Black Locust

Black Locust (Robinia pseudoacacia), also called Common Locust, False Acacia, Green Locust, Pea Flower Locust, White Locust.

The black locust is a deciduous, flowering tree in the Fabaceae or bean family. It may grow 30 to 50 feet tall, and 20 to 35 feet wide, and its trunk may measure up to 3 to 4 feet in diameter. While it is a native forest tree in the mountains of North Carolina, it is widely planted throughout the state and is now naturalized to all soil and water conditions, except for saturated soils. In other parts of the state, black locusts can be found in thickets, on clay banks, in waste areas, or along fence rows. Unfortunately, the black locust has become invasive in the Midwestern United States. It is a relatively short-lived tree with about a 90-year life span.

Black locust grows in a variety of soil types and can fix nitrogen in the soil, so it is sometimes used to reclaim soil banks in abandoned sites. This is a good plant for difficult sites, transplants well, and tends to reseed. It develops shoots from roots. It is moderately salt tolerant. Species trees are sometimes considered to be somewhat weedy. Trees send out long underground root suckers that not only become a maintenance problem but also can disrupt nearby gardening areas. The trees can be pruned and kept at shrub size by cutting them to the ground every 2-3 years, but pruning should be avoided in the spring as the tree will bleed.

The leaves are alternate, pinnately compound with 7 to 23 leaflets. The bark is gray or light brown, thick, fibrous, heavily ridged, and furrowed. In spring and early summer, fragrant, pea-like white flowers, borne in 5-inch long hanging clusters, mature. The tree produces a 2 to 4-inch flattened legume which contains 4 to 8 red-brown seeds.

Identification

The leaf of the Black locust is noted for its attractive alternate, compound leaves and pendant racemes of pea-like flowers. It has pinnate dark blue-green leaves, 8 to 14 inches long, with each leaf having up to 23 lance-shaped to ovate leaflets. Leaves turn uneventful yellow in fall.

The bark is brown to gray, deeply furrowed, and scaly.

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

The flowers are followed by smooth, flat, purple-brown, flat, seed pods measuring 2 to 4 inches long, and each bearing 4 to 8 red-brown kidney-shaped seeds. These display from July to November, and the pods can persist on the tree through the winter.

Habitat and cultural conditions

Light: Full sun (6 or more hours of direct sunlight a day); Partial Shade (Direct sunlight only part of the day, 2-6 hours). Soil texture: Clay; High Organic Matter; Loam (Silt); Sand. Soil pH: Acid (<6.0); Alkaline (>8.0); Neutral (6.0-8.0). Drainage: Good Drainage; Moist; Occasionally Dry.

Uses and significance

This tree is rarely seen in residential areas and is not recommended for the home landscape because it tends to be weedy, suckering, thorny, and has brittle branches that can break off in the wind.

The wood from this native is yellow, coarse grained, hard, strong, and naturally rot resistant and is often used to make fence posts and rails, mine timbers, and landscaping ties.

This is a larval host plant for several butterflies. Clouded Sulphur (Colias philodice) larvae appear from May to October in the north and from March to November in the south. There are 3 flights in the north and 4 to 5 flights in the south (but it rarely uses this plant as a host in North Carolina). Zarucco Duskywing (Erynnis zarucco) has three broods in the deep south from March-October, appearing as early as January in Florida. Silver-Spotted Skipper (Epargyreus clarus) appears from May-September with three-four broods in the deep south, two broods in the east, and one brood in the north and west. The buds and catkins are eaten by birds. It provides an excellent leaf cover for birds in wetland sites. Bees are attracted to its flowers and rabbits depend on the sprouts and seeds during the winter.

Insects, diseases, and other problems

It is susceptible to locust borer (often fatal) and locust leaf miner (brown foliage). Other insect problems include caterpillars, weevils, scales, and whiteflies. Possible disease problems include canker, powdery mildew, leaf spots, wood rots, and verticillium wilt. Small branches of the Black locust fall easily during storms. Some of its leaves are yellow and fall off during times of drought.

Cultivars / varieties

• 'Frisia' — Golden yellow leaves in the fall.
• 'Lace Lady' — Eight to ten feet high, smaller spines, but does not produce flowers.
• 'Purple Robe' — Compact and rounded with rose-pink flowers.
• 'Tortuosa' — Slow growing and compact, but fewer and smaller flowers.
• 'Umbraculifera' — Attractive leaves, thornless, but rarely flowers.

Plant details

Further reading

• N.C. Extension Gardener Plant Toolbox: Robinia pseudoacacia — primary source for the plant details above