Prickly Acacia (Acacia nilotica)
Prickly Acacia (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. The globular flowerheads are golden-yellow, about 10 mm in diameter in groups of 2-6 in the leaf axils. The seed pods are grey-green, 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.
For further information and assistance with identification of Prickly Acacia contact the herbarium in your state or territory.
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.
- Prickly Acacia (Acacia nilotica) is one of four prickle bushes that are weeds throughout semi-arid Australia.
- It 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.
- Chemical and mechanical control can be integrated with fire, grazing management and biological control to combat Prickly Acacia.
How it spreads:
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.
Where it grows:
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.
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.
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.
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.
Prickly Acacia is native to the tropics and subtropics of Africa (Ethiopia, Somalia) through to Pakistan, India and Burma.
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.