How to Plant and Grow Black Locust

The black locust tree is prized (and despised) for its rapid growth. At maturity, a black locust tree can easily reach 30 to 40 feet tall and about 20 feet wide, making it a lush and leafy living screen. It can, however, grow much taller (sometimes 90 feet) and spread rapidly in welcoming landscapes. So, be sure to consider the liability of black locust trees before planting.

More than a quick-growing tree, the black locust is beloved for its ropelike clusters of pink and white flowers in midspring. These flowers are frequent stopping points for bees and black locust honey is valued for its rich flavor.

Black locust trees feature irregular branches with stout thorns and feathery leaves. The tree also produces long, leathery seed pods that appear in the fall and persist throughout the winter. The seeds themselves are resilient and can remain viable where they fall for up to 10 years. Once established, black locust trees reproduce by root suckering and stump sprouting—a practice that can lead to quick colonies of trees that crowd out other species.  

It's also worth noting that (as with many trees in the Robinia genus) leaves, bark, flowers, seeds, and seed pods of the black locust tree are considered toxic to dogs, cats, livestock, and humans.

Where to Plant Black Locust

Black locust trees can grow in almost any type of soil but prefer moist, well-drained soil and access to lots of sun. The flowers of the black locust tree are considered a valuable food source for bees while the seeds provide nourishment for birds and other wildlife. The fast-growing trees need ample space to grow and should not be planted close to homes or other structures.  

Black locust trees are also nitrogen-fixing trees—which means they change the balance of the soil that surrounds them by converting atmospheric nitrogen into forms usable by other plants. This has both positive and negative effects as it can enrich poor soil but can also open the door to non-native plants that would otherwise not grow in such a space.

In the past, black locust trees were most frequently used for roadside plantings, property windbreaks, and privacy hedges. Today, the quick-growing tree is primarily used for erosion control and landscape reclamation projects, particularly in arid climates.

How and When to Plant Black Locust

The best time to plant a black locust tree is in the spring. Choose a spot and dig a hole twice as deep and twice as wide as the plant’s root ball. Place your tree in the center of the hole and fill the hole halfway. Add a little water and then continue backfilling the hole. Tamp down the soil to secure the tree and water thoroughly.

Black Locust Care Tips

Black locust trees thrive (sometimes too well) in full sun and well-drained soils. They tolerate drought, salt, and poor soil and can become invasive in some climates if not kept in check.

Light

Black locust trees prefer full sun but tolerate light shade.

Soil and Water

Black locust is known for its ability to grow in almost any type of soil. Dry, sandy soil—no problem. Compacted soil in an urban area—no problem. Boggy soil that dries occasionally? It grows here too. The only soil that limits black locust growth is soil that is consistently soggy. Streamside and ravine planting spots are often too wet for black locust trees, but a rain garden, which dries from time to time, is a great planting area.

Black locust trees are also drought tolerant once established and may only need watering once or twice a month (if rainfall is absent). Young, freshly planted trees may need weekly or daily supplemental watering (depending on climate) to help establish a strong root system.

Temperature and Humidity

The deciduous tree is native to the moist forests of North America, so it tolerates moderate to relatively high humidity. Black locust trees also thrive in temperatures between 32˚and 90˚ Fahrenheit but are hardy in zones 4-9. This means they can tolerate a wide range of temperatures including those well below zero and above 100˚. That said, extreme heat can cause stunted growth, wilting, yellowing, and scorching. Extreme cold and frost can damage shoots, cause dark spots, and lead to crooked or diminished growth.

Fertilizer

Black locust trees rarely (if ever) require fertilization. In fact, they are often used as pioneer plants in poor soils to pave the way for other plants. This is because black locust trees have nitrogen-fixing capabilities, which allow them to break through poor soils, aerating them and making them more hospitable. They also produce leaf litter with a higher nitrogen content than most native tree species.

Pruning

Despite its robust growth, the black locust tree doesn’t require much pruning. You can, however, prune the tree to shape its canopy—just be sure to watch out for the thorns at the base of each leafstalk. Use sharp pruning shears to remove crossed or congested limbs but avoid removing the tree’s lower limbs as this might unnecessarily stress the tree.

