Quick Facts

Quick facts

• Prickly Acacia (Vachellia nilotica subsp. indica) is native to India and Pakistan and is now a weed of national significance (WONS) in semi-arid Australia.
• It is a thorny shrub or small tree that now covers large tracts of Queensland’s grasslands and also infests watercourses and woodlands.
• Preventing spread (e.g. by quarantining cattle) is the most cost-effective way of managing Prickly Acacia.
• An healthy adult Prickly Acacia tree is capable of as many as 200 000 seeds per year.
• Chemical and mechanical control can be integrated with fire, grazing management and biological control to combat Prickly Acacia.

What Does It Look Like?

What is it?

Prickly Acacia (Vachellia nilotica) [as Acacia nilotica] is a small, thorny, shrub or spreading tree generally growing to about 4-7 m high, occasionally to 10 m. It is usually single stemmed. The deep taproot also has several branches near the surface. The bark of young trees has a tinge of orange and/or green. Young trees usually have long white straight spines 10-50 mm long in pairs at the base of each leaf; older trees have dark, rough bark and tend to lose most of their thorns. Leaves are 30-40 mm long, each comprising up of 10-25 pairs of very small (3-6 mm) leaflets along its length.

Flowers occur as globular flowerheads which are golden-yellow in colour, about 10 mm in diameter in groups of 2-6 in the leaf axils.

Fruit are seed pods which are grey-green in colour, covered in fine hairs and generally 100-200 mm long. The pods are an important distinguishing feature of the plant, having deep constrictions between the seeds that gives them a necklace-like appearance (Gracie 1998; CRC 2003).

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

Flower colour

Yellow

Growth form (weed type/habit)

Tree

Where it currently grows? Preferred habitat

Prickly Acacia grows best on cracking clay soils that have high water holding capacity, but can also grow on sandy soil in areas of higher rainfall. It grows best around waterways and on seasonally inundated floodplains receiving 350-1500 mm of annual rainfall. Young plants may be found growing in areas of sufficient moisture such as around creeks, river levees, bores and dams. Once established they form dense, thorny thickets which become impenetrable to both man and animals. Its growth is also particularly suited to the heavy cracking clay soils of the Barkly Tablelands and is capable of rendering valuable perennial pastures significantly less productive (Gracie 1998; CRC 2003).

Are there similar species?

Three plants are commonly confused with Prickly Acacia. Mesquite (Prosopis spp.) has a very similar leaf structure and growth habit to Prickly Acacia. It may be distinguished by having a cylindrical rather than globular flowerhead and zig-zag reddish branches instead of straight yellowish branches. The seed pods are darker with much less constriction between the seeds. Needle Bush (Vachellia farnesiana) [as Acacia farnesiana] has small, scarcely constricted, cigar shaped pods and zig-zag stems. Mimosa (Mimosa pigra) is a small shrub with leaflets that are sensitive to touch and close when brushed. Prickly Acacia leaflets are not touch-sensitive (Gracie 1998).

Why Is It A Weed?

What are its impacts?

Prickly Acacia is a Weed of National Significance. It is regarded as one of the worst weeds in Australia because of its invasiveness, potential for spread, and economic and environmental impacts.

Agriculture: Prickly Acacia could potentially infest vast tracts of grasslands and woodlands throughout Australia. The economic impacts of Prickly Acacia on Queensland’s grazing industry are estimated at $5 million per year. Even at medium densities, it halves the primary productivity of grasslands, interferes with stock mustering and restricts stock access to water. Control costs considerably outweigh its benefits as a shade tree and drought fodder (CRC 2003).

Native ecosystems: Prickly Acacia dramatically alters the ecological balance of grasslands and thereby threatens biodiversity, particularly in the Mitchell Grass Downs in Queensland, home to 25 rare and threatened animal species and two endangered plant communities. Even a moderate canopy cover of Prickly Acacia reduces grass cover markedly and changes the relative abundance of native plant species in favour of forbs and annual grasses, in turn affecting the native fauna and overall ecology of the system. Infestations also have an impact on tourism and land use by indigenous people (Mackey 1998; CRC 2003).

See Victorian Resources Online (at http://www.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/impact_prickly_acacia ) for a more detailed impact assessment.

How does it spread?

Although capable of regenerating from cut stumps, Prickly Acacia only reproduces by seeds. A healthy, medium-sized tree in a well watered environment can produce as many as 200 000 seeds per year. Seeds may be washed downstream in fast flowing water, but long distance spread in Australia is mainly attributed to consumption of seeds by cattle, which readily eat the nutritious, ripe seed pods. At least 40% of the seeds consumed in this way are viable after being excreted, which is normally up to six days after consumption. Manure assists germination by providing extra moisture and nutrients. Cattle spread viable seeds more effectively than either goats or sheep, which tend to chew the seeds. Transported stock also spread seeds interstate due to the time taken for seeds to pass through the animal’s digestive tract. Seed numbers of 20 000 per hectare are common throughout the heavy clay soils that support Prickly Acacia infestations (Gracie 1998; CRC 2003).

What is its history in Australia?

