The term “locust tree” commonly refers to two distinct species: the Black Locust (Robinia pseudoacacia) and the Honeylocust (Gleditsia triacanthos). Both are members of the legume family, but their blooms are vastly different, often leading to confusion. This article addresses when these trees typically flower and what to expect from the brief display.
Regional Timing of the Bloom
The precise moment a locust tree blooms is tied to local climate and environmental cues, not a fixed calendar date. The flowering period for the Black Locust generally begins in late spring and extends into early summer, typically falling between mid-May and early June across much of its range.
The bloom is triggered by growing degree days, a measure of accumulated warmth over time. For the Black Locust, flowering often commences when accumulated warmth reaches a threshold, sometimes cited around 140 growing degree days. Warmer southern regions may experience the bloom earlier, sometimes in late April, while northern areas may see it delayed until early June.
The entire flowering event is remarkably brief, typically lasting only one to two weeks. This short duration means the window for experiencing the bloom is narrow and highly dependent on local weather conditions. A sudden cold snap or heavy rain can prematurely end the display.
Distinctive Features of the Flowers
The flowers of the Black Locust offer a powerful sensory experience during their short tenure. The blooms emerge in dense, drooping clusters called racemes, which can be 4 to 8 inches long and cascade down the branches. Each individual flower is pea-like in shape, generally creamy white, and often features a small, pale yellow spot on the uppermost petal.
The most celebrated feature is the intense, pervasive fragrance, which can be detected from a significant distance. The aroma is intensely sweet and is frequently compared to orange blossoms, vanilla, or rich honey musk. This potent scent strongly attracts pollinators, particularly honeybees, making the Black Locust a substantial source for honey production.
The sheer volume of these fragrant clusters creates a spectacular visual display, transforming the canopy into a curtain of white. This heavy flowering is a reliable indicator of late spring, but its short duration makes the event fleeting. Once the petals drop, the intense fragrance quickly disappears until the following year.
Key Differences Between Locust Tree Species
While both trees share the common name “locust,” the Black Locust (Robinia pseudoacacia) and the Honeylocust (Gleditsia triacanthos) have radically different floral and structural characteristics. The Black Locust produces the showy, highly fragrant white clusters most people associate with the bloom. The Honeylocust, by contrast, has a far more inconspicuous flower.
Honeylocust flowers are small, greenish-yellow, or cream-colored, appearing in short clusters at the base of the leaves in late spring. While they emit a subtle fragrance, they are not visually prominent and rarely draw attention like the cascades of the Black Locust. The difference in bloom is the easiest way to distinguish the two species.
Structural Differences
Beyond the flowers, other physical features aid in identification. The Honeylocust is known for its formidable, branched thorns that emerge directly from the trunk and branches, often reaching several inches in length, though many cultivated varieties are thornless. The Black Locust typically has smaller, paired thorns or prickles located only at the base of the leaf stems on younger growth. The bark of the Honeylocust is shaggy and less deeply furrowed than the dark, rugged, and deeply ridged bark of a mature Black Locust.
Post-Bloom Development
Once the brief flowering period is over, both locust species begin to develop their characteristic seed pods. These pods are a defining trait of the Fabaceae or legume family. The appearance of the maturing pods provides another clear distinction between the two trees throughout the summer and winter.
The Black Locust produces relatively short, flat, dark brown seed pods, typically measuring between 2 and 4 inches long. These pods contain several small, hard seeds and ripen in the autumn, often persisting on the branches throughout the winter.
The Honeylocust is known for its dramatically longer seed pods, which can grow from 12 to 18 inches in length. These pods are flat, often twisted or curved, and start out green before maturing to a dark reddish-brown in the fall. They are significantly larger and more noticeable than the Black Locust’s pods.