Black locust
Also known as
False acacia, Locust tree, Ship-mast locust, Yellow locust, Common locust, Black acacia, Post locust
Keep this plant’s care guide close at hand.
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Robinia pseudoacacia, commonly called black locust, is a fast-growing, deciduous leguminous tree native to the Appalachian and Ozark regions of the eastern United States. It typically reaches 12–25 m (40–80 ft) tall with deeply furrowed dark bark, paired short spines on young shoots, and pinnate leaves that fold at dusk. In late spring, it bears showy, pendulous racemes of fragrant, white, pea-like flowers rich in nectar, valuable to pollinators and honey production. The tree fixes atmospheric nitrogen, enabling it to thrive on poor, sandy, or rocky soils and to colonize disturbed sites via root suckers. Its dense, rot-resistant heartwood was historically prized for fence posts and shipbuilding. While durable and drought-tolerant once established, the species can be brittle in storms and is considered invasive in many regions outside its native range, where it spreads aggressively and alters soil chemistry. All parts—especially bark, seeds, and young shoots—are toxic if ingested.
Taxonomy
Properties & Requirements
How to care for Black locust
Water
Water deeply but infrequently. For the first two growing seasons, soak the root zone once per week during dry periods, delivering 15–25 liters (4–7 gallons) for young trees and allowing the top 5–8 cm (2–3 in) of soil to dry between waterings. Use a wide watering basin extending to the dripline. Mulch 5–8 cm (2–3 in) with wood chips, keeping mulch off the trunk, to conserve moisture and moderate soil temperature. Once established, black locust is notably drought-tolerant and rarely needs supplemental irrigation.
Light
Provide full sun—at least 6–8 hours of direct light daily—for best flowering, compact growth, and trunk strength. It tolerates light shade but will bloom poorly, elongate, and shed lower limbs over time. In dense shade, it performs poorly and may lean toward light. Urban reflectance and heat are tolerated, but ensure adequate rooting volume and drainage. Black locust is an outdoor landscape tree and is not suitable for indoor cultivation.
Nutrients
As a nitrogen-fixing legume, black locust generally needs little to no nitrogen fertilizer. At planting, mix in well-rotted compost or a balanced, low-N slow-release product (e.g., 5-10-10) to support root establishment. In nutrient-poor soils, a light application in early spring can aid young trees. Avoid high-nitrogen fertilizers, which encourage weak, sappy growth and more suckering. Where soils lack compatible rhizobia, consider an appropriate legume inoculant at planting. Mature, established trees typically require no fertilization.
Temperature
Hardy in USDA Zones 4–8, black locust tolerates winter lows around −29 to −34°C (−20 to −30°F) and hot summers. It withstands heat and drought but dislikes prolonged waterlogging or high-salinity soils. Late spring frosts can singe new growth and flowers but usually do not harm overall vigor. Not suited to tropical, frost-free climates. Provide wind exposure appropriate for a tree with somewhat brittle wood—structural pruning helps reduce storm breakage.
