prickly acacia

Vachellia nilotica
habit (Photo: Land Protection, QDNRW)
habit in flower (Photo: Chris Gardiner)
habit in fruit (Photo: Chris Gardiner)
older stems covered in tiny whitish spots (Photo: Sheldon Navie)
twice-compound leaves (Photo: Sheldon Navie)
leaves and flower clusters (Photo: Land Protection, QDNRW)
close-up of globular flower clusters and flower buds (Photo: Chris Gardiner)
mature fruit (Photo: Land Protection, QDNRW)
old fruit on the ground (Photo: Chris Gardiner)
seedlings (Photo: Chris Gardiner)
Tree
Alternate
Compound
Yellow
Green

A thorny shrub or small tree with twice-compound leaves. A pair of greyish-coloured spines (2-50 mm long) is usually present near the base of each leaf. Its yellow or golden-yellow flowers are borne in small dense globular clusters (8-15 mm across) in the leaf forks. Its large, elongated, and greyish-green pods (6-25 cm long) are deeply constricted between each of the seeds (i.e. they resemble a string of pearls).

Common names 
Also known as: algaroba, babul, babul acacia, black thorn, blackthorn, Egyptian acacia, Egyptian thorn, gum arabic tree, gum-arabic tree, Indian gum-arabic tree, Nile acacia, prickly acacia, prickly mimosa, scented pod acacia, scented-thorn, thorny acacia,&nbsp,
Family 
Mimosaceae
Deciduous 
No
Flowering time 
Autumn-Winter
Native/Exotic 
Exotic
Origin 
Prickly acacia (Acacia nilotica) is native to the arid and semi-arid regions of Africa and western Asia. The sub-species that is found in Australia (i.e. Acacia nilotica subsp. indica) is thought to be native to the Indian sub-continent (i.e. India, P
Notifiable 
No
State declaration 
Category 3 - Must not be distributed or disposed. This means it must not be released into the environment unless the distribution or disposal is authorised in a regulation or under a permit.
Council declaration 
As per State Declaration
Known distribution 

This species is widely naturalised in Queensland and is most common and widespread in the north-western and central-western areas of this state. Major infestations occur from Barcaldine north to Hughenden and west to Longreach, Winton and Julia Creek.. It is also naturalised to a lesser extent in the northern and eastern parts of the Northern Territory, in northern Western Australia, and in north-eastern South Australia. In the north-east of the Northern Territory is is found mostly on properties along the Barkly Highway. In northern Western Australia it has recently been found infesting a few areas near Kununurra in the Kimberley region. In north-eastern South Australia there is a single large infestation at cordillo Downs. Scattered infestations are also present in other parts of Queensland (i.e. along the coast and in southern Queensland) and the Northern Territory (i.e. in the Katherine district). There are also reports of it being present in some inland regions of northern New South Wales, although no herbarium specimens have been collected from this area. Prickly acacia (Acacia nilotica) has become naturalised in other parts of the world, includign eastern Indonesia and the Galapagos Islands. This species is also considered to be a weed in some parts of southern Africa (i.e. within its native range).

Habitat 

Prickly acacia (Acacia nilotica subsp. indica) prefers semi-arid, warmer temperate and sub-tropical regions but is also found in tropical environments and will grow near water sources in arid areas. It is most commonly found growing in grasslands, pastures and open woodlands. Within these plant communities it inhabits floodplains, open plains, gullies, areas near waterways (i.e. creeks and streams) and areas near other water sources (i.e. dams and bores). It also grows on recently cleared land, near stockyards and farm buildings, and along roadsides.

Habit 

A large spreading multi-stemmed shrub or small upright (i.e. erect) single-stemmed tree usually growing 2.5-7 m tall, but occasionally reaching up to 14 m in height.

