PSC 2620: Woody Trees and ShrubCourse HomeWeek 5

Robinia psuedoacacia - Black Locust

Plant Viewer
The leaf of the Locust can be up to 14 inches long with 7-19 elliptic leaflets per leaf. Panicles of fragrant white blossoms appear in late spring.
The Locust has hard and very sharp thorns. Pea-pod fruit emerges by late summer.

Plant Description

Black Locust is a large, deciduous shade tree reaching 40-50 feet tall, with a narrower spread. It is a tree that is both loved and hated; its tough constitution makes it ideal for use in many tough locations, but it suffers from wind and snow breakage and tendency to spread is problematic in most residential and commercial landscapes. Additionally, its 1/2 inch long thorns make it a tree that one wants to keep at arms length, especially while young or low-branched.

Hanging racemes of white blossoms, 4-8 inches long, appear in the late Spring. The flowers are very fragrant, and a tree in full bloom is quite showy. Individual flowers are similar to pea flowers. Flat pea pods form during summer and mature by fall. The 2-4 inch long pods turn brown and hang on the tree into winter.

The odd pinnately compound leaves are true green, and grow to be 6-14 inches long. There are 7-19 leaflets per leaf, each leaflet being elliptic in shape with a smooth margin. In fall the leaves turn a dull yellow and drop.

Landscape Use

Robinia psuedoacacia is a tough and hardy tree that can tolerate nearly any landscape condition. Its tendency to colonize and its wild look make it unattractive for most residential or commercial landscapes though. It is best used in situations such as highway landscaping and reclamation projects.

Points of interest

Black Locust is susceptible to several different diseases and insects, most troublesome being leaf spots, borders and scale.

Notable Cultivars

Umbraculifera A fine specimen tree, it only reaches 15-20 feet high and has a dense rounded, umbrella-shaped canopy. It is nearly thornless, but also produces very few flowers.

Purple Robe One of the most popular and attractive cultivars. It has bright purple blossoms in the spring and blooms for two weeks or longer. The leaves emerge a rusty red in spring before fading to rich green in summer. It is nearly seedless. Grows to 40 feet high.


Twisty Baby (Lacy Lady) A smaller form with heavily contorted branches and stems. The leaves appear to be more pendulous than on the regular species. It does not flower. Reaches 12 feet high and wide.