– by David Schroeder
Black locust’s (Robinia pseudoacacia) home range was initially in the north-central U.S. However, it was widely planted and is now naturalized in most of the eastern part of the country, including Connecticut. It probably owes its origin in Connecticut to farmers who planted the trees in an attempt to produce decay-resistant fence posts. It escaped cultivation and soon was found throughout the state. Black locust will grow on a wide range of soils, and in Connecticut is usually found on poor, open sites. It is very shade intolerant and fairly fast growing on good sites, where it may attain a height of 60 feet. A rather unusual characteristic of black locust is its ability to contribute nitrogen to the soil via nitrogen- fixing bacteria on its roots. It can actually improve the nutrient value of soil and has been used in the south to re-vegetate spoil-banks. When black locust trees are cut they sprout from the stump and roots and can quickly spread through an area.
The blue-green leaves of black locust are 8 to 12 inches long, pinnately compound with 7 to 9 leaflets. In the spring it produces white, fragrant flowers in a dropping cluster up to six inches long. Twigs are armed with prominent, paired, stipular spines that are sharp to the touch. The fruit is a brown, flat legume approximately 4 inches long. Bark on older trees is black and deeply furrowed. The inner bark is poisonous, and livestock and children have become sick or died after chewing on the twigs.
Black locust was introduced into Germany as early as the 1600s and has become widely distributed in Europe. There are indications that in some areas it is regarded as an invasive species.
In Connecticut , black locust is used primarily for fence posts and fence rails. Some lumberyards still carry black locust posts. However, most people today use plastic or pressure treated posts. Black locust is excellent fuelwood surpassing hickory in heat value. Since the wood shrinks very little, it was used early on in the history of this country for tree nails in wood ship construction. Because of its excellent durability, black locust was also used for railroad ties and mine timbers before chemical preservatives were available.
Black locust is not a particularly valuable wildlife tree. Deer will browse the twigs and some small mammals, primarily squirrels, eat the seeds. It has been planted on mine spoil areas to provide cover for wildlife. Because of the rather showy flowers, black locust has been planted as an ornamental. It is attacked by a major pest, the locust borer, which has discouraged some from planting it for posts or as an ornamental.
This article originally appeared in the December 2003 ECFLA/WDLT Newsletter.