Four species of thorn-bearing trees have been used in New York as landscape trees because of their attractive shapes, pretty flowers or ability to attract wildlife. But the hazards to people and pets posed by the thorns of hawthorn, honey locust, black locust and Russian olive have pushed these trees out of favor. If you want to keep these thorn trees in your landscapes, install a barrier around the tree trunk, prune low branches and promptly pick up all fallen twigs or branches.
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The hawthorn (Crategus linnaeus) is a small tree that will grow in U.S. Department of Agriculture plant hardiness zones 4 to 8. It thrives through most of New York state. Its distinguishing characteristics are stiff, zigzagging twigs armed with many sharp thorns up to 2 inches long and clusters of cherry-like red fruits with a center seed that ripen in early autumn. The tree has a scaly bark that's dark brown to dark gray and oval simple leaves.
The honey locust (Gleditsia triacanthos) is native to western New York state but has been introduced statewide. This tree will grow in USDA zones 3 to 8. Its twigs and branches bear numerous sharp thorns that can grow up to 4 inches long. Its blackish bark has firm, broad ridges that curl over. It has compound leaves that are around 7 inches long with 18-28 leaflets. Its reddish-brown seed pods grow up to 18 inches long and often twist into a spiral.
The black locust (Robinus pseudoacacia) was introduced into New York by early European settlers who valued its hard, durable wood for fence building. The tree bears sharp half-inch-long thorns at the base of each of its compound leaves. Its bark is deeply furrowed. The thorns secrete a toxin that can cause allergic reactions.The leaves grow up to 14 inches long with seven to 19 leaflets. The flat, smooth seed pods grow up to 4 inches long. This tree will grow in USDA zones 4 to 8, including most of New York.
The Russian olive (Elaeagnus angustifolia) was brought into New York in the late 19th century as an ornamental tree prized for its clusters of fragrant white blossoms with yellow centers that attract hummingbirds. It grows in USDA zones 3 to 8. It bears inch-long sharp thorns at the base of each cluster of its elongated silver-green leaves. The tree bears clusters of silver-green to yellow berries that resemble olives. Birds and mammals relish the berries, which contain seeds that pass unharmed through the digestive tract to be deposited far and wide.