Black Locust Tree: Everything You Need to Know

Many people think that the hickory tree’s wood is the strongest and sturdiest in North America.  That is wrong. It is the black locust, a member of the Fabaceae or pea family, that can lay claim to that title.  Couple the strength of its timber with its near invulnerability to wood rot, and you have the perfect medium for home construction and improvement.  Black locust wood is commonly used to build decks, outdoor furniture, and fence posts.

In bloom, the beautiful white flowers produce an intoxicatingly sweet smell that permeates the air in the spring.  The flowers’ nectar is a favorite of bees and produces some of the best wild honey you’ll ever taste.

This tree’s fast growth rate of 3 to 4 feet per season is primarily attributed to its ability to fix nitrogen in its stout root system.  The roots grow vigorously and are prized for their ability to control erosion.

Black locusts are amazing trees to have in your landscape.  Not only are they beautiful to behold, but they are also able to provide benefits to other plants. 

All in all, black locusts could be the perfect addition to your landscape.  If you know how to grow these trees correctly, you can reap incredible benefits.  So check out this article and discover the ins and outs of caring for these fantastic deciduous trees. 

General Information

The black locust, Robinia pseudoacacia, is a hardwood native to sections of the Appalachian Mountains and the Ozarks.  It has been naturalized throughout North America.  You find it growing in every state in the lower 48.  It was transplanted across the Atlantic Ocean and is now naturalized in Europe, Asia, and Southern Africa. 

Although it is native to North America, in sections of the western United States, New England, and the Midwest, it is now considered an invasive species.

In the open plains of the Midwest, for example, black locusts can overtake the landscape and dominate it with unwanted shade. This will change the grassland ecosystem into a forest.  The grasses beneath the canopy thin out over time and are eventually no longer viable to harvest.  This can be catastrophic to grain farmers in the plains. 

The shade-intolerant black locust grows very straight to heights between 50 and 100 feet and has a diameter of 2 to 4 feet.  Most live to be around 60 years old.  However, some specimens can grow taller than 150 feet, have a diameter of more than 5 feet, and live to be 100 years old. 

The branches grow erratically and without any sense of order.  When the limbs are very young, they are coated in a silvery-white down that quickly disappears.  It fades to pale green and then darkens into a red or greenish-brown. 

The bark is ridged and furrowed into grooves that run up the trunk and across one another to form a diamond-like shape.  The interior of these grooves is tinted reddish-orange, and the surrounding bark is a maroonish-gray.

The compound leaves are pinnate and feather-like in appearance.  Each leaf grows between 6 and 18 inches long and contains 7 to 21 individual leaflets that grow in opposing pairs with a single terminating leaflet.  Each leaflet is 1 to 2 ½ inches long and ½ to 1 inch wide and rounded at both ends.  The leaves appear in early spring and are a dark blue-green on the upper surface and a much lighter blue-green underneath.  In the fall, the leaves turn to a pale yellow.  The contrast in the surfaces of the leaves is striking and adds to the tree’s mystique and elegance when they blow in the wind.

After the leaves have appeared in May or June, cream-colored flowers will appear for 1 to 2 weeks.  They grow in loose 4- to 8-inch clumps that sag towards the earth.  The individual flowers are irregularly shaped and resemble a butterfly’s wings.  The creamy white color is interrupted by a splotch of pale yellow in the center.  The flowers are very broad at their opening and can hold large amounts of nectar.

The tree has spines that grow in pairs at the base of each leaf and can be anywhere from ¼ to 1 inch long.  The spine grows out from a broad base and acutely tapers to a fine, sharp purple point. 

The black locust is a member of the pea family.  So it should come as no surprise that its seed pods have the smooth, pea-like characteristic of legumes.  Each pod is 2 to 4 inches long and about half an inch wide.  The fruit has 4 to 10 dark orange-brown seeds that ripen in late fall.  The fruit will hang on the branch until early spring.

Care Guide

Let’s look at the optimal conditions you’ll need to create for healthy, sustained growth of your black locust.

Sunlight Requirements

Black locusts are considered a pioneer species because they colonize an area before other forms of vegetation.  This means that over the years, there was little to no competition for sunlight. Thus, the species has developed into a very shade-intolerant hardwood. 

It prefers full sun conditions but can survive in partial sun.  To be exact, it prefers 6 hours or more of direct sunlight each day but can live with between 4 and 6 hours of unfiltered sunlight per day.

Soil Conditions

Black locusts are hardy and resilient trees that have adapted to a variety of soil conditions.  The nitrogen-fixing roots prefer well-drained soil that is deep and dry, and only somewhat moist.  It has been shown to exhibit even healthier growth in soils that are calcareous or that contain limestone. 

