Black Locust
Robinia pseudoacacia
NEMBA category
English: Black Locust, False Acacia, Yellow Locust, White Locust
Afrikaans: Witakasia, Valsakasia
Robinia pseudoacacia, commonly known as Black Locust, is a medium-sized, fast-growing deciduous tree with a somewhat open, irregular crown. Its bark is dark brown and deeply furrowed, often forming diamond shapes. The branches are typically thorny, especially on younger growth. Its leaves are feathery, composed of many oval leaflets, and turn yellow in autumn. In late spring to early summer, it produces beautiful, fragrant white, pea-like flowers in drooping clusters, which are followed by flattened, reddish-brown seed pods that often hang on the tree through winter.
This tree is a problematic invasive species in South Africa, as it forms dense, thorny thickets through prolific root suckering, displacing indigenous vegetation and altering ecosystems. All parts of the plant, especially the seeds and bark, are poisonous.
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Origin:
Native to the southeastern United States, primarily the Appalachian Mountains and Ozark Plateau.
Distinguishing features:
Paired thorns: Sharp, stout, paired thorns (modified stipules) are typically found at the base of the leaves, especially on younger stems.
Feathery leaves with oval leaflets: Leaves are compound, with 7-19 oval, smooth-margined leaflets arranged along a central stalk. They are bright green and turn yellow in autumn.
Fragrant white pea-like flowers: Showy, white, fragrant flowers with a yellow blotch on the standard petal are borne in drooping clusters (racemes).
Flat, reddish-brown pods: Produces flattened, smooth, dark reddish-brown seed pods (7-10 cm long) that contain 3-8 seeds and often persist on the tree through winter.
Deeply furrowed bark: Mature bark is dark reddish-brown to grey-black and deeply furrowed, often forming diamond patterns.
Root suckering: Readily produces new shoots (suckers) from its root system, often forming dense, thorny thickets.
Overall appearance:
A medium-sized, deciduous tree, often appearing upright with a narrow or somewhat irregular crown. It is recognised by its thorny branches, distinctive compound leaves, showy white flowers, and flattened seed pods. It frequently forms dense, clonal stands due to its aggressive suckering habit.
Height:
Typically grows 10 to 25 metres tall, but can reach up to 30 metres in ideal conditions.
Width:
The crown can spread 5 to 12 metres wide. Due to its extensive root suckering, it can form thickets covering much larger areas.
Leaves:
Deciduous, alternate, and pinnately compound (feathery), 20 to 30 centimetres long. Each leaf consists of 7 to 19 (typically 9-19) oval to elliptical leaflets, 3 to 5 centimetres long, arranged along a central stalk. The leaflets are dull green, smooth-margined, and often have a small bristle tip. They turn yellow in autumn.
Flowers:
Showy, fragrant, white, pea-like flowers, 1.5 to 2 centimetres long, with a yellow blotch on the upper petal (standard). They are borne in dense, drooping clusters (racemes), 10 to 20 centimetres long, appearing from leaf axils. The flowers are bisexual.
Flowering season (South Africa):
Late spring to early summer (typically September to November).
Fruits:
Flat, linear-oblong pods (legumes), 7 to 10 centimetres long and 1 to 1.5 centimetres wide. They are smooth, thin-walled, dark reddish-brown, and usually contain 3 to 8 seeds. The pods often persist on the tree through winter.
Seeds:
Small, hard, kidney-shaped to oblong, dark brown to blackish seeds, about 3 to 5 millimetres long. They have a hard, impermeable coat and can remain viable in the soil for a long time.
Bark:
On young stems, the bark is smooth and greenish-brown. On mature trees, it becomes dark reddish-brown to greyish-black, deeply furrowed with prominent ridges that often cross to form diamond-shaped patterns.
Thorns:
Prominent, stiff, and sharp thorns (spines), 1 to 2.5 centimetres long, are typically found in pairs at the base of the leaves on young shoots and branches. They can persist on older wood.
Wood:
Hard, dense, strong, durable, and highly resistant to decay, even in contact with soil. It is valued for fence posts, flooring, furniture, and firewood.
