Natives to North America, locust trees are well-known for their strong wood and fall foliage. This article provides a brief overview about the common types of locusts that are also popular as landscape trees.
Locust wood is so hard and durable that it is used for making rails and fence posts. As a young man, President Abraham Lincoln used to split rails from locust trees. Thus he earned the nickname Rail Splitter.
Locusts are fast-growing, hardy trees that belong to the pea family Fabaceae or Leguminosae. Most of the locust species are classified into two genera – Gleditsia and Robinia. While the genus Gleditsia has 12 species, there are around ten species in the genus Robinia. The most popular among them are Robinia pseudoacacia (black locust) and Gleditsia triacanthos (honey locust). Both are deciduous, and are grown as landscape trees. You may also come across different locust cultivars.
Even the carob tree (Ceratonia siliqua) and the African locust bean tree (Parkia biglobosa) are called locust trees. They too belong to the family Fabaceae. Though there are more than 20 species of locust trees, two species are very popular. They are black locust and honey locust trees. Here is a brief overview about some of the characteristic features of these trees.
Otherwise known as false acacia, the black locust tree (Robinia pseudoacacia), is fast growing and hardy. It can grow to a height of around 25 meters and a diameter of around one meter. You may come across some of the very old black locust trees that have a height of around 50 meters and a diameter of 1.5 meters.
The black locust tree produces white flowers that are intensely fragrant; and are arranged on axillary, pendulous racemes. You may also come across some black locusts with pink or purple flowers. The flowers are produced in hanging clusters that can be four to ten inches long. Each flower is around an inch in length. Black locust flowers are consumed in some regions.
Leaves and Seed Pods
The leaves are pinnately compound, with a length of around 25 centimeters. Each leaf has nine to nineteen leaflets, that are roughly oval. The leaflets resemble our thumbprints, in size and shape. Each leaf has a single leaflet at the tip. The leaves turn yellowish during autumn. The legume fruit contains seeds. As compared to some other locust species, the seed pods of the black locust are small and light.
Bark and Thorns
A mature black locust tree produces numerous branches, and has a dark and deeply furrowed bark. One of the characteristic features of this tree is the short, prickly thorns that are located at the base of the leaves. The thorns of black locust trees are short, when compared to that of honey locusts. They do not have the branched thorns that are seen on the trunk of honey locust trees.
Though they are mainly grown for ornamental purposes, black locusts are much valued for their hard and durable wood. In some regions, the black locust is cultivated as a honey plant. In other words, the blooms of black locust is a source of nectar for honey bees in that area. Apart from the flowers, the bark, seed pods and every other part of this tree are considered toxic; but it is also said that the toxicity can be nullified through cooking. It has also been contended that the tender seed pods as well as the seeds can be boiled and consumed.
The honey locust tree (Gleditsia triacanthos) is otherwise known as sweet bean, sweet locust, and honey shuck. It is a fast-growing tree that grows to a height of around 30 meters and a diameter of around one meter. The honey locust is grown for ornamental purposes, as it has an attractive fall foliage. Some varieties of honey locusts, like Gleditsia triacanthos inermis, do not have thorns and seed pods. The image below depicts a sunburst honey locust tree that has no thorns and seed pods. You may also come across honey locust cultivars that are very popular as landscape trees.
The bright green leaves are pinnately compound, but you may also find compound pinnate leaves in honey locust trees. Unlike black locusts, most of the leaves of honey locusts have no leaflet at the tip. The bright green leaves turn yellow during early autumn. Both black and honey locust trees produce new leaves during late spring. However, honey locusts develop new leaves, slightly earlier than the black ones.
Honey locusts carry thorns on their branches, and at the base of leaves. These thorns are longer (around 3 to 10 cm); and are seen in dense clusters. The young and tender thorns are green and soft. As they age, the thorns turn harder and reddish brown. Fully mature thorns are brittle, and are usually ash gray in color. The tree is also known as thorny locust.
The strong-scented, creamy-green flowers are very small; and develop in clusters. Flowers of honey locusts are often found as inconspicuous spikes that develop from the base of leaf axils. While the male and female flowers are produced in different trees, some of them have both types in the same tree. The male flowers are found as dense clusters, whereas the female flowers are loosely arranged on the rachis.
The female honey locust trees produce long, flat and twisted fruits (or seed pods). The pale green seed pods turn reddish-brown and black, when they mature. As they ripen, the seed pods produce a strong smell. The sticky pulp inside the pods are edible. These seed pods fall off the tree during winter.
Unlike black locusts, honey locust trees are not honey plants. The name may be derived from the sweetness of the pulp that is also used for making beer. Its timber is highly valued for making furniture, as it is hard and durable. The seeds pods are used as fodder for livestock. Like black locust, different parts of the honey locust tree are also used for medicinal purposes.
These are some of the interesting facts about locust trees. A basic understanding about their characteristic features may prove helpful for locust tree identification. In order to identify a locust tree; check the size of the tree, the color and texture of its bark, shape and arrangement of leaflets, flower color and structure, and nature of seed pods and thorns.