APC (2020). Australian Plant Census, Council of Heads of Australasian Herbaria (CHAH). Available at: https://biodiversity.org.au/nsl/services/apc. [accessed 16/11/2020]
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Black Locust (Robinia pseudoacacia) is a fast-growing, deciduous, root suckering tree, reaching 10-15-metres tall, but occasionally growing to 17 metres in height. Stems and branches have stout spines (or thorns), and are produced uniformly in pairs. These spines are stipules (small appendage at the bases of leaves) that becoming spiny, 5–25 mm long. New growth may also occur from root suckers and is particularly spiny (Jeanes 1996; Hussey & Keighery 1997). Suckering clumps can eventually form dense thickets. The leaves are 8-15 cm long overall, divided in to 5 to 10 pairs of leaflets (11-21 leaflets in total), each oval to obovate (egg-shaped with the widest part near the tip) in out line, which are 15-50 mm long to10–30 mm wide with entire margins (no teeth), and obtuse (bluntly pointed) to emarginate ( notched at apex), with a mucronate (small sharp point). Leave have a petiole (leaf stalk) 20–40 mm long.
Flowers are white, pink or purplish with green sometimes yellow markings in the centre, 15–20 mm long, typically pea-shaped, sweet scented, produced in pendulous (hanging) many flowered racemes (stalked flower heads), in spring (Sep.–Nov). Each flower is on pedicels (flower stalk) 5–10 mm long.
Fruits are brown woody, flattish pods, also pendulous (hanging), glabrous (smooth without hairs) that are 30-80 mm long, 1 0–15 mm wide, and flattish, green turning light brown to eventually dark brown. Pods contains 4-10 mottled blackish seeds each about 5 mm long.
For further information and assistance with the identification of Black Locust, contact the herbarium in your state or territory.
Black Locust (Robinia pseudoacacia) occurs principally on river flats, in lowland grassland and grassy woodland. It is also persistent in abandoned gardens (Carr et al. 1992; Jeanes 1996; Thorp, J.R. & Wilson, M. 1998). It occurs in gardens and and sucker throughout and under fences roadsides in country and urban areas.
Prefers full sun and well drained soils and invades dry and sandy sites, ex-pasture, disturbed forest, disturbed shrubland, forest edges, plantation forest and roadsides.
In Australia, Black Locust is most likely to be confused with Honey Locust (Gleditsia triacanthos) which is a widely grown ornamental and a common street tree. It differs from Black Locust in having more leaflets (normally more than 25) per leaf, each of which is obscurely toothed along the leaf edge. Like Black Locust, Honey Locust also has spines, but these are produced separately from the leaves and not in pairs whereas in Black Locust the spines are paired at the leaf-bases (however in both species, spines are not always present). The flowers of Honey Locust are quite different from Black Locust, not pea-like, but very tiny and yellowish, with 8 segments (4 petals; 4 sepals, or modified leaves) of more or less equal size and shape produced in spikes 5-10 cm long. Golden Locust is also naturalised to some extent in Queensland, New South Wales and Victoria (AVH 2020; Wiecek 1991).
Black Locust (Robinia pseudoacacia) can produce a large amount of root suckers to form quite dense thickets or copses that exclude native and other vegetation (DPI NSW (2019; Weiseler 2008). Black Locust has become a locally serious weed in Europe and in the united States of America outside of its natural range. In parts of Germany it has been responsible for the conversion of grassland into woodland (Thorp, J.R. & Wilson, M. 1998). Suckering dense clumps are all interconnected and further disturbance encourages further dense suckering.
Agriculture: Suckering plants can invade open paddocks, reducing carrying capacity. In addition, all parts of the plant including the leaves, wood, roots, pods and seeds are toxic to stock (Blood 2001). Black Locust is also fatal to horses and in one case a woman gave her horses some Robinia pseudoacacia to graze on. Within two hours both horses were in distress, shaking, frothing at the mouth and in much pain. The vet was able to save one of the horses, with the older horse put down after hours of agony. The owner was previously unaware that the plant was poisonous (Thomson 2007). Horses feeding on Robinia pseudoacacia bark died after 4 hours (Wink & Van Wyk, 2008)
Native ecosystems: In Victoria, it has invaded lowland grassland and grassy woodlands, as well as riparian vegetation (Carr et al. 1992). Will invade grassy open areas, and suckers can invade edge of open woodlands, and vegetation on roadsides .