Prickly Acacia first spread widely in Australia in the early 1900s, when it was planted as a shade and ornamental tree in the Bowen and Rockhampton districts of Queensland. In 1926 the Queensland Department of Agriculture recommended planting Prickly Acacia for shade and fodder for sheep in western Queensland. However, the introduction of cattle into the area, and good wet seasons in the 1950s and 1970s, contributed to an explosion in the abundance and range of Prickly Acacia throughout the Mitchell Grass Downs of central and western Queensland (CRC 2003).

How To Manage It?

Best practice management

Control of Prickly Acacia is possible by mechanical, chemical, ecological and biological means. It is recommended that the least affected areas of Prickly Acacia be controlled first to ensure the best value for money.

Chemical control: is most effective after the wet season when soil moisture is still high, and may be effected by basal bark spray (suitable for stems up to 100 mm in diameter, cut-stump technique or foliar (or overall), spraying (especially effective on seedlings and young plants up to 2 m tall as a follow-up to other forms of control)). Soil-applied herbicides placed as close to the trunk as possible can also be highly effective, especially when applied before rainfall (i.e. October-November for central Queensland).

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

Non-chemical control: Mechanical control should be conducted before the seed pods are dropped (the months before October to January), and will be particularly effective during drought because seedling regrowth is minimised. Large areas with a scattered to medium density of trees with trunk diameters less than 150 mm can be grubbed, cutting the root to at least 300 mm below the soil surface to prevent regeneration. Pushing and stickraking of Prickly Acacia are suited to large areas of medium-density infestation.

Chaining, or double-chain pulling, is especially useful for larger trees (greater than 40 mm trunk diameter) in established very dense stands of Prickly Acacia. Chaining is best suited to the second year of drought or before the first seed pod drop following drought. All forms of mechanical control will require follow-up to check for regrowth. Mechanical and chemical controls are both suitable as follow-ups.

Fire is effective against Prickly Acacia seedlings but mature trees are highly fire resistant.

Biological control: Prickly Acacia has also been recognised as a target for biological control through a cross-jurisdictional government process. This allows activities to be undertaken to develop effective biological controls. Some native insect species that attack native acacias will also attack actively growing Prickly Acacia. One of these insects – the root eating Cicada Cicadema oldfieldi – is thought to be one of the causes of the Prickly Acacia dieback observed occasionally during drought conditions. A biological control program has been operating since 1980. Of 260 insect species known to attack Prickly Acacia, 17 are likely to only feed on Prickly Acacia and are therefore potentially suitable for introduction to Australia. So far, the Queensland Department of Natural Resources and Water has released six insect species into Prickly Acacia infestations across the state. However, only a few of these species have established and their impacts have not been significant. Further research continues into the suitability and potential for success of other biocontrol species (CRC 2003).

For further information see the Prickly Acacia Best Practice Management Manual (available at https://www.weeds.org.au/WoNS/pricklyacacia).

Does it have a biological control agent?

YES. Six agents released with two establishing: the seed-feeding beetle (Bruchidius sahlbergi) and the leaf-feeding moth (Chiasmia assimilis) (Harvey,  et al 2023).

When does it grow? (lifecycle/growth calendar)

Prickly Acacia germinates following rainfall in the wet season. Although 95% of seed is dead after two years, some seeds may still germinate up to 15 years after seed drop. Germination is aided when seeds are disturbed, e.g. by fire or by passing through the digestive system of animals. Seedlings grow rapidly near water but more slowly in open grasslands. Trees can flower and fruit two to three years after germination, and more quickly after high rainfall years (CRC 2003). Below average rainfall following germination generally leads to very low establishment rates of seedlings (Mackey 1998).

Prickly Acacia flowers between March and June, with pods forming between July and December. Most leaf fall corresponds to this dry period between June and November. Seed pods drop from October to January (CRC 2003).

Where Is It Found?

Which states and territories is it found?

NT, QLD, SA, WA

What areas within states and territories is it found?

At present over 6.6 million ha of arid and semi-arid Queensland are infested with Prickly Acacia, mainly in the Mitchell Grass Downs. Scattered infestations have been found along the Queensland coast between Bowen and Maryborough, the Barkly Tablelands and Arnhem Land in the Northern Territory, Cordillo Downs Station in northeastern South Australia and on the Durack River and nearby areas, west of Wyndham, Western Australia. It is now estimated to cover 6.5 million hectares in Queensland and is recorded around Winton, Richmond, Cloncurry and Hughenden, with lighter infestations around Longreach, Bowen and Rockhampton. In the Territory, there are fewer than 10 outbreaks recorded, covering an estimated 600 ha in total, most of which occur on properties along the Barkly Highway. A minor outbreak is also recorded in the Katherine district (Gracie 1998; CRC 2003).

Where does it originate?

Prickly Acacia is native to the tropics and subtropics of Africa (Ethiopia, Somalia) through to Pakistan, India and Burma (CRC 2003).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

YES

Where is it a declared weed?

ACT, NSW, NT, QLD, SA, VIC, WA

Is it on the National Alert List for Environmental Weeds?

NO

Is it on the Agricultural Sleeper List?

NO

Names And Taxonomy

Main scientific name

Vachellia nilotica

Other scientific names (synonyms)?

• Acacia arabica (Lam.) Willd.
• Acacia nilotica (L.) Willd. ex Delile
• Mimosa arabica Lam.
• Mimosa nilotica L.

Does it have other known common name(s)?

Blackthorn, Prickly Mimosa, Black Piquant, Babul