Common Name: black locust
Type: Tree
Family: Fabaceae
Zone: 4 to 8
Height: 30.00 to 40.00 feet
Spread: 20.00 to 30.00 feet
Bloom Time: May
Bloom Description: Violet purple
Sun: Full sun
Water: Dry to medium
Maintenance: Medium
Suggested Use: Street Tree, Flowering Tree, Naturalize
Flower: Showy, Fragrant
Fruit: Showy
Other: Thorns
Tolerate: Deer, Drought, Clay Soil
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. Spreads by root suckers. Promptly remove suckers as they appear unless naturalization is desired.
Noteworthy Characteristics
Robinia is a genus of between maybe 6 to 30 species (many are unresolved) of trees and shrubs from North America. They bear attractive flowers but many have spines, bristles or thorns.
Genus name honors Jean Robin (1550-1629), of Paris, gardener to Henri IV and Louis XIII of France, who received new plants from Canada.
‘Purple Robe’ is a medium sized, suckering, deciduous tree that typically grows to 30-40’ tall with an oval-upright habit. It is noted for its attractive compound leaves and pendant racemes of violet purple pea-like flowers. It is advertised as thornless, but plants with thorns are found in commerce. Pinnate bronze-green leaves, with each leaf having up to 23 lance-shaped to ovate leaflets, are attractive during the growing season. Leaves emerge an interesting bronze-red in spring, but eventually turn an uneventful yellow in fall. Fragrant wisteria-like violet purple flowers in pendant racemes (to 8” long) bloom in mid to late spring. Bees are attracted to the flowers. Flowers are followed by smooth, flat, purple-brown seed pods (to 4” long). U.S. Plant Patent PP2,454 issued November 24, 1964. Some confusion exists over the nomenclature of this plant. It is reportedly a hybrid cross between R. ambigua ‘Decaisneana’ and R. hispida ‘Monument’. However, some consider the plant to be a cross between R. pseudoacacia and R. hispida. In commerce, it is synonymous with and frequently sold as R. pseudoacacia ‘Purple Robe’.
Problems
‘Purple Robe’ is considered to be a very easy-to-grow tree. It thrives in many difficult growing conditions. It is also sometimes considered to be somewhat weedy. Trees send out underground root suckers that not only become a maintenance problem but also can disrupt nearby gardening areas. 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.
Uses
Violet purple spring flowers make this an interesting tree for the landscape. It is considered to be a good selection for difficult areas in the landscape where other trees will not grow well. May be used as a street tree.