Robinia pseudoacacia, commonly known in its native territory as black locust,is a medium-sized hardwood deciduous tree, belonging to the tribe Robinieae of the legume family Fabaceae. It is native to a few small areas of the United States, but it 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, such as the temperate east coast of Australia where the cultivar "Frisia" (Golden Robinia) was widely planted as a street tree before being classed as a weed. Another common name is false acacia,a literal translation of the specific name (pseudo [Greek ψευδο-] meaning fake or false and acacia referring to the genus of plants with the same name).
The roots of black locust contain nodules that allow it to fix nitrogen, as is common within the pea family. Trees reach a typical height of 12–30 metres (40–100 feet) with a diameter of 0.61–1.22 m (2–4 ft).[7] It is a very upright tree with a straight trunk and narrow crown that grows scraggly with age.[8] The bark is a reddish black and gray and tinged with red or orange in the grooves; it is deeply furrowed into grooves and ridges which run up and down the trunk and often cross and form diamond shapes.[8] The branches are typically zig-zaggy and may have ridges and grooves or may be round.When young, the branches are at first coated with white silvery down; this soon disappears, and they become pale green and afterward reddish or greenish brown.
The dark blue-green leaves are lighter on the underside,[citation needed] and are compound, meaning that each leaf contains many smaller leaflike structures called leaflets, which are roughly paired on either side of the stem that runs through the leaf (rachis). There is typically one leaflet at the tip of the leaf (odd pinnate), and the leaves are alternately arranged on the stem. Each leaf is 15–36 cm (6–14 in) long and contains 9–19 leaflets, each being 2.5–5.1 cm (1–2 in) long, and .65–1.9 cm (1⁄4–3⁄4 in) wide. The leaflets are rounded or slightly indented at the tip and typically rounded at the base. The leaves come out of the bud folded in half, yellow green, covered with silvery down which soon disappears. Each leaflet initially has a minute stipel, which quickly falls, and is connected to the (rachis) by a short stem or petiolule. The leaves are attached to the branch with slender hairy petioles which are grooved and swollen at the base. The stipules are linear, downy, membranous at first and occasionally develop into prickles. The leaves appear relatively late in spring, and turn a clear pale yellow in autumn. The leaflets fold together in wet weather and at night (nyctinasty), as some change of position at night is a habit of the entire leguminous family.
Young trees are often spiny, especially on root suckers and branches near the ground; mature trees often lack spines. R. pseudoacacia is quite variable in the number of spines present, as some trees are densely prickly and other trees have no prickles at all. The spines typically remain on the tree until the young thin bark to which they are attached is replaced by the thicker mature bark. They develop from stipules (small leaf-like structures that grow at the base of leaves), and since stipules are paired at the base of leaves, the spines will be paired at the bases of leaves. They range from .65–2 cm (1⁄4–3⁄4 in) in length and are somewhat triangular with a flared base and sharp point. Their color is of a dark purple and they adhere only to the bark.
The winter buds are minute, naked (having no scales covering them), 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 the buds are forming they are covered by the swollen base of the petiole.The large flowers open in May or June for 7–10 days, after the leaves have developed. They are arranged in loose drooping clumps (racemes) which are typically 10–20 cm (4–8 in) long.The flowers themselves are cream-white (rarely pink or purple) with a pale yellow blotch in the center and imperfectly papilionaceous in shape. They are about 2.5 cm (1 in) wide, very fragrant, and produce large amounts of nectar. Each flower is perfect, having both stamens and a pistil (male and female parts). There are 10 stamens enclosed within the petals; these are fused together in a diadelphous configuration, where the filaments of 9 are all joined to form a tube and one stamen is separate and above the joined stamens. The single ovary is superior and contains several ovules. Below each flower is a calyx which looks like leafy tube between the flower and the stem. It is made from fused sepals and is dark green and may be blotched with red. The pedicels (stems which connect the flower to the branch) are slender, 1.3 cm (1⁄2 in), dark red or reddish green.
