Black locust

Robinier faux-acacia

Robinia pseudoacacia L.Fabaceae (legume family)

Origin: Eastern USA


Black locust is a medium-sized tree with distinctive ropy bark and a pair of spines at each leaf or branch node.

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Leaves are pinnately compound with 3 to 9 pairs of oval leaflets and a single leaflet at the tip.

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Flowers are white, pea-like and fragrant, borne in large showy clusters.

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Fruits are flat reddish-brown pods that hang singly or in clusters.

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Black locust is a medium-sized tree, up to 25 m (82') in height and 60 cm (2') in diameter.

The crown is open; the trunk is often forked and crooked.

Young bark is smooth with conspicuous lenticels (pores), and spines.

Bark on mature trees is brown and deeply furrowed with a ropy texture consisting of long, forking ridges.

Depending on the variety of tree, there may be a pair of short spines at each leaf or branch node.

Twigs are smooth and reddish-brown, and may be slightly zig-zagged.

Clusters of 3 or 4 very small buds are embedded in the bark, covered by the base of the leaf stalk.

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Leaves are pinnately compound, with a single leaflet at the tip and 3 - 9 pairs of leaflets along a central stalk (rachis). The rachis, which is hairy, is 20 - 30 cm (8" - 12") long.

The oval leaflets are 3 - 5 cm (1 1/8" - 2") long. Some may be bristle tipped. The upper surface is dull green, the underside paler. Leaflets droop and fold at night or in rainy weather.

Leaves on new shoots are arranged alternately on the branch.

Leaves turn pale yellow in the fall.

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The white pea-like flowers are borne in clusters about 14 cm (5 1/2") long.

The white flowers are about 1.5 - 2.5 cm (5/8" - 1") long, with a yellowish green blotch on the upper petal. They are fragrant and pollinated by insects.

Flowers emerge in early summer after the leaves have developed.

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Fruits are flat reddish-brown pods, 7 - 10 cm (2 3/4" - 4") long.

Pods hang in clusters from a central stalk and mature in the fall.

Each pod contains 4 - 8 dark seeds, 3 - 5 mm (1/8" - 3/16") long.

Pods may remain on the tree during winter.

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Black locust is native to the eastern United States, especially on the lower slopes of the Appalachian Mountains. It has been extensively planted as an ornamental tree and, having escaped cultivation, has become naturalized in areas well beyond its range. Because it is an early successional plant adapted to dry, open conditions it has become invasive, especially in prairie and savanna habitats across North America.

Derivation of names

The genus name, Robinia, commemorates Jean Robin (1550-1629) and his son, Vespasien Robin (1579-1662), who were herbalists to King Henry IV of France and among the first to plant black locust in Europe. Since its initial introduction, black locust has become one of the most widely planted New World trees in Europe and is now naturalized there in many areas. The species name, pseudoacacia, means "false" acacia, referring to the fact that despite the many features it shares with species of Acacia, ( the shape of the pods, pinnately compound leaves, and spines ) it is NOT an Acacia.


Like many members of the bean family, Fabaceae, black locust roots bear nodules that are inhabited by bacteria that transform atmospheric nitrogen into compounds usable by plants.

Commercial use

Although black locust wood is very hard and rot resistant, it has limited commercial value. It has been used to manufacture fence posts and railway ties, and it was previously used in the shipbuilding industry.

Wildlife value

Many species of wildlife, including rabbits, squirrels, mourning doves, and pheasants, are able to eat the seeds without ill effects. However, the inner bark, seeds, and leaves are toxic to cattle and humans and should never be eaten. The fragrant flowers attract bees, which produce a good honey from their nectar.

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Black locust IN TORONTO

Black locust's place in Toronto's urban forest

Black locust has been mistakenly planted as a native tree. It is found in parks, ravines and forest edges throughout the city and spreads aggressively in areas where mowing has been discontinued.

Landscape value and potential for home planting

Black locust is a fast-growing medium-sized tree suitable for growing in yards. It requires sunny sites and does not tolerate shade. The fragrant flowers are eye-catching and increase urban biodiversity by attracting insects. Many cultivars are available. Due to its potential invasiveness black locust should not be planted adjacent to ravines or other natural areas.

Pests and diseases: Black locust is attacked by the locust borer (Megacyllene robiniae), which eats through the tree's wood, both weakening the tree and allowing other destructive pests to attack the tree. The fastest-growing specimens of black locust are the most resistant to the locust borer. Locust leaf miner turns the tree's foliage brown.

A pink-flowered cultivar of this tree is available for planting through the City of Toronto's street tree program.

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WHERE CAN I SEE Black locust?

Links to maps at Canadian Tree Tours:

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