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Black Locust Robinia pseudoacacia

 

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Black Locust, leaves & flowers
credit: Botaurus stellaris

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Family: Fabaceae, Pea view all from this family



Description Robinia pseudoacacia, commonly known as the Black Locust or False Acacia, 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.

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 (reminiscent of orange blossoms) flowers are white, borne in pendulous racemes 8–20 cm long, and are considered edible (dipped in batter; deep-fried). The fruit is a legume 5–10 cm long, containing 4–10 seeds.

Although similar in general appearance to the 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.

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 seventy 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.

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. The wood has a specific gravity 0.7333, and a weight of approximately 45.7 pounds per cubic foot.
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, 21-40 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, gibbous, 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.


Warning Seeds are poisonous and can be fatal to humans and animals. Leaves, twigs, and bark may also be toxic. Sensitivity to a toxin varies with a personís age, weight, physical condition, and individual susceptibility. Children are most vulnerable because of their curiosity and small size. Toxicity can vary in a plant according to season, the plantís different parts, and its stage of growth; and plants can absorb toxic substances, such as herbicides, pesticides, and pollutants from the water, air, and soil.


Habitat Cities, suburbs & towns, Scrub, shrub & brushlands, Canyons & valleys, Watersides (fresh), Fields.


Range Rocky Mountains, Mid-Atlantic, Southeast, Florida, Southwest, Great Lakes, New England, California, Texas, Northwest, Plains.


Comments Some consider this a serious pest tree because it readily escapes from cultivation and invades areas where it isn't wanted. It is susceptible to borers, weakened trees are prone to collapse. Prune in late summer or fall to prevent borer attack. Though troublesome in the wrong place, fast-growing black locust is a hardy, durable, no-maintenance tree that controls erosion with root suckers and enriches the soil by fixing nitrogen. Sucker removal is necessary to maintain a single stem. Tolerates drought, salt spray, and sterile soils.


Exposure Preference Sun.


Flower June


Native Distribution Georgia to Louisiana & Arkansas, n. to Pennsylvania, West Virginia, s. Indiana, s. Illinois, Louisiana & e. Oklahoma; naturalized elsewhere


Site Preference Woods; thickets; fence rows


Soil Preference Moist, rich to dry, rocky soils. pH 5.1-7.7.


Wildlife Value Flowers are a favorite of honeybees and hummingbirds.


 

 

 

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