Black Locust Tree Guide: Thorns, Wood, Bark, Flowers, Areas (Beware This Tree)

The Black Locust Tree has the richest cultural history of most any native North American tree, dating back further than its European migration in the 1600s. But in recent years, the dangers of the tree have begun to overshadow its past honor.

This deciduous, hardwood tree was integral to U.S. agriculture and economics for many years and was cultivated internationally for the beauty and fragrance of its spring flowers.

However, the Black Locust tree’s leaves, seed pods (beans), flowers and bark are all poisonous, making them dangerous to have around some animals.

Read on to find out why the black locust was so beloved by many and why opinions of this tree have changed over time.

In this article, learn how to identify the Black Locust Tree by its flowers, bark, leaves, thorns and other characteristics that can be dangerous if consumed.

What Is Robinia Pseudoacacia? (False Acacia)

Robinia pseudoacacia²² is the scientific name for what is commonly known as the Black Locust Tree, or false acacia. The Black Locust Tree is a native tree, originally found in the Appalachian Mountains in the eastern United States and in the Ozarks toward central U.S.

Over time, the tree has been distributed and established across the country as well as in several European and Asian countries.^{3,10,15,20,21,22,28,29,36}

This medium-size tree boasts some of the strongest and most durable hardwood of native North American tree species. Historically, the wood was used to make railroad ties, mining timbers, tree nails used in ship-building and fence posts which could withstand the test of time.²

The primary function of Black Locust Trees today is as a soil stabilizer. The Black Locust Tree can survive and thrive in very dry, depleted soils.

Its abundant root system spreads rapidly, making it ideal for erosion control. Additionally, it bears root nodules which are super effective as nitrogen fixers, enriching the surrounding soil.^2,25

Ironically, one of the Black Locust Trees greatest strengths (its roots) is also one of its primary drawbacks. The thick roots and their habit of spreading through root suckers, much like Willow Tree species, can smother out surrounding plant life.²

It has been theorized that the Black Locust Tree could be used effectively to nourish and fertilize foraging plants, stock crops and fruit and nut-bearing trees such as the Black Walnut Tree, Hickory Tree, and even Cherry Tree.²⁵

Note that a Black Locust Tree grown in livestock pastures must be coppiced before fruiting to prevent animals being poisoned by the fruit.⁷

Black Locust Trees are relatively short-lived, topping out at around 90 years, and older tree specimens are often scraggly and unattractive.¹⁵

The Black Locust Tree makes excellent firewood when fully dried.⁷

10 Facts About the Black Locust Tree

Here are ten interesting facts about the Black Locust Tree:

1. The Black Locust Tree was one of the first native North American trees to be cultivated in Europe, dating back to the 1600s when it was brought to France.¹
2. It was a favorite tree in colonial America, particularly with Jamestown settlers.⁶
3. The fragrant, pea-shaped flowers are the only non-toxic part of the tree. The flowers are edible and can be used to make delicious marmalade and fritters.²
4. Much like the white Mulberry Tree, the Black Locust Tree is considered invasive in many areas, particularly in midwestern states.⁸
5. The Black Locust Tree has an orange inner bark which sometimes becomes more apparent as the tree ages.¹⁰
6. The tree must be pruned deliberately and carefully, as Black Locust Trees bleed excessively when pruned in the springtime.²⁸
7. The Black Locust Tree typically sports a pair of short, stout thorns at the base of each leafstalk.
8. The Black Locust Tree is often confused with its distant relative, the Honey Locust Tree.
9. The species is credited with helping the American fleet to victory over the British in the War of 1812, because the strong yet pliant tree nails from the black locust were used in American ships.¹⁹
10. Many towns and historic estates across the country are named for the Black Locust Tree.¹⁹

Other Types of Robinia Trees

The Robinia³⁶ genus is a grouping of several species of flowering plants commonly known as locusts.

Though there are other plant species called locusts, only Robinia³⁶ species are considered true locusts.

Aside from the Black Locust Tree, there are a handful of other Robinia³⁶ trees including the Rose-Acacia or Bristly Locust (Robinia hispida),³¹ the Clammy Locust (Robinia viscosa),³⁷ the New Mexican Locust (Robinia neomexicana),³⁵ and the extinct