The common name “locust tree” is often used broadly in North America, leading to confusion about how many types exist. All species commonly called locust trees belong to the Fabaceae, or pea family, characterized by fast-growing, nitrogen-fixing trees that produce legume seed pods. Early colonists gave the designation because they thought the trees resembled the Old World Carob tree, whose pods were known as “locust beans.” When people refer to a locust tree, they are typically thinking of one of two distinct genera native to the United States.
The Two Primary Locust Tree Families
The question of how many types of locust trees there are is answered by looking at the two primary genera that dominate the landscape: Robinia and Gleditsia. These include the Black Locust and the Honey Locust, respectively. While both are classified within the pea family, they possess significant differences in morphology and ecological roles. The Robinia genus is smaller, containing mostly shrubby species, with the Black Locust being its most significant tree representative.
The Gleditsia genus is more diverse, including species like the Water Locust (Gleditsia aquatica), but the Honey Locust (Gleditsia triacanthos) is the most widely recognized. Most other named “locust” varieties, such as ‘Shademaster’ or ‘Purple Robe,’ are simply cultivated forms or hybrids of these two main types. Understanding the characteristics of Black Locust and Honey Locust is the main way to distinguish between the types of trees found in yards and forests.
Characteristics of the Black Locust
The Black Locust (Robinia pseudoacacia) is a deciduous tree capable of reaching heights of 50 to 80 feet. It thrives in poor, disturbed soils, which it improves through a symbiotic relationship with nitrogen-fixing bacteria in its root nodules. This makes it a valuable species for land reclamation and erosion control projects, especially in areas like former mine sites.
A distinguishing feature of the Black Locust is its highly fragrant, showy white flowers that bloom in pendulous clusters during late spring or early summer. These pea-like blossoms are a major source of nectar, making the tree popular among beekeepers for producing high-quality honey. The fruit is a small, flat, dark reddish-brown seed pod, typically two to four inches long, containing four to eight seeds.
The wood of the Black Locust is exceptionally dense and resistant to rot, making it highly prized for fence posts, railroad ties, and heavy-duty outdoor construction. Young Black Locust trees and their new branches typically bear sharp, paired thorns. Nearly all parts of the Black Locust—including the bark, leaves, and seeds—contain toxic compounds and should not be consumed by humans or livestock. The tree’s tendency to spread aggressively by root suckers has led to it being considered an invasive species in many regions outside its native range.
Characteristics of the Honey Locust
The Honey Locust (Gleditsia triacanthos) is another fast-growing member of the pea family, often reaching heights of 70 to 100 feet. Unlike the Black Locust, its flowers are small, greenish-yellow, and largely inconspicuous, though they still attract insects. The most dramatic feature of the wild Honey Locust is the presence of massive, often branched thorns that emerge directly from the trunk and branches.
These formidable thorns, which can be up to eight inches long, are reliable identifiers for the wild type of this tree. However, the Honey Locust is popular for urban landscaping because many modern cultivars, such as ‘Shademaster’ and ‘Sunburst,’ have been selectively bred to be entirely thornless. The tree is also valued for its compound leaves, which cast a light, dappled shade that allows turfgrass to grow underneath, making it a desirable street tree.
The tree gets its common name from its distinctive seed pods, which are large, flat, and often twisted or curled. The pods contain a sweet, thick, sticky pulp that is safe for consumption by wildlife and was historically used by Native Americans as a sweetening agent. The Honey Locust is highly tolerant of drought, salt, and urban pollution, contributing to its widespread use in city planning across North America.
Distinguishing Between Black Locust and Honey Locust
Differentiating between the two main types of locust trees is straightforward once a few key characteristics are observed. The most noticeable difference is in their thorns: Black Locusts have small, paired thorns on younger twigs, while the wild Honey Locust has massive, multi-branched thorns emerging from the main trunk and older limbs.
Another distinction is found in the flowers and seed pods. Black Locust produces showy, fragrant white flowers in drooping clusters, followed by short, dark, non-twisting pods. The Honey Locust has small, inconspicuous greenish flowers and develops long, flat, often curved or twisted pods up to 18 inches long.
The Black Locust is a nitrogen-fixer, enriching the surrounding soil, a trait the Honey Locust generally lacks. Correct identification is also important because the Honey Locust’s pod pulp is sweet, whereas most parts of the Black Locust contain toxins.