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Black locust

Botanical Name:

Robinia pseudoacacia

Common Name:

Black locust,Honey locust,False Acacia,Common Robinia,Bastard Acacia,Fragrant White Locust,Locust,Yellow Locust,Common Locust


Black locust reaches a typical height of 40–100 feet (12–30 m) with a diameter of 2–4 feet (0.61–1.22 m). Exceptionally, it may grow up to 52 metres (171 ft) tall and 1.6 metres (5.2 ft) diameter in very old trees. It is a very upright tree with a straight trunk and narrow crown that grows scraggly with age. The dark blue-green compound leaves with a contrasting lighter underside give this tree a beautiful appearance in the wind and contribute to its grace. Black locust is a shade-intolerant species and therefore is typical of young woodlands and disturbed areas where sunlight is plentiful and the soil is dry. In this sense, black locust can often grow as a weed tree. It also often spreads by underground shoots or suckers, which contributes to the weedy character of this species. Young trees are often spiny, but mature trees often lack spines. In the early summer black locust flowers; the flowers are large and appear in large, intensely fragrant clusters reminiscent of orange blossoms. 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. Although similar in general appearance to the honey locust, the black locust lacks that tree's characteristic long branched thorns on the trunk, having instead the pairs of short prickles at the base of each leaf; the leaflets are also much broader than honey locust. It may also resemble Styphnolobium japonicum, which has smaller flower spikes and lacks spines. Detailed descriptionEdit   One black locust leaf showing 13 leaflets 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 roots of black locust contain nodules that allow it to fix nitrogen, as is common within the pea family. The branches are typically zig-zaggy and may have ridges and grooves or may be round. When young, they are at first coated with white silvery down; this soon disappears, and they become pale green and afterward reddish or greenish brown. Spines may or may not be present on young trees, root suckers, and branches near the ground; typically, branches high above the ground rarely contain 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 .25–.8 inches (0.64–2.03 cm) 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. Wood is pale yellowish brown, heavy, hard, strong, close-grained, and very durable in contact with the ground. The wood has a specific gravity of 0.7333, and a weight of approximately 732 kg per cubic metre (45.7 pounds per cubic foot). The leaves are compound, meaning that each leaf contains many smaller leaf like 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 6–14 inches (15–36 cm) long and contains 9-19 leaflets, each being 1–2 inches (2.5–5.1 cm)long, and .25–.75 inches (0.64–1.91 cm) 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. The leaf color of the fully grown leaves is a dull dark green above and paler beneath. In the fall the leaves turn a clear pale yellow.  Closeup of flowers The 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 4–8 inches (10–20 cm) 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 1 inch (2.5 cm) 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, .5 inches (1.3 cm), dark red or reddish green. The fruit is a typical legume fruit, being a flat and smooth pea-like pod 2–4 inches (5.1–10.2 cm) long and .5 inches (1.3 cm) 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 25500 seeds per pound. Winter buds: 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. Cotyledons are oval in shape and fleshy.

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Bloom Time

Spring, Summer, Apr, May, Jun

Plant Type

Trees, Native Plant, Poisonous, Tree, Perennial, Not climbing


Woody, Perennial

Plant Height

36-72 ft.


Fragrant wisteria-like white flowers in pendant racemes (to 8” long) bloom from April to June. They are pea-like and very fragrant.


The flowers are followed by smooth, flat, purple-brown seed pods (to 4-5” long). These display from July to November.

How to Grow


Low, Average, Medium


Full Sun, Full sun (6 or more hours of direct sunlight a day), Partial Shade (Direct sunlight only part of the day, 2-6 hours), Sun


Moist, rich to dry, rocky soils.


Black Locust is widely planted for ornament and shelterbelts. It is also used for erosion control, particularly on strip-mined areas. Although it grows rapidly and spreads by sprouts like a weed, it is short-lived.









This species naturalizes easily and is considered an invasive weed in many of its non-native areas of establishment. Its brittle branches (subject to breaking in winds), vicious thorns, rampant root sprouts and copious seeding make this species a garden thug.

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