Roots:
Develops a strong, shallow, and aggressive lateral root system that produces numerous suckers (new shoots) over wide areas, leading to the formation of dense, clonal thickets. It also has nitrogen-fixing root nodules, enriching the soil.
Soil type:
Highly adaptable to a wide range of soil conditions, including poor, sandy, rocky, or degraded soils. It prefers well-drained loams but can tolerate acidic soils (pH 4.8) and some drought once established. It does not tolerate waterlogging.
Aspect:
Prefers full sun. It is very tolerant of a wide range of temperatures, including cold winters, and can withstand moderate drought. It is a pioneer species often found in disturbed areas.
Edibility:
Highly toxic. All parts of the plant, particularly the bark and seeds, contain toxic proteins (toxalbumins like robin and phasin). Ingestion can cause severe symptoms in humans and livestock. The flowers are considered less toxic and are sometimes used to make fritters or honey in its native range, but caution is advised.
Toxicity:
Highly toxic to humans and most livestock (especially horses, cattle, and chickens) if ingested. Symptoms of poisoning include nausea, vomiting, abdominal pain, weakness, lethargy, dehydration, and potentially severe neurological issues or death. Skin punctures from thorns can also cause local irritation or more severe reactions.
Uses:
Historically and currently used for its extremely durable wood (fence posts, railway sleepers, furniture). Also planted for erosion control, soil improvement (nitrogen fixation), and as an ornamental tree in parks and gardens for its attractive flowers and rapid growth. Its flowers are a source of high-quality honey for bees.
Ecology:
As an alien invasive plant in South Africa, Robinia pseudoacacia is a significant threat to natural ecosystems. Its rapid growth, prolific root suckering, and efficient seed production allow it to form dense, impenetrable thickets that outcompete and displace indigenous vegetation, reducing biodiversity. It can alter soil nitrogen levels through its nitrogen-fixing ability, which can favour other invasive species. Its widespread root system makes eradication difficult. It invades forests, woodlands, grasslands, savannas, roadsides, and disturbed areas. Its toxicity to livestock also reduces the carrying capacity of grazing lands.
Distribution and habitat:
Widely cultivated and extensively naturalised and invasive throughout most of South Africa. It has invaded all nine provinces, particularly in areas with temperate climates. It is common in the Eastern Cape (especially in disturbed areas, along roads, in grasslands, and near plantations around Kenton-on-Sea), Western Cape, Free State, KwaZulu-Natal, Mpumalanga, and Limpopo. It invades forests, woodlands, grasslands, savannas, roadsides, abandoned farmlands, and riparian zones.
Derivation of name:
The genus name Robinia honours Jean Robin (1550–1629) and his son Vespasien Robin (1579–1662), French herbalists who first cultivated this tree in Europe.
The species epithet pseudoacacia is derived from the Greek word 'pseudes' (false) and 'acacia', because the tree superficially resembles true acacia species, though it belongs to a different genus within the pea family.
"Black Locust" refers to its dark, furrowed bark and its resemblance to the European carob tree, which was also called "locust" historically.
Historical aspects:
Robinia pseudoacacia was one of the earliest North American trees introduced to Europe in the early 17th century. It was brought to South Africa for its valuable, durable timber, rapid growth, and as an ornamental plant. Its ability to grow in poor soils and fix nitrogen also made it attractive for land reclamation. However, its aggressive suckering and prolific seeding quickly led to its escape from cultivation, establishing it as a widespread and problematic invader, impacting natural habitats and agricultural land across the country.
Interesting facts:
This tree is super tough! Its wood is so incredibly strong and resistant to rot that it was historically used for fence posts and even railway sleepers!
Despite its name, it's not a 'true' acacia, but it totally looks like one with its feathery leaves and thorns, which is why it's sometimes called 'False Acacia'!
It's a bit of a trickster: while its white flowers smell sweet and are great for bees to make honey, almost all other parts of the tree, especially the seeds and bark, are very poisonous! So no tasting the leaves or seeds!
The Black Locust is a soil superhero – it has special bacteria in its roots that pull nitrogen from the air and put it into the soil, acting like a natural fertiliser!
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Common name
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Plant family (derivation)