The fruit is a typical legume fruit, being a flat and smooth pea-like pod 5.1–10.2 cm (2–4 in) long and 1.3 cm (1⁄2 in) broad. The fruit usually contains 4–8 seeds.[8] The seeds are dark orange brown with irregular markings. They ripen late in autumn and hang on the branches until early spring.There are typically 25,500 seeds per pound.Cotyledons are oval in shape and fleshy.
Black locust reproduces both sexually via flowers, and asexually via root suckers. The flowers are pollinated by insects, primarily by Hymenopteran insects. The physical construction of the flower separates the male and female parts so that self-pollination will not typically occur. The seedlings grow rapidly but they have a thick seed coat which means that not all seeds will germinate. The seed coat can be weakened via hot water, sulfuric acid, or be mechanically scarified, which will allow a greater quantity of the seeds to grow.The seeds are produced in good crops every year or every other year.[citation needed]
Root suckers are an important method of local reproduction of this tree. The roots may grow suckers after damage (by being hit with a lawn mower or otherwise damaged) or after no damage at all. The suckers are stems which grow from the roots, directly into the air and may grow into full trees. The main trunk also has the capability to grow sprouts and will do so after being cut down. This makes removal of black locust difficult as the suckers need to be continually removed from both the trunk and roots or the tree will regrow. This is considered an asexual form of reproduction.
The suckers allow black locust to grow into colonies which often exclude other species. These colonies may form dense thickets which shade out competition. Black locust has been found to have either 2n=20 or 2n=22 chromosomes.
The black locust is native to the eastern United States, but the exact native range is not accurately known as the tree has been cultivated and is currently found across the continent, in all the lower 48 states, eastern Canada, and British Columbia.The native range is thought to be two separate populations, one centered about the Appalachian Mountains, from Pennsylvania to northern Georgia, and a second westward focused around the Ozark Plateau and Ouachita Mountains of Arkansas, Oklahoma and Missouri.
Black locust's current range has been expanded by humans distributing the tree for landscaping and now includes Pakistan, India, Australia, Canada, China, Europe, Northern and South Africa, temperate regions in Asia, New Zealand, Southern South America.
Black locust is planted for firewood, as it grows rapidly, is highly resilient in a variety of soils, and it grows back rapidly after harvest from the existing root system (i.e. coppicing).
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.[citation needed] If the black locust is cut, split, and cured while relatively young (within 10 years), thus minimizing beetle damage, "spitting" problems are minimal.
The wood is extremely hard, being one of the hardest woods in Northern America with a Janka hardness test of 1,700 lbf (7,560 N)[44] and specific gravity of 0.733 (733 kilograms per cubic metre or 45.7 pounds per cubic foot). It is very resistant to rot, and durable, making it prized for furniture, flooring, paneling, fence posts, and small watercraft. Black Locust is a highly durable organic wood product that does not require chemical treatment to preserve its beauty for 50 years or longer.[45] Wet, newly cut planks have an offensive odor which disappears with seasoning. Black locust is still in use in some rustic handrail systems. 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. Flavonoids in the heartwood allow the wood to last over 100 years in soil.
With a light yellowish color and strength, the wood was much used for decorative inlays and banding in furniture in England and France in the 17th and 18th centuries, under the name "acacia" or "Virginia acacia".
In 1900, the value of Robinia pseudoacacia was reported to be practically destroyed in nearly all parts of the U.S. 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.
In Asia, many black locusts, called cihuai (zh:刺槐), yanghuai (foreign huai zh:洋槐, against native huai zh:国槐) or simply "acacias", were planted in Dalian, Liaoning, China, during its Russian and Japanese occupation, and are loved by the local people: there is Acacia Avenue (槐树大道) in downtown; the Acacia Flower Festival (槐花節) is celebrated every year in May; and acacia honey is collected in the suburbs by bee keepers.
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