Its ability to flourish in damaged soils can be seen in the wide variety of pH levels it can grow in.  The preferred pH level for black locusts falls between 4.6 and 8.2.  So, as long as your soil isn’t saturated and poorly drained, your tree will probably find a way to grow.

Water Requirements

After you plant your black locust, you’ll need to water it regularly to ensure that the roots establish themselves and spread outward.  Ensure the soil remains moist but not soggy through your tree’s first growing season.   

It’s crucial that your water drains properly and doesn’t puddle in the root zone. This can drown the roots.

Mature trees are moderately drought tolerant.  They will benefit from supplemental watering during extended dry periods, but they usually receive sufficient water from the environment under normal weather conditions. 

Lay down 2 to 3 inches of organic mulch around the base of the tree to help the soil retain its moisture.  This will keep the water from evaporating too quickly and keep the roots happy.

Fertilization

Your black locust will probably never need to be fertilized with a nitrogen-heavy product.  Its ability to fix its own nitrogen means that it has its own built-in fertilizer factory. 

However, if you noticed something amiss, you should test the soil to understand what’s going on.  You may find it necessary to use urea or some other ammonia-based product to counterbalance the nitrogen’s effect on the pH level.  Repeated nitrogen-fixing can result in soil with a raised pH level. 

Pruning

Prune back dead, diseased, or damaged branches as soon as you notice them.  Cut them back until you reach living tissue and bandage the exposed wood to prevent disease. 

In the fall, inspect the branches to determine if there is any contact between two adjacent branches.  If so, remove the weaker of the two branches because the contact can wound the tree and make it vulnerable to disease.

Growing Tips

Here are a few tricks of the trade to keep in mind when planting your black locust.

Pests

Be mindful of the locust borer and the black locust leaf miner.  These pests won’t kill the tree, but if they are allowed to gorge themselves unchecked, you’ll have an ugly brown tree by the middle of summer.  Although the hardwood will heal by the next growing season, you’ll still have to stare at a barren tree for the remainder of the year.

Also, the locust borer will expose the tree’s wood and make it susceptible to disease and internal decay, which you will probably not notice until it’s too late.  Everything will appear normal until a steady wind blows through.  Then, the weakened internal structure can cause limbs and even the base to snap.  So, inspect the bark for evidence of the pest and treat it accordingly.

Treat the Seeds Before Planting

The thick seed coat and water’s inability to permeate it are prohibitive to germination.  In some cases, the germination rate can be as low as 20%. 

So, what can you do?

Well, there are three methods for weakening the seed coat.

1. Soak the seed for 12 to 20 hours in water just below the boiling point (about 160℉).
2. Treat the seed with sulphuric acid.
3. Scar, scratch, or wear down the seed coat mechanically. To do this:
4. Agitate the seed in a revolving drum or another container that is filled with sand or small gravel.
5. Shake the seed in a sealed jar.
6. Blowe the seed against a curved surface covered with a fine-grit sandpaper.

To ensure your success, I recommend using method three, followed by method one.

Tips for Planting Germinated Seedlings

Loosen the Soil

Before planting, ensure that the soil you’re placing the seedlings in is broken up and loose.  In this environment, the roots will grow more efficiently in a lateral direction. It also stimulates good top development.

In contrast, placing a seedling in a hole with solid soil does not allow for healthy growth. It would essentially just sit inertly in the ground.

If you’re planting multiple seedlings over a large area, you should cultivate the ground with a plow or roto-tiller to loosen the soil. 

When to Plant

The best time to plant seedlings is in early spring, before the leaf buds begin to swell.  If you wait too late into the spring, the surrounding vegetation will leaf out and force the seedling into undesirable shaded conditions.

You can plant seedlings in the fall so long as the soil has been loosened and it has rained recently to soak the soil to a considerable depth.  Do not plant seedlings in the fall if you have not prepared the soil because the winter’s frost will injure them.

Check the Roots

Inspect your seedlings for threadlike, spindling roots and discard those that you find. It’s likely that they are from weak or undersized seeds and won’t do well even in fertile, prepared soil.

Planting and Setting the Seedlings

Keep the seedling’s roots moist while transplanting them from the nursery to the planting site.

At the planting site, place the seedlings firmly into the ground, as you would do with cabbage or a tomato plant.  Spread the roots out fully.  Roots with bunched or curled roots will not grow well, even in soil that has been prepared. 

Once you have set the roots into the soil, backfill the hole until the roots are covered.  Firm up the soil with your foot to ensure the roots are lying flat and the plant is firmly set into the ground.

If you follow these tips, your black locust is off to a good start and you will soon be able to enjoy the shade of this exotic-looking tree.

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