Impact and control methods 

Prickly acacia (Acacia nilotica subsp. indica) is a Weed of National Significance (WoNS). It is one of 20 plants regarded as the worst weeds in Australia because of their invasiveness, potential for spread, and economic and environmental impacts. This species has largely spread from deliberate plantings as a fodder and shade plant for livestock, but may also have spread from garden plantings in some areas. Many of these plantings were often located near water points (i.e. near waterways and other water sources), from which it has spread out into adjacent grasslands and pastures. This spread was accelerated by the introduction of cattle to these areas, and in some areas infestations now cover entire properties. As well as being one of the most serious weeds in this country, this species is also regarded as one of Australia's worst environmental weeds. It forms dense thorny thickets in grasslands, open woodlands and open forests. Little grows under the canopy in these thickets, as this larger shrub outcompetes pasture grasses for water. Such loss of groundcover then facilitates water and wind erosion and leads to soil degradation. Grassland and open savanna communities in northern Australia are under the most serious threat, and vast areas of the north-eastern sector of the mitchell grass plains in Queensland are suffering from a very serious invasion by this species. In fact, aproximately seven million hectares of the mitchell grass plains in northern Australia have been infested to varying degrees by prickly acacia (Acacia nilotica subsp. indica). It has also been predicted that this species has the capacity to invade a further 50 million hectares of native grassland communities in northern Australia. Failure to adequately manage this species in the future could lead to much of these grasslands being converted into a dense shrubland dominated by this exotic woody weed.The establishment of this woody species in grassland communities causes a significant change in vegetation structure, and is likely to result in substantial changes in the species composition of both the flora and fauna. The increased density of trees eliminates much of the normal ground vegetation from grassland communities and threatens their integrity and biodiversity. Prickly acacia (Acacia nilotica subsp. indica) has been observed to change grasslands from being dominated by a relatively dense cover of perennial grasses to having a relatively sparse groundcover of short-lived ephemeral forbs and annual grasses. It is very likely that this change will seriously disadvantage numerous perennial grass species endemic to the mitchell grass plains and ultimately threaten their survival.Such changes to the vegetation are also thought to pose a threat to the survival of about 25 rare and threatened animal species that are found in the area. The Julia Creek dunnart (Sminthopsis douglasi), a small carnivorous masrupial, is one of these threatened animal species. It is restricted to the mitchell grass plains of north-western Queensland and requires these grasslands as a habitat in which to live. Research has shown that several native grassland lizards, that rely on adequate groundcover for their survival, were absent from areas heavily infested by prickly acacia (Acacia nilotica subsp. indica). This included at least one species of skink (i.e. Ctenotus agrestis) that is only found on the mitchell grass plains. Similar patterns have also been observed in the bird popluation of these areas, with predominantly grassland species being replaced by woodland species in infested areas. The observed changes in animal populations are thought to be caused by the loss of food sources and shelter. Similarly, the spread of prickly acacia (Acacia nilotica subsp. indica) along waterways may also reduce the availability of nesting sites for waterbirds, which cannot nest in these prickly thickets. Dense infestations may also restrict the movement of wildlife and harbour feral animals.

Stem and leaves 

The stems are whitish and either hairy or occasionally hairless when young, but turn a darker colour (i.e. grey to brown) as they age and become woody. They are usually covered in tiny whitish-coloured raised spots (i.e. lenticels). The trunks of mature trees are rough with fissured bark towards their base that is brown, reddish-brown or black in colour. Pairs of stout greyish-coloured spines (2-50 mm long, occasionally up to 10 cm in length) are borne at the base of each leaf (i.e. near the leaf axils). Sometimes these spines may be inconspicuous (i.e. they may occasionally appear to be absent from some branches), and they are usually absent from the trunks and older branches. The twice-compound (i.e. bipinnate) leaves are dark green in colour and have a feathery appearance. They are borne on a relatively short leaf stalk (i.e. petiole) 4-20 mm long, which is finely hairy and sometimes has a small raised structure (i.e. gland) just below the lowest pair of leaf branchlets (i.e. pinnae). The extension of the leaf stalk (i.e. rachis) is 8-68 mm long and bears 2-10 pairs of small leaf branchlets (i.e. pinnae). A small raised gland is usually present on the rachis, at the junction of the uppermost one or two pairs of leaf branchlets (i.e. pinnae). Each of the leaf branchlets (1-5 cm long) bears numerous (7-30) pairs of small, oblong or narrowly oblong leaflets (i.e. pinnules). These leaflets (2-7 mm long and 0.5-2 mm wide) are usually hairless (i.e. glabrous), but sometimes have somewhat hairy (i.e. ciliate) margins, and have rounded tips (i.e. obtuse apices).

Flowers and fruits 

The small yellow or golden-yellow flowers are densely arranged into small globular clusters (6-15 mm across). Each individual flower has four or five relatively inconspicuous petals and sepals and numerous conspicuous stamens that give the flower clusters a very fluffy appearance. The globular flower clusters contain 30-50 of these flowers and are borne on hairy stalks (i.e. peduncles). These stalks (7-32 mm long) emanate from the leaf forks (i.e. axils) and usually have some tiny bracts about half way along them. The flower clusters are borne in groups of two to six in each leaf fork (i.e. axil). Flowering occurs throughout the year, but is most abundant during autumn and winter (i.e. from March to July). The fruit is an elongated pod (6-25 cm long and 4-17 mm wide) that is swollen around each seed and strongly constricted between the seeds (i.e. it is torulose), and therefore resembles a string of pearls. These greyish-green pods are covered in tiny soft hairs (i.e. thye are pubescent), are somewhat flattened, and turn green to brown in colour when mature. However, very old pods may turn dark brown or black in colour after being shed from the plant. They contain several (8-15) seeds, do not split open at maturity (i.e. they are indehiscent), and are most commonly present during late winter, spring and summer (i.e. from May to January). The brown or blackish-brown coloured seeds (6-7 mm long and 4.5-6.5 mm wide) are almost round in shape (i.e. sub-globular) and somewhat flattened. They are smooth in texture and have a hard seed coat.

Reproduction and dispersal 

This plant reproduces by seed, which can remain dormant in the soil for long periods. It is also capable of regenerating from the crown when trees are cut, or after being damaged by fire. Seeds are most often dispersed after being eaten by large animals and then passing through their digestive systems intact. Livestock, such and cattle and sheep, are the most common dispersal vectors. Cattle are thought to be particularly important in the spread of this species because they readily eat the mature seed pods and pass about 50% of the seeds intact. Transportation of livestock that have recently ingested seeds can lead to dispersal of this species over very long distances. Seeds can also be spread significant distances by water, particularly during floods after heavy rain, and may be dispersed by in mud adhering to the legs of animals or vehicles. Strong winds have also been seen to carry the seed pods over relatively short distances (i.e. less than 20 metres).

Similar species 

Prickly acacia (Acacia nilotica subsp. indica) can be confused with several other thorny bushes, including mimosa bush (Acacia farnesiana), karroo thorn (Acacia karroo), the mesquites (Prosopis spp.), cutch tree (Acacia catechu var. sundra) and parkinsonia (Parkinsonia aculeata). These species can be distinguished by the following differences: prickly acacia (Acacia nilotica subsp. indica) is a thorny plant with twice-compound (i.e. bipinnate) leaves having several to numerous (3-10) pairs of branchlets. Its fluffy bright yellow flowers are borne in small globular clusters (8-15 mm across) in the leaf forks. The large fruit are elongated in shape (6-25 cm long and 4-17 mm wide), somewhat flattened, softly hairy (i.e. pubescent), strongly constricted between the seeds (i.e. torulose), and turn greyish-green to in colour as they mature.mimosa bush (Acacia farnesiana) is a thorny plant with twice-compound (i.e. bipinnate) leaves having few (2-4) pairs of branchlets. Its fluffy bright yellow flowers are borne in small globular clusters (11-15 mm across) in the leaf forks. The large fruit are only somewhat elongated in shape (5-7 cm long and 12-15 mm wide), almost cylindrical, often slightly curved, hairless (i.e. glabrous), and turn dark brown in colour as they mature.karroo thorn (Acacia karroo) is a thorny plant with twice-compound (i.e. bipinnate) leaves having few to several (2-7) pairs of branchlets. Its fluffy bright yellow flowers are borne in small globular clusters (about 10 mm across) in the leaf forks. The large fruit are elongated in shape (6-16 cm long and 6-10 mm wide), somewhat flattened, sickle-shaped or curved, smooth and hairless (i.e. glabrous), have slight constrictions between each of the seeds, and turn dark brown in colour as they mature. The mesquites (Prosopis spp.) are thorny plants with twice-compound (i.e. bipinnate) leaves having few (1-5) pairs of branchlets. Its fluffy pale yellow, yellow, greenish-yellow or whitish coloured flowers are borne in elongated clusters (5-14 cm long) in the leaf forks. The large fruit are very elongated in shape (8-20 cm long and 7-15 mm wide), almost cylindrical, have slight constrictions between each of the seeds, and turn yellowish in colour as they mature. Clutch tree (Acacia catechu var. sundra) is a thorny plant with twice-compound (i.e. bipinnate) leaves having numerous (7-24) pairs of branchlets. Its fluffy pale yellow or whitish coloured flowers are borne in elongated clusters (35-75 mm long) in the leaf forks. The large fruit are elongated in shape (5-12.5 cm long and 15-20 mm wide), flattened, softly hairy in the lower part (i.e. partly pubescent), and turn dark brown in colour as they mature. Parkinsonia (Parkinsonia aculeata) is a thorny plant with twice-compound (i.e. bipinnate) leaves having few (1-3) pairs of branchlets. Its bright yellow flowers (about 2 cm across) have five obvious petals and are borne in loose elongated clusters. The large fruit are somewhat elongated in shape (3-13 cm long and about 7 mm wide), almost cylindrical, strongly constricted between the seeds (i.e. torulose), hairless (i.e. glaborus), and turn light brown in colour as they mature. Prickly acacia (Acacia nilotica subsp. indica), mimosa bush (Acacia farnesiana), the mesquites (Prosopis spp.) and parkinsonia (Parkinsonia aculeata) are very often confused. These species are all relatively common and widespread, found in the same areas, and all have spines, compound leaves, yellow flowers and elongated seed pods. See the Queensland Department of Natural Resources and Mines fact sheet on Identification of Prickle Bushes for more help with distinguishing between these species.

Replacement species 
Brisbane wattle (Acacia fimbriata); flat-steemed wattle (Acacia complanata); hickory wattle (Acacia disparrima)