Black Locust (Robinia pseudoacacia) is a tree 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 and Asia and is considered an invasive species in some areas. A less frequently used common name is False Acacia, which is a literal translation of the specific epithet. It was introduced into Britain in 1636.
With a trunk up to 0.8 m diameter (exceptionally 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 thorns 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 flowers
are white, borne in pendulous racemes
8–20 cm long, and are considered edible. The fruit
is a legume
5–10 cm long, containing 4–10 seeds
Although similar in general appearance to Honey locust, it lacks that tree's characteristic long branched spines on the trunk, instead having the pairs of short thorns at the base of each leaf; the leaflets are also much broader.
Native from Pennsylvania to northern Georgia and westward as far as Arkansas and Oklahoma, but has been widely spread. Reaches the height of seventy feet with a trunk three or four feet in diameter, with brittle branches that form an oblong narrow head. Spreads by underground shoots. The leaflets fold together in wet weather, also at night; some change of position at night is the habit of the entire leguminous family.
- Bark: Dark gray brown tinged with red, deeply furrowed, surface inclined to scale. Branchlets at first coated with white silvery down. This soon disappears and they become pale green, afterward reddish brown. Prickles develop from stipules, are short, somewhat triangular, dilated at base, sharp, dark purple, adhering only to the bark, but persistent.
- Wood: Pale yellowish brown; heavy, hard, strong, close-grained and very durable in contact with the ground. Sp. gr., 0.7333; weight of cu. ft., 45.70 lbs.
- Winter buds: Minute, naked, three or four together, protected in a depression by a scale-like covering lined on the inner surface with a thick coat of tomentum and opening in early spring; when forming are covered by the swollen base of the petiole.
- Leaves: Parallel, compound, odd-pinnate, eight to fourteen inches long, with slender hairy petioles, grooved and swollen at the base. Leaflets petiolate, seven to nine, one to two inches long, one-half to three-fourths of an inch broad, emarginate or rounded at apex. They come out of the bud conduplicate, yellow green, covered with silvery down which soon disappears; when full grown are dull dark green above, paler beneath. Feather-veined, midvein prominent. In autumn they turn a clear pale yellow. Leafs out relatively late in spring. Stipules linear, downy, membranous at first, ultimately developing into hard woody prickles, straight or slightly curved. Each leaflet has a minute stipel which quickly falls and a short petiole.
- Flowers: May or June, after the leaves. Papilionaceous. Perfect, borne in loose drooping racemes four to five inches long, cream-white, about an inch long, nectar bearing, fragrant. Pedicels slender, half an inch long, dark red or reddish green.
- Calyx: Campanulate, givvous, hairy, five-toothed, slightly two-lipped, dark green blotched with red, especially on the upper side teeth valvate in bud.
- Corolla: Imperfectly papilionaceous, petals inserted upon a tubular disk; standard white with pale yellow blotch; wings white, oblong-falcate; keel petals incurved, obtuse, united below.
- Stamens: Ten, inserted, with the petals, diadelphous, nine inferior, united into a tube which is cleft on the upper side, superior one free at the base. Anthers two-celled, cells opening longitudinally.
- Pistil: Ovary superior, linear-oblong, stipitate, one-celled; style inflexed, long, slender, bearded; stigma capitate; ovules several, two-ranked.
- Fruit: legume two-valved, smooth three to four inches long and half an inch broad, usually four to eight seeded. Ripens late in autumn and hangs on the branches until early spring. Seeds dark orange brown with irregular markings. Cotyledons oval, fleshy.
Black locust is a major honey plant
in eastern USA, and, having been taken and planted in France
, is the source of the renowned acacia monofloral honey
from France. Flowering starts after 140 growing degree days
In Europe it is often planted alongside 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.
Black locust has nitrogen-fixing bacteria on its root system; for this reason it can grow on poor soils and is an early colonizer of disturbed areas.
In 1900 it was reported that the value of Robinia pseudacacia is practically destroyed in nearly all parts of the United States beyond the mountain forests which are its home, by the 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.
The wood is extremely hard, resistant to rot and long lasting, making it prized for fence posts and small watercraft. As a young man, Abraham Lincoln spent a lot of time splitting rails and fence posts from black locust logs. 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 the most rot-resistant local tree, 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". It is most easily ignited by insertion into a hot stove with an established coal bed. For best results it should be seasoned like any other hardwood, however 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 ten 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.
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.
Like the honey locust, the black locust reproduces through its distinctive hanging pods. Black locust's pods are smaller and lighter, and thus easily carried long distances by the wind. Unlike the pods of the honey locust, but like those of the related European Laburnum,
the black locust's pods are toxic
. In fact, every part of the tree, especially the bark, is considered toxic, with the exception of the flowers. However, various reports have suggested that the seeds and the young pods of the black locust can be edible when cooked, since the poisons that are contained in this plant are decomposed by heat. Horses that consume the plant show signs of anorexia, depression, diarrhea, 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, which is also native to Syria, is supposed to be the true locust of the New Testament; the fruit of this tree may be found in the shops under the name of St. John's bread.
Robinia is now a North American genus, but traces of it are found in the Eocene and Miocene rocks of Europe.