Robinia pseudoacacia, commonly known as black locust, is a medium-sized hardwood deciduous tree, belonging to the tribe Robinieae of the legume family. 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). Although fossilized traces of the genus were found in Europe, the species itself is native to a few small areas of the United States, but has been widely planted and naturalized elsewhere, including temperate North America, Eurasia, and Africa. It is considered an invasive species in some regions.
Description
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 0.6–1.2 m (2–4 ft). It is a very upright tree with a straight trunk and narrow crown that grows scraggly with age. 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. 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, 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 cm (1–2 in) long and 0.6–2 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 are up to 2 cm (3⁄4 in) long and somewhat triangular with a flared base and sharp point. Their color is 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 5–10 cm (2–4 in) long and 1.3 cm (1⁄2 in) broad. The fruit usually contains 4–8 seeds. 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.
Reproduction and dispersal
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