Robinia pseudoacacia, commonly known in its native territory as black locust, is a medium-sized hardwood deciduous tree, belonging to the tribe Robinieae of the legume family Fabaceae. It is native to a few small areas of the United States, but it has been widely planted and naturalized elsewhere in temperate North America, Europe, Southern Africa and Asia and is considered an invasive species in some areas, such as the temperate east coast of Australia.
Black Locust trees grow to a height of 40 to 100 feet (12 to 30 meters) when mature. In optimal conditions, they might reach even taller heights. The spread or width of the canopy can be around 20 to 35 feet. Trees growing in forests or close proximity to other trees might grow taller but with a narrower spread as they compete for light.
The trunk diameter can grow to be between 1 to 4 feet with exceptional specimens potentially growing larger. The bark is deeply furrowed and dark brown or black. Leaves are pinnately compound with small, oval leaflets.
Black Locusts produce highly fragrant, white flowers in drooping racemes (clusters) in late spring. These flowers are not only aesthetically pleasing but are also a good nectar source for honey production. After flowering, it produces flat, brown pods that contain seeds. These pods can hang on the tree into winter. The seeds are dark orange brown with irregular markings. They ripen late in autumn and hang on the branches until early spring.
Under favorable conditions, it can grow more than 24 inches (60 cm) annually. The tree tends to reach maturity in terms of height within about 10 to 15 years, though it will continue to fill out and grow thicker for several more years. On average, they might live for 60 to 100 years. However, some trees can live longer under ideal conditions, potentially up to 120 years or more, but this is less common.
Black Locust is fairly resistant to many diseases and pests, which contributes to its longevity. However, it can be susceptible to certain issues like locust borer, heart rot, and leaf miners, which can shorten its life if not managed. Proper care, including adequate watering in dry conditions, pruning to remove dead or diseased wood, and protection from severe physical damage, can extend the life of the tree.
Trees in urban settings or under stress from pollution, poor soil, or mechanical damage might have a reduced lifespan. Conversely, those in optimal rural or forest conditions might live towards the longer end of this spectrum.
Black Locust has the ability to fix nitrogen from the atmosphere, thanks to symbiotic bacteria in its root nodules. This trait allows it to grow in poor soils where other plants might struggle and can improve soil fertility over time. Outside its native range, Black Locust can become invasive, altering ecosystems by shading out other plants and changing soil chemistry.
Parts of the tree, especially the bark, leaves, and seeds, are toxic if ingested. They contain toxalbumins like robin, which can cause gastrointestinal distress, weakness, and in severe cases, could be fatal.
The wood is extremely hard, resistant to rot, and durable, making it ideal for outdoor uses like fence posts, poles, railroad ties, and even in boat building. It’s also used for firewood because it burns slowly with little visible flame or smoke and has a high heat content.
Black locust can be easily propagated from roots, softwood, or hardwood. Cultivars may be grafted, ensuring that parent and daughter plants will be genetically identical. In Europe, it is often planted along streets and in parks, especially in large cities, because it tolerates pollution well.
Cultivars
Several cultivars exist, ‘Frisia’ being one of the most planted ones.
- ‘Decaisneana’ has been considered a cultivar but is more accurately a hybrid (R. pseudoacacia x R. viscosa). It has light rose-pink colored flowers and small or no prickles.
- ‘Frisia’, a selection with bright yellow-green leaves and red prickles, is occasionally planted as an ornamental tree.
- ‘Purple robe’ has dark rose-pink flowers and bronze red new growth. The flowers tend to last longer than on the wild tree.
- ‘Tortuosa’, a small tree with curved and distorted branches.
- ‘Unifoliola’, a plant with fewer leaflets, no prickles, and a shorter height.
Black Locust (Robinia pseudoacacia) Growing Condition/Environment
- Black Locust grows well in USDA hardiness zones 3 through 8.
- Prefers well-drained soils; it can grow in sandy, loamy, or clay soils. However, it does particularly well in limestone-based soils.
- While it is drought-tolerant once established, consistent moisture can promote better growth, especially in younger trees. However, it does not do well in waterlogged or extremely wet conditions.
- It thrives in temperate climates with warm summers and can withstand cold winters, but very severe or prolonged frost can damage young trees or shoots.
- Shows some resistance to salt, which can be beneficial for roadside plantings where de-icing salts are used.
- When planting, consider its potential size; trees should be spaced at least 15 to 20 feet apart if you’re aiming for a grove or forest setting. For individual landscape trees, provide enough space for its canopy spread.