Locust trees, belonging to the genera Robinia (Black Locust) and Gleditsia (Honey Locust), are known for their fragrant spring flowers. Predicting the bloom time requires identifying the specific species, as the timing and appearance differ significantly between the two most common varieties. The precise moment of flowering is influenced by both the tree’s genetics and the local environment.
Distinguishing Between Black and Honey Locust
Identifying the type of locust tree is essential for predicting its bloom time, as the two major species have distinct physical features. The Black Locust (Robinia pseudoacacia) features deeply furrowed, dark gray bark when mature. Its leaves are pinnately compound, consisting of a central stem with 7 to 19 small, oval leaflets. This species is armed with small, sharp thorns, usually less than one inch long, which grow in pairs at the base of the leaf stems on new growth.
In contrast, the Honey Locust (Gleditsia triacanthos) has darker, smoother bark when young, later developing thin, scaly plates. Its leaves are often bipinnately compound, consisting of smaller leaflets that create a fine, delicate texture. The thorns of the Honey Locust are large, frequently growing in branched clusters directly from the trunk and branches, sometimes reaching eight inches in length. However, many popular cultivated varieties have been selectively bred to be entirely thornless.
Specific Bloom Timing for Each Variety
The timing of the floral display is a key difference between the two locust species. The Black Locust produces showy, highly fragrant flowers, blooming in late spring to early summer. The typical window is May to early June, though this varies based on geographical location. These flowers appear as creamy white, pea-like blossoms in long, drooping clusters called racemes.
The Black Locust flowering period is brief, generally lasting only seven to fourteen days. The Honey Locust flowers are also fragrant but are less conspicuous, usually blooming in May and June. Its flowers are small, greenish-yellow, and appear in short, hanging clusters. Since they blend with the newly emerged foliage, the Honey Locust bloom is less of a visual spectacle than the Black Locust.
Environmental Factors That Influence Bloom
While genetics determine the typical bloom window, environmental conditions influence whether a tree blooms earlier or later than average. Spring temperatures play a significant role in this timing. Prolonged warm weather in early spring may accelerate flower bud development, causing the bloom to arrive early. Conversely, an unexpected late frost can severely damage newly formed flower buds, potentially delaying or preventing the bloom entirely for that season.
The tree’s climate zone also dictates its timeline, with trees in southern regions blooming weeks ahead of those in northern latitudes. Tree maturity affects bloom reliability; Black Locusts typically flower around three years of age, while Honey Locusts often take ten years to reach consistent seed-bearing age. Soil moisture is another factor, as severe drought stress can cause a tree to conserve energy by reducing or skipping floral production.