Complete Guide to Black Locust Tree – What you NEED to know

Identification and characteristics of the Black Locust tree

Identifying a Black Locust trees is quit simple once you know what to look for. To make a positive identification, look for irregular branching and crown structure, the bark will have deep furrows that run at irregular angles, small thorns are present on young twigs & branches, and they have alternating compound odd-pinnate leaves.

Crown

Mature Black Locust Trees will grow on average 50′-80′ tall (15m-24m) and have an irregular crown shape. The branching on a Black Locust tree is typically upwards.

Black Locust bark and trunk

The Black Locust trunk will reach 1.5′-3′ diameter at maturity. While the bark of the Black Locust Tree will have shallow furrows and rough when young, changing to deep furrows that runs at strange angles and forks. Black Locust bark on mature trees is generally identifiable from a distance, as it is quite unique.

The bark along branches is brown with white spots called lenticels, and new growth will be red in color. As the branch grows and matures, it will become more rough and furrowed.

Thorns

Thorns occur along the smaller branches and twigs which are about 1/2″ long, and occur in pairs. These thorns are quite sharp, and can cause minor injury if you grab a branch or sapling.

Why Black Locust Trees have thorns

The thorns of a Black Locust Tree are a product of evolution, and act as a deterrent to mammals browsing the foliage. Deer and rabbits are less likely to eat the young branches and trees if they have to contend with sharp thorns.

Leaves

Black Locust Tree leaves are compound alternate leaves in an overall arrangement that is approximately 6-18″ long by ~4″ wide. The individual leaflets are about 1-2″ long by roughly half as wide, with a rounded shape. Leaves will have a light green color early, changing to blueish-green color when mature.

In Autumn, the leaves of the Black Locust will turn yellow. In my yard I don’t bother raking leaves, as the leaflets are quite small. They do not smother the grass.

Flowers

In late Spring on Black Locust Trees, groups of white flowers will form and hang down from the branches in beautiful arrangements. The overall cluster will be 4-7″ long, and look like dangling cluster of pea-like flowers that are very fragrant. An individual flower is approximately 1″ long with 5 petals that are white in color. Blooming will last for about 2 weeks.

Seed Production

The flowers will give way to seedpods that are approximately 2-4″ long by 1/2″ wide and hang down much like the preceding flowers. Each pod will contain roughly 2-12 seeds that are hard, 1/4″ diameter, and have a rounded shape but very flat.

Pods will begin to ripen in September and split open, dispersing the seed. Pods also can travel a bit by being blown off in wind storms, where they can also open. I have Black Locust trees near my property and often find the occasional pod dozens to maybe 50 yards (m) away from any tree.

Root System

On average the root system of Black Locust Trees is wide spreading shallow lateral roots that can run 50′ or more from a tree. But in drier areas the Black Locust Tree can produce deep vertical roots that reach 20-25′ deep into the ground. [1]

But I have personally found Black Locust roots that ran 50-75′ into my lawn from the Black Locust trees that surround my property. So, in typical temperate areas, you should expect shallow roots that extend well beyond the canopy.

Growth rate

The growth rate of the Black Locust tree typically ranges from 2-4′ per year depending on conditions. The fastest growth rate will occur in full sun and well-draining soil.^[1]

How to identify a Black Locust Tree in Winter

Mature Black Locust trees are not difficult to identify in winter due to their irregular shape, distinct bark, small isolated thorns. In Winter it is also likely that you can find seed pods near the drip-line of the tree. Also, Black Locust trees do not survive full shade conditions, the crown or side of the tree should have sun exposure.

Grow and care for Black Locust Trees

How to care for

Black Locust trees do not need special care once established. They have proven themselves to be adaptable to a wide variety of environments and climates. If you plant a Black Locust Tree in a location they prefer, basically no care will be required.

Sunlight Requirements

The sunlight requirements for Black Locust Trees is what is known as full sun, which is at least six hours of direct sunlight per day. Black Locust Trees are intolerant of shade.

In general you will not find Black Locust Trees within mature forests, but only on the perimeter or edge. This is because most other deciduous trees will grow taller than Black Locust, and thus shade them out. Oak, Maple, Pine will all out-compete Black Locust, so if it cannot get the sunlight it requires, it will die. [1]

Moisture Requirements

Black Locust Trees prefer moist to medium moist soil that drains well. In general, Black Locust Trees do not like dry or drought prone locations.

If you are unsure about your soil, you can learn to test your soil drainage here.

