Black locust's fast growth and tolerance of poor soil could be a problem for homeowners. These hardwood trees reproduce as clones from their own widespread roots. At maturity a grove of black locust could reach heights of 100 feet, with each tree in the grove a genetic copy of the original. Pruning focuses on controlling rather than shaping the tree.
Pruning Black Locust
Prune during the winter dormancy to cause the least injury to the tree. To be practical, some pruning must be done throughout the growing season as well. A seedling could grow--on average--to about 50 inches in height the first season. Sucker shoots might be even more vigorous.
Only prune young trees if limbs are damaged. Black locust quickly grows tall with a naturally high crown. As height increases and most limbs are above head level, lower limbs can be clipped off. Use limb loppers to remove low limbs at the junction with the main trunk. Heavy loppers could be needed even for small branches because black locust ranks as one of the hardest North American woods.
Watch for sucker shoots emerging near the tree as the black locust matures. Suckers sprout from the tree's roots and unless clipped off regularly will eventually match the parent tree in size. With limb loppers cut the sucker shoots off just below ground level--leaving even a small knob of this tough wood could ruin a lawn mower if caught by the blade.
Mow the area beneath the black locust regularly. Lawn mowers could be the best defense against the tree's root suckers. If left alone, root suckers come back with a vengence--usually studded with thorns the next time. Small green shoots can be controlled by mowing, but occasionally the clusters of short spikes that remain should be clipped short by hand.
Repeat pruning of low branches and root suckers as necessary through the season. Both branches and suckers will regrow, and second or third growth will almost certainly bear thorns. Wear eye and hand protection against the spikes and be sure to remove clippings from the yard before mowing. Thorns on suckers and twigs could puncture the tires of a riding mower.