 cm long) are flat, grey-green and covered in fine hairs. Mature pods darken to green or brown and have deep, irregular constrictions between each seed (8–10 seeds per pod). Seeds have a very hard brown seed coat.

Habitat

Prickly acacia grows best in areas that have cracking clay soil with high water holding capacity. However, it can also grow on sandy soils when water is abundant and is commonly found on waterways and seasonal floodplains (350–1500 mm per annum).

Acknowledgements

Adapted by AnDi Communications from the CRC for Australian Weed Management Weed Management Guide: Prickly acacia.

Reviewed by Peter Gray. Edited by Elissa van Oousterhout, Birgitte Verbeek.

References

Spies P & March N (2004) Prickly Acacia National Case Studies Manual, Department of Natural Resources, Mines and Energy

Declared Plant Policy (Government of South Australia), Prickly acacia (Acacia nilotica subsp.indica).

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Control

Your local council weeds officer will assist with identification and information on control, removal and eradication of this weed. Infestations can be spread by inappropriate control activities. Individual plants can be manually removed but prickly acacia is capable of regenerating from cut stumps so all stumps and root material should be removed.

Herbicide options

WARNING - ALWAYS READ THE LABEL
Users of agricultural or veterinary chemical products must always read the label and any permit, before using the product, and strictly comply with the directions on the label and the conditions of any permit. Users are not absolved from compliance with the directions on the label or the conditions of the permit by reason of any statement made or not made in this information. To view permits or product labels go to the Australian Pesticides and Veterinary Medicines Authority website www.apvma.gov.au

See Using herbicides for more information.


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: 7 days.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate


Fluroxypyr 333 g/L (Starane™ Advanced)
Rate: 900 mL per 100 L of diesel
Comments: Basal bark cut stump application
Withholding period: 7 days.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate


Picloram 44.7 g/kg + 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


Triclopyr 240 g/L + Picloram 120 g/L (Access™ )
Rate: 1.0 L in 60 L of diesel
Comments: Basal bark/cut stump application.
Withholding period: Nil
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate


Triclopyr 600 g/L (Garlon® 600)
Rate: 1.0 L in 120 L of diesel
Comments: Basal bark/cut stump application.
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 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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For technical advice and assistance with identification please contact your local council weeds officer.
For further information call the NSW Invasive Plants and Animals Enquiry Line on 1800 680 244 or send an email to weeds@dpi.nsw.gov.au

Reviewed 2017