Soil Requirements

Black Locust Trees are very adaptable in that they can grow in a wide variety of soils as long as they drain well. But in general sand-loam or silt-loam soils will be better than clay for Black Locust Trees.

We have guides to teach you how to check what type of soil you have by hand, and a more precise method using a jar here.

Maintenance

As a general rule Black Locust Trees do not require special maintenance. The Autumn Leaves are small enough where they don’t even need to be raked up, as they are too small to smother the grass. You may wish to prune your tree though, but just make sure you do so when insects are not active, such as Winter.

But, eventually the lateral roots will begin producing saplings as off-shoots from the root. These saplings will need to be mowed periodically if they are not wanted.

How to grow Black Locust Tree from seed

Black Locust Trees are easy to grow from seed. There are no cold-stratification requirements, but seeds will need to be scarified in order to allow for germination.

Scarify Black Locust Tree seeds by rubbing them on sandpaper, or using a hot water treatment. [10] In my own experiments I have found sandpaper to be more effective. But I will relay both methods here:

Scarification by sandpaper

For sandpaper, rub the seed over the sandpaper until you notice a white spot on the seed. This white spot indicates that the hard outer shell of the seed has been worn away by the sand paper. Soak the now-scarified seed for 24 hours to reduce germination time.

Scarification by hot water

To perform this method you will need a small container, water, and some seeds. Bring a small amount of water to a boil on a stove. Then, remove the water from the heat and wait about 30 seconds. Then, pour this water into a coffee cup with the seeds inside. Allow the seeds to soak for 24 hours.

If at the end of 24 hours you can’t noticeably see that the seeds are larger, this means that they probably did not imbibe water. You should repeat the hot water soak again.

Planting Black Locust Tree Seeds

Fill a large container with moist potting soil. Then, scarify the seeds and soak them for 24 hours. Plant the now-scarified seeds approximately 1/4″ deep. If the seeds were properly scarified, you can expect germination within a few days.

Raise the seedlings until they develop several sets of true leaves, then transplant them to their final location. The young seedling/saplings should be protected from deer and rabbits.

Wildlife, Pests, and diseases of Black Locust Trees

Fauna Associations of Black Locust Tree

Insects

The Black Locust Tree is ecologically valuable as it feeds over 70 species of insects including beetles, borers, galls, walkingstick, leaf feeders, sucking insects, seed beetles, and over 15 species of moth. Some of these insects are quite damaging such as the Locust Borer and the Locust Twig Borer. [11]

Pollinators

The flowers of Black Locust Tree are mainly pollinated by bumblebees. However, Hummingbirds, butterflies, honeybees, and moths have been documented to visit the flowers. [12] Although the butterflies and moths are not effective at pollinating, just nectar feeding.

Black Locust Trees host several moth species including the Black-Spotted Prominent, Black Zale, Honest Pero, Locust Underwing, Locust Leafrolle, and the Orange Wing.

Mammals

Black Locust Trees and their seeds are toxic to most mammals. However, Deer will occasionally browse the foliage and eat saplings. Squirrels and chipmunks will sometimes eat the seeds as well.

The dense thickets of Black Locust that often form in abandoned areas provide excellent cover for Deer, Coyote, Fox, Skunk, game birds and other woodland mammals.

Birds

The seeds of the Black Locust Tree are a source of food for larger birds such as Bob-White Quail, Turkey, and other game birds. Additionally, the heart rot fungus, while damaging to the tree makes it an excellent habitat for woodpeckers to build cavity nests, such as the Pileated, Flicker, and Yellow or Red-bellied woodpeckers. [1] [13]

Black Locust Trees are toxic to dogs, cats, and horses

The bark, seed pods, and leaves of Black Locust Trees are toxic to dogs, cats, and horses. [14] Symptoms include depression, weakness, vomiting, and respiratory problems. The poisoning will damage the kidneys and liver. [15] If you suspect your pet has consumed any part of the Black Locust Tree, you should seek veterinary care immediately.

Pests

There are two frequent insect pests that can negatively effect the Black Locust and it’s potential lumber value – the Locust Borer and Locust Twig Borer. But there are numerous other insects that harm the tree such as leaf miners, suckers, and various boring insects.

