Robinia pseudoacacia, also known in its native territory as black locust, is a tree of the genus Robinia in the subfamily Faboideae of the pea family Fabaceae. It is native to the southeastern United States, but has been widely planted and naturalized elsewhere in temperate North America, Europe, Southern Africa and Asia and is considered an invasive species in some areas. Another common name is false acacia, a literal translation of the specific name. It was introduced into Britain in 1636.
It has a trunk up to 0.3 m diameter (exeptionally up to 52 m tall and 1.6 m diameter in very old trees), with thick, deeply furrowed blackish bark. The leaves are 10-25 cm long, pinnate with 9-19 oval leaflets, 2-5 cm long and 1.5-3 cm broad. Each leaf usually has a pair of short spines at the base, 1-2 mm long or absent on adult crown shoots, up to 2 cm long on vigorous young plants. The intensely fragrant (reminiscent of orange blossoms) flowers are white to lavender or purple, borne in pendulous racemes 8-20 cm long, and are edible. The fruit is a legume 5-10 cm long, containing four to 10 seeds.
Although similar in general appearance to the honey locust, it lacks that tree-s characteristic long branched thorns on the trunk, instead having the pairs of short spines at the base of each leaf; the leaflets are also much broader. Resemblance to Styphnolobium japonicum which has smaller flower spikes.
The black locust is native in the United States from Pennsylvania to northern Georgia and westward as far as Arkansas and Oklahoma, but has been widely spread. The tree reaches a height of 70 feet, with a trunk three or four feet in diameter and brittle branches that form an oblong narrow head. It spreads by underground shoots. The leaflets fold together in wet weather and at night; some change of position at night is a habit of the entire leguminous family.
Black locust is a major honey plant in the eastern US, and has been planted in European countries. In many European countries, it is the source of the renowned acacia honey. Flowering starts after 140 growing degree days. However, its blooming period is short (about 10 days) and it does not consistently produce a honey crop year after year. Weather conditions can have quite an effect on the amount of nectar collected, as well; in Ohio for example, good locust honey flow happens in one of five years.
In Europe, it is often planted along streets and in parks, especially in large cities, because it tolerates pollution well. The species is unsuitable for small gardens due to its large size and rapid growth, but the cultivar -Frisia-, a selection with bright yellow-green leaves, is occasionally planted as an ornamental tree.
In South Africa, it is regarded as a weed because of its habit of freely suckering from roots near the surface and aggravated by cutting of the main stem.
Black locust has nitrogen-fixing bacteria on its root system, so it can grow on poor soils and is an early colonizer of disturbed areas.
In 1900, the value of Robinia pseudoacacia was reported to be practically destroyed in nearly all parts of the United States beyond the mountain forests which are its home by locust borers which riddle the trunk and branches. Were it not for these insects, it would be one of the most valuable timber trees that could be planted in the northern and middle states. Young trees grow quickly and vigorously for a number of years, but soon become stunted and diseased, and rarely live long enough to attain any commercial value.
Black locust leaves contain flavone glycosides characterised by spectroscopic and chemical methods as the 7-O-β-d-glucuronopyranosyl-(1 - 2)[α-l-rhamnopyranosyl-(1 - 6)]-β-d-glucopyranosides of acacetin (5,7-dihydroxy-4-methoxyflavone), apigenin (5,7,4-trihydroxyflavone), diosmetin (5,7,3-trihydroxy-4-methoxyflavone) and luteolin (5,7,3,4-tetrahydroxyflavone).
The wood is extremely hard, resistant to rot, and durable, making it prized for furniture, flooring, paneling, fence posts, and small watercraft. Wet, newly cut planks have an offensive odour which disappears with seasoning. As a young man, Abraham Lincoln spent much of his time splitting rails and fence posts from black locust logs. Black locust is still in use in rustic handrail systems. Flavonoids in the heartwood allow the wood to last over 100 years in soil. In the Netherlands and some other parts of Europe, black locust is one of the most rot-resistant local trees, and projects have started to limit the use of tropical wood by promoting this tree and creating plantations. It is one of the heaviest and hardest woods in North America.
Black locust is highly valued as firewood for wood-burning stoves; it burns slowly, with little visible flame or smoke, and has a higher heat content than any other species that grows widely in the Eastern United States, comparable to the heat content of anthracite. For best results, it should be seasoned like any other hardwood, but black locust is also popular because of its ability to burn even when wet. In fireplaces, it can be less satisfactory because knots and beetle damage make the wood prone to -spitting coals for distances of up to several feet. If the black locust is cut, split, and cured while relatively young (within 10 years), thus minimizing beetle damage, -spitting problems are minimal.
It is also planted for firewood because it grows rapidly, is highly resilient in a variety of soils, and it grows back even faster from its stump after harvest by using the existing root system. (see coppicing)
With fertilizer prices rising, the importance of black locust as a nitrogen-fixing species is also noteworthy. The mass application of fertilizers in agriculture and forestry is increasingly expensive; therefore nitrogen-fixing tree and shrub species are gaining importance in managed forestry.
In traditional medicine of India, different parts of R. pseudoacacia are used as laxative, antispasmodic, and diuretic.
Black locust-s pods are small and light, and easily carried long distances. Although the bark and leaves are toxic, various reports suggest that the seeds and the young pods of the black locust are edible. Shelled seeds are safe to harvest from summer through fall, and are edible both raw and/or boiled. Due to the small nature of the seeds, shelling them efficiently can prove tedious and difficult. In France and in Italy, R. pseudoacacia flowers are eaten as beignets after being coated in batter and fried in oil; they are also eaten in Japan, largely as tempura.[better source needed] Important constituents of the plant are the toxalbumin robin, which loses its toxicity when heated, and robinin, a nontoxic glucoside. Horses that consume the plant show signs of anorexia, depression, incontinence, colic, weakness, and cardiac arrhythmia. Symptoms usually occur about 1 hour following consumption, and immediate veterinary attention is required.
The name 'locust' is said to have been given to Robinia by Jesuit missionaries, who fancied that this was the tree that supported St. John in the wilderness, but it is native only to North America. The locust tree of Spain (Ceratonia siliqua or carob tree), which is also native to Syria and the entire Mediterranean basin, is supposed to be the true locust of the New Testament.
Robinia is now a North American genus, but traces of it are found in the Eocene and Miocene rocks of Europe.