Name that Tree

By Gil Medeiros, Fairfax Master Gardener
Black Locust FlowersIf you selected Black Locust, you are correct!

You are probably wondering why the native Black Locust is not usually cultivated in its native range but is commonly used in cultivated landscapes in Europe. The problem is that the Black Locust is prone to several severe pest problems in the eastern U.S. but not in Europe. There are borers and leaf-miners, but the worst problem is infestation by the conk fungus (Phellinus robineae). It causes the tree trunks to deteriorate and fail catastrophically in windstorms, damaging cars, buildings and other nearby structures. Some extension services advise removal of Black Locusts, particularly from the sides of streets.

Black Locust Leaves

Black Locust leaves similar in appearance to those of garden peas

Here is another oddity: the conk fungus damages LIVE trees, but harvested locust wood is among the most durable and decay resistant on the planet! Go figure. It has been said that Locust wood will last longer than stone. Locust wood is commonly used for fence posts, the underground portions of which are subject to all manner of insect and microbial attacks.

And get this — Black Locust has become an invasive species in some parts of CHINA where it is cultivated for its timber. So if you have ever wondered if some of our native species can naturalize on other continents, as Eurasian species do here, the answer is yes! Turnabout is fair play I suppose.

Locust Pods

Black Locust fruit. Notice the similarity to peapods

If you have a Black Locust along your back fence, you probably appreciate the beautiful flowers it produces in May. The white racemes produce a pleasant fragrance. When bees pollinate the flowers, the resulting honey is highly prized.

Black Locust is a pioneer species that grows quickly in cleared or fire-damaged areas. That is why it can be found so commonly on roadsides. It can survive in a wide variety of soils. The literature reports that it can grow in soil around mine reclamation sites where the pH is as low as 2.5! This is astonishing. Consider this: horticultural vinegar (20% acetic acid) commonly used to kill top growth of weeds has a pH of approximately 2.2!

Black Locust Bark

Bark of a mature black Locust

Because it is a legume, Black Locust can even produce some of its own nitrogen nutrients in bacterial nodules that grow on its roots. Therefore, it can grow in barren soils. Black Locust cannot survive, however, in soil that is permanently wet.

To identify the tree, look for dark gray, deeply furrowed bark on older trees and oval, oppositely attached leaves. The leaves strongly resemble those of pea plants. Not surprising, the fruit strongly resembles a peapod.

Black Locust, University of Kentucky, Department of Horticulture
Plant of the Week: – Black Locust, Latin: Robinia pseudoacacia, University of Arkansas Cooperative
   Extension Service