Culture
Grow in average, dry to medium, well-drained soils in full sun. Tolerates some light shade, but avoid shady locations. Tolerates a wide range of soils including sandy or nearly barren ones. Best performance is in moist, organically rich loams. Good drought tolerance. Fixes nitrogen. Avoid pruning in spring when it tends to bleed.
Noteworthy Characteristics
Robinia pseudoacacia, commonly called black locust, is medium sized, suckering, deciduous tree that typically grows to 30-50’ (less frequently to 80’) tall. Although originally native to the Allegheny Mountains, it has escaped gardens and naturalized over time to cover much of the United States and southern Canada plus parts of Europe, Asia and South America. At its best, it will grow as a broadly columnar single trunk tree with a narrow oblong crown. It also will grow in suckering thickets. It is noted for its attractive compound leaves and pendant racemes of pea-like flowers. Branches are usually armed with short paired spines (to 1.25” long). Pinnate dark blue-green leaves, with each leaf having up to 23 lance-shaped to ovate leaflets. Leaves turn uneventful yellow in fall. Fragrant wisteria-like white flowers in pendant racemes (to 8” long) bloom in late spring. Bees are attracted to the flowers. Flowers are followed by smooth, flat, purple-brown seed pods (to 4-5” long). This species is also commonly called common locust, yellow locust, white locust, green locust, pea flower locust and false acacia.
Genus name honors Jean Robin (1550-1629), of Paris, gardener to Henri IV and Louis XIII of France, who received new plants from Canada.
Specific epithet means false acacia.
'Umbraculifera' is usually sold as (1) a large, rounded, multi-stemmed shrub or (2) a small, densely rounded tree, often topgrafted to a standard locust at 6-8' high. Tree form may grow up to 20' tall by 20' wide. Attractive, pinnately compound leaves. Rarely flowers. Thornless.
Problems
Black locust is generally considered to be a very easy-to-grow tree. It thrives in many difficult growing conditions. It is susceptible to locust borer (often fatal) and locust leaf miner (browns foliage). Other insect problems include caterpillars, weevils, scale and whiteflies. Possible disease problems include canker, powdery mildew, leaf spots, wood rots and verticillium wilt.
'Umbraculifera' grown as a tree can be top heavy.
Uses
Tree may be clipped to globular shape and used in formal gardens. Shrub is effective in shrub borders or backgrounds.