Locust Borer

The Locust Borer is a type of longhorn beetle native to North America that exclusively damage Black Locust Trees. The adults are roughly 3/4″ long, yellow-black in color with a sort of zig-zag stripe pattern on their wings, and straight line horizontal stripes on their head. [16]

Lifecycle

Adults are visible from August through October where they frequent Goldenrod flowers during the morning. From afternoon to early evening they can be found laying eggs on Black Locust Tree trunks inside the deep valleys of bark, on or near wounds. [17]

These eggs will hatch and burrow into the bark to hibernate during the winter. When trees begin to leaf out in Spring, you can see sap oozing from the entry holes. In Spring and summer, it will tunnel throughout the tree eventually exiting as an adult in late Summer, completing the cycle.

Like most pests and diseases, trees grown on poor sites are more susceptible to attack and damage. It has been noted that the Locust Borer does play an important role in the succession of forests within Appalachia. [18] [19]

Locust Twig Borer

The Locust Twig Borer (Ecdytolopha insiticiana) is a small moth with ~1″ wing span whose larvae will bore into the center of Black Locust twigs, causing damage and twig death. They can be particularly damaging to young saplings. [20]

One day several years ago I was removing some unwanted Black Locust seedlings. I pulled one, and accidentally snapped it off, reveling a Locust Twig Borer larvae inside.

Fungus

Black Locust is frequently attacked by several fungi while alive that severely rot the heart wood, making lumber worthless. These fungi all gain entry to the tree from wounds, most frequently caused by the Locust Borer and Locust Twig Borer.

There are numerous fungal agents, but three will be presented here that are the most frequent.

Black Locust and Heart Rot Fungus

Heart-Rot Fungus Phellinus rimosus is a fungus that resembles a semi-circle shelf that is generally 2-10 inches diameter and 1-2″ thick. Typically the upper surface is brown to black in color, and the margin will bear spores and be yellowish brown in color to brown. Like most tree fungi, it gains entry to the tree from wounds. [21]

Another heart-rot fungus that effects Black Locust, Perenniporia robiniophila is a white-shelf like fungus appearing near the base of trees. It’s scientific name robiniophila means ‘locust loving’ but will also effect Hackberry. Like other fungi, it will gain entry from wounds caused by insects. [22]

Perenniporia fraxinea is an inconspicuous white-rot fungus that will severely damage the heart wood, making a tree not mechanically stable. It attacks the base of stems and the trunk base, and while visible fungus/mushroom is present and visible, it is a dark gray or brown color rendering it inconspicuous. Excessive nitrogen in the soil or from pet urine may accelerate the growth, as research indicates the fungus feeds from high nitrogen. [23]

Aggressive or invasive nature of Black Locust Trees

Black Locust is aggressive or invasive in many sites that it colonizes. As a pioneer species, it is quick to establish itself via it’s rapid growth rate when a sapling, out competing other species.

How a Black Locust Tree spreads

As a general rule, Black Locust Trees will primarily spread via suckering roots that are sent out from the mother tree. These roots sprout new seedlings that rapidly grow, and eventually a dense thicket is formed. This primarily happens in open fields. By year six Black Locust Trees can also produce seeds, furthering it’s spread.

Where Black Locust can be aggressive

Black Locust Trees are most likely to become aggressive in open areas with full sun, where they can form dense thickets from suckering roots. They are not a threat to established forests, as they will not survive in shade of taller trees.

However, once established as the dominant tree, Black Locust will remain in areas outside of it’s native range. Research in Europe has found that other, taller species of tree will not out-compete already established Black Locust forests. This is not an issue in North America, as the natural pests of the tree (Locust Borer, Heart Rot) will naturally thin Black Locust thickets. [24]

How to Control Black Locust Trees

There are two effective methods for controlling Black Locust Trees stop them from spreading in invaded areas. The first, and most environmentally friendly method is a combination of mechanical and herbicide application. The second is strong herbicide application to the basal bark.

But in general, a living Black Locust Tree will continually send out suckering roots that sprout new trees. Additionally, there is some spread from seed dispersal in the fall. The only way to truly stop a Black Locust Tree from spreading is to kill it, or plant it where it’s roots can not expand.

Herbicide Control

As a note for the following two subheadings – were PPE if using herbicide (long clothes, rubber gloves, goggles), only work on calm days, and make sure you follow all directions on the label!

Mechanical / Herbicide Control of Black Locust Trees

The most environmentally friendly method of killing any tree is to cut and paint the stump. You use a chainsaw to cut the tree down, and then immediately apply a coat of 20% Glysophate or Garlon 3A (triclopyr formulation). In this way the herbicide only contacts the tree you wish to kill. [25]

I’ve written a detailed guide on this method here. I have personally killed many invasive trees and shrubs using this method. It is effective, and just about the most environmentally friendly method to kill a tree or shrub that re-sprouts from roots.