Urban areas: Individual trees can grow without suckering long into maturity, and do not sucker. Suckering occurs when roots are disturbed, or the tree is cut down, then prolific suckering can occur.
Black locust is toxic to humans (Blood 2001) and can cause discomfort and irritation, but is reported as not life-threatening (DPI NSW 2019). All parts of the plant are poisonous, especially the pods, seeds, bark and leaves. It can cause dizziness, nausea, vomiting, diarrhea, convulsions and drowsiness.
What to do if poisoning occurs:
Black Locust reproduces both by seed and by root suckers. Root suckers usually are more important to reproduction than are seedlings. Individual plants of Black Locust produce root suckers when the roots or foliage are disturbed, resulting in the development of thick copses. Sprouting is an important mechanism for colonising areas that have herbaceous plant cover but no woody canopy. Grasses can form an impenetrable barrier that prevents establishment of Black Locust seedlings, but root sprouts are able to colonise these areas. Black Locust seeds are reported to disperse mostly by gravity and wind. They have a hard coat which is responsible for their longevity. Water or mass movement of soil containing seeds may assist in long distance dispersal. Although Black Locust may produce copious amounts of seed, few germinate (Thorp & Wilson 1998; University of California 2008; Wieseler 2008).
Black Locust was probably introduced into Australia as an ornamental tree, although it is valued in the United States of America for its durable timber and elsewhere as a good honey plant (Weiseler 2008). The earliest herbarium record in Australia for this species appears to be 1918 (Wirabara Forest, South Australia), although some undated collections may be older than this. The majority of herbarium records of this species are relatively recent collections and are from collections made after 1980 (AVH 2020).
Black Locust (Robinia pseudoacacia) is a suckering tree, re-sprouting from underground roots. Disturbance of roots or removal of tree normally results in dense suckering clumps. Unless the near impossible task of removing all roots, suckering will ocurr. After above ground control the only feasible means of control is l Chemical, with follow-up control required.
Chemical control: Wieseler (2008) reports control by applying 15:1 water-glyphosate solution to cut stems and stumps, although follow-up applications may be required. More effective control may be achieved by applying a 25% triclopyr solution in basal oil (3:1 oil:triclopyr) applied immediately to cut stumps (Wieseler 2008). Standard chemical control techniques for woody weeds (e.g. 'drill and fill' or 'frill and fill' as outlined in Muyt 2001) are likely to be effective. More effective control may be achieved by applying a 25% triclopyr solution in basal oil (3:1 oil:triclopyr) applied immediately to cut stumps (Wieseler 2008).
For summary on herbicide use, see: (Brown & Bettink 2009-; DiTomaso & Kyser et al. 2013; DPI NSW 2019; Wieseler 2008). Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au
Non-Chemical control: Seedling rarely occur, but where young seedlings appear, seedling with tap root can be physically removed. Larger trees that are cut down will sucker, and removal of suckers will be ongoing. Chemical long term control may be the only way to successful treat Black Locust, preventing ongoing sucking.
Black Locust (Robinia pseudoacacia) flowers in spring and fruits ripen in late summer. Growth from root suckers occurs as new leaves develop in spring (National Herbarium of Victoria 2008; VicFlora (2016). Seedlings are intolerant of shade and herbaceous competition, but, once established, they are capable of growing over 1 m per year on better sites. Saplings begin producing seed at about six years. Most seed production occurs between fifteen and forty years of age. Seed production continues until about age sixty. Heavier seed crops are produced at one- to two-year intervals (University of California 2008).
In the United States, Black locust sprouts leaves in late spring and shoots elongate rapidly. Trees reach mature heights in twenty to forty years, but are short-lived. Trees often die from forty years of age, and rarely live more than 100 years (University of California 2008).
Black locust produces shoots from stump and roots. Shoot production is stimulated by root and foliage damage. Root suckers first appear when stems are four to five years old. Shoot production is greatest in full sun (University of California 2008).
ACT, NSW, QLD, SA, TAS, VIC, WA
Black Locust (Robinia pseudoacacia) occurs in scattered localities from south-eastern Queensland to Adelaide, South Australia, mostly on and near the Dividing Range, but extending as far inland as Roma in Queensland and West Wyalong and Deniliquin in New South Wales. It occurs also between Perth and Albany in Western Australia, and near Launceston and Hobart, Tasmania (AVH 2020).
Black Locust is native to the United States of America (GRIN 2020).
False Acacia, Locust Tree, Yellow Locust, Robinia