The best time to prune your black locust tree is in late winter when the tree is still dormant or in the summer. Diseased, dead, or damaged limbs can be removed as needed. Avoid pruning in the spring as the tree may bleed—a stress response that causes excessive sap to flow from the wound. This could, in turn, draw harmful insects that may further damage the tree.

Physical attempts to control black locust reproduction (like pulling, mowing, and burning) often encourage suckering and colonization. If you wish to eliminate or control black locust production, you will need to apply an herbicide to cut stems and stumps.

Pests and Problems

Borers and black locust leaf miners regularly threaten black locust trees. Borers and leaf miners rarely kill a tree, but they do give it a brown appearance in late summer after they have ravaged the foliage or wood. However, the tree often fully recovers the following growing season.

In many areas, black locust trees are plagued by internal decay. This can kill a tree or weaken its structure. Once weakened, black locust trees can easily break apart in windstorms and become a liability. For this reason, it is critical to monitor the health of black locust trees in urban and suburban landscapes. A weak tree can damage property or endanger people.

How to Propagate Black Locust

The easiest way to propagate black locust is to dig up and transplant suckers. You can also propagate the trees via seed or with hardwood cuttings taken from plants with established root systems.

How to Propagate via Cuttings

In late winter or early spring, use sharp shears to cut a 6- to 12-inch section from a well-established black locust tree. Allow the cutting to dry out in cool, dry sand for three weeks. After that, apply a root hormone powder to the cutting and prepare a 5-gallon pot filled with equal parts sand and perlite. Plant your cutting deep enough that the top end is just slightly below the surface of the soil. Place the pot in full sunlight and keep the soil consistently moist until it begins to sprout.

How to Propagate Via Seed

If you wish to propagate black locust from seed, collect the seeds in early fall before they are dispersed by the wind. You must scar or break the hard seed coat to allow germination. This can be done by rubbing the seeds with 120-grit sandpaper or by soaking them in boiling water for 24 to 48 hours. Once the seeds are scarred, plant the swollen seeds in moist potting soil (about 1/4 inch deep). Place the container in a cool, dark location and keep the soil evenly moist as the seeds germinate. Once they sprout, place your container in a cool, but sunny spot. Continue to keep the soil moist until you transplant your seedlings into the ground in the spring.

Types of Black Locust

Common Black Locust

In it's wild form, Robinia pseudoacacia is fast growing and hardy in zones 4-9. It offers fragrant white flowers and typically reaches 80 feet tall and 50 feet wide at maturity.

Golden black locust

Robinia pseudoacacia 'Frisia' has white flowers and bright golden-yellow foliage in spring and summer that turns orange in fall. It grows 50 feet tall and 25 feet wide in zones 4-9.

'Purple Robe' Black Locust

Robinia pseudoacacia ‘Purple Robe’ features rosy-pink and purple flowers and foliage that matures into a bronze-green color. It typically grows 30 to 40 feet tall in zones 4-8.

'Lace Lady' Black Locust

Robinia pseudoacacia 'Lace Lady' (a.k.a., 'Twisty Baby') is a compact black locust cultivar that grows only 20 feet tall and features twisted, zig-zag limbs. It rarely flowers and is hardy in zones 4-8.

Companion Plants for Black Locust

Arborvitae

Arborvitae trees are classic hedge plants beloved for their evergreen foliage and dense growth habit. Arborvitaes love black locust trees. In fact, studies have shown that some cultivars fare better when grown in proximity to black locust trees. Like black locust trees, easy-care arborvitaes grow best in consistently moist but well-drained, loamy soil with a neutral to slightly alkaline pH (6.5 to 8.0). They also prefer full sun but can tolerate partial shade and are hardy in zones 2-7.

Blanket Flower

Like black locust trees, blanket flowers can easily adapt to poor soils and drought conditions. Blanket flowers are also native to the tough prairie soil of the western United States. They thrive in zones 3-10 and are not particular about soil pH. Blanket flowers like full sun. They will grow in partial shade but may grow leggy and produce fewer flowers.

Mock Orange

Mock orange is cold-hardy in USDA zones 4-8. The shrubs need full-sun exposure to produce the best flowers but can tolerate partial shade. Like black locust trees, they grow best in moist, well-draining soil and offer fragrant flowers that draw in bees and butterflies. Also like black locust trees, they grow rapidly (about 2 feet per year). At maturity, they are typically 8 to 10 feet tall.