Basal Bark Herbicide Application

Successful herbicide application on basal bark has been successfully used for many years on smaller trees with thinner bark. To kill Black Locusts chemically, Garlon 4 (trilopyr mixture) and mineral oil (1:5 ratio) and using a pump sprayer and spray from the base of the tree to a height of 15″ (0.5 m). [25]

When spraying, apply enough herbicide until it begins to run off to the ground. After application, walk backwards from the tree to ensure you don’t pick up herbicide on clothing.

Mechanical Control

Since Black Locust Trees primarily spread via suckering roots, and have the ability to re-sprout, mechanical removal methods are often not effective. If a single mature tree is present, the trees will resprout from the roots. However, if an area contains only small trees, then several years of mowing or goat grazing can be effective control.

Control of Black Locust seedlings in residential yards

If you have Black Locust Trees that border your property you will likely have multiple trees sprout up in the middle of your yard each year. My own yard is surrounded by Black Locust Trees on two sides, and when we moved in late 2016 trees sprouting up were a problem.

Regular mowing of your yard will kill any seedlings that sprout up. But I found in times of drought, the tree seedlings will grow while the grass goes dormant. This gives the appearance of, well, small trees growing in your yard.

Removing the suckering roots

I was able to drastically slow the spread of Black Locust Trees in my yard by removing the suckering roots from the Black Locust Trees in Autumn. This was much easier than it sounds. To get rid of the roots, or at least the majority of the root, you need to locate a sapling, use a shovel to expose the root, and start pulling.

It is important to do this in Fall, when grass roots are not as strong, and the ground is generally moist. But locate the root and pull. Follow the root until you can pull no more, then cut it with a pruning saw or spade. You will create a trench, but be able to remove huge amounts of root material.

Doing this greatly reduced the amount of saplings/seedlings that would sprout in my yard. I estimate that some of the roots I removed were over 50′ long. Also of note, the trees that produced these roots were no more than 17 years old at the time of me pulling up the roots.

Uses of Black Locust Trees

Fence Posts

Famous for being rot-resistant, Black Locust Trees have long been sought after and used by farmers for fence posts. There are anecdotes of fence posts lasting for upwards of 80 years. There is a saying that your fence will rust away before the Black Locust fence-post rots.

Black Locust Wood

With a Janka Hardness of 1700lbf, Black Locust Wood is one of the hardest commercially available lumbers in North America, rivaling Hickory and second only to Osage Orange. This combined with it’s rot resistance make it particularly suited to certain applications. Additionally, Black Locust Lumber is very beautiful honey colored wood. [26] [27]

Although dimensional lumber availability is somewhat limited, there are several companies that offer it as decking. Frequently small lumber mills have 4×4 fence posts and live edge slabs available. Hobbyist woodworkers can often find value in the appropriate piece of firewood, making tool handles or mallets.

Related ==> Learn to make your own DIY mallet here

Erosion Control

The Black Locust Tree’s rapid growth rate as a sapling combined with it’s propensity to grow in almost any well-drained soil condition make it an excellent tree to stabilize soil banks. It can be utilized on roadsides, slopes, or other disturbed areas.

Land Reclamation

Black Locust Trees have been used extensively in Appalachia, and worldwide for land reclamation at abandoned coal mines and similar sites. Their ability to tolerate poor soil conditions and a wide pH range make it possible to return desolate lands to nature, bringing homes to a variety of insects and wildlife. [5]

Honey

Black Locust Flowers are reputed to produce some of the finest tasting honey in the world. Although the blossoms only last for 1-2 weeks, which will keep that honey in short supply!

Native American Uses of Black Locust Trees

There are 10 uses of Black Locust Trees documented by several tribes, but mainly the Cherokee. These uses included chewing the bark as emetic, crushed root used to treat toothaches, and the bark was also used as a tea (even though toxic). [28]

In addition to several medicinal uses, the Black Locust Tree was used by the Cherokee as a building material for making bows, blowgun darts, fence posts/sills. This further shows the durability of the Black Locust was known for hundreds if not thousands of years.

Value of Black Locust Trees

Depending on the quality quantity of the tree, Black Locust Trees can be worth money. Black Locust Trees can be of value for fence posts and lumber, if the tree isn’t effected by Heart Rot fungus. You can frequently find local and amateur sawmills selling 4×4 locust posts, as well as rough split fence posts.

Read More About Native Trees Here