mesquite or algarroba
Native to North and South America, mesquite is a shrub or tree that can form dense, impenetrable thickets. Three mesquite species and a mesquite hybrid have been recorded in Queensland. Once used for soil stabilisation and as a shade tree around homesteads, mesquite has spread significantly in Queensland.
This species is widespread throughout northern Australia, particularly in inland regions. It is most common in the north-western areas of Queensland and in the central regions of the Northern Territory. Scattered populations occur in southern and central Queensland, inland northern New South Wales, coastal northern Western Australia and other parts of the Northern Territory.
Algaroba (Prosopis pallida) is mostly found in wetter habitats within semi-arid and arid environments (i.e. along creeks, in gullies, in intermittent stream-beds, etc.). It also grows along roadsides, in pastures, in open woodlands and near habitation.
A spiny tree usually growing less than 10 m tall, but occasionally reaching up to 15 m in height.
Algaroba (Prosopis pallida) is regarded as a significant environmental weed in Queensland, the Northern Territory and Western Australia, and as apotential environmental weed or "sleeper weed" in other parts of Australia. This species is actively managed by community groups in Queensland and the mesquites (Prosopis spp.), as a group, are also one of the 20 Weeds of National Significance (WoNS) in Australia.
The bark of older branches is usually rough, fissured, and grey, brown or blackish in colour. Younger branches have smooth bark, are green or reddish in colour, and often have a zig-zagging pattern to their growth. They are sometimes sparsely hairy (i.e. puberulent), but generally become hairless (i.e. glabrous) with age. The stems are almost always armed with a pair of spines (3-17 mm long) just above each leaf fork (i.e. axil).
Mesquite leaves are fern-like. They occur at each point where the branch changes direction (the zig-zag) in groups of one to three pairs, often with one or two thorns. The leaves are alternately arranged, twice-compound (i.e. bipinnate), and fern-like in appearance. They are borne on finely hairy stalks (i.e. puberulous petioles) 8-20 mm long and usually have two to four pairs of branchlets (i.e. pinnae). Each branchlet (i.e. pinna) is 2-5 cm long and bears 7-15 pairs of oblong to slightly elongated leaflets (i.e. pinnules). These leaflets (2-10 mm long and 1.4-4 mm wide) are generally about 2-4 times longer than they are broad. They are usually somewhat hairy (i.e. pubescent) on both sides and have weakly pointed or rounded tips (i.e. acute or obtuse apices).
The flowers are arranged in elongated, cylindrical, clusters (i.e. racemes). These flower clusters (8-12 cm long) are borne on stalks (i.e. peduncles) 4-30 mm long and two to five of them can arise from a single leaf fork (i.e. axil). The small flowers are greenish, whitish, cream or more commonly yellowish in colour. They consist of five inconspicuous sepals (0.5-1 mm long) that are joined into a short tube (i.e. calyx tube), five small petals (1.5-3 mm long) that are joined together at the base, and 10 prominent stamens (5-6 mm long). Flowering occurs mostly during summer.
The fruit is a large elongated pod (6-25 cm long and 7-15 mm wide) that often has slight constrictions between each of the seeds. These pods are hairless (i.e. glabrous) and turn from green to yellowish-brown as they mature. They contain 5-20 hard seeds that are embedded in a sugary pulp. The seeds (4-7 mm long and 2.5-5 mm wide) are oval (i.e. ellipsoid) in shape, glossy in appearance, and brown or reddish-brown in colour.
Algaroba (Prosopis pallida) reproduces mainly by seed, however existing stands can become more dense through suckering and layering.
Seeds are most commonly dispersed after being eaten by animals. They are also spread by floodwaters, in mud attached to vehicles and machinery, and by other human activities.
It is very difficult to distinguish between individual mesquite species (Prosopis spp.), as hybrids and intermediates between them often occur in the field. The mesquites (Prosopis spp.) can also be confused with several other woody weeds, including prickly acacia (Acacia nilotica), mimosa bush (Acacia farnesiana) and parkinsonia (Parkinsonia aculeata). All of these species have spines, twice-compound (i.e. bipinnate) leaves, yellow flowers and elongated seed pods. However, algaroba (Prosopis pallida) can be distinguished from these species by its elongated cylindrical flower clusters. Prickly acacia (Acacia nilotica ) can also be distinguished by its strongly constricted seed pods, mimosa bush (Acacia farnesiana) by its shorter (less than 6 cm long) pods, and parkinsonia (Parkinsonia aculeata) by its greenish-coloured stems and drooping strap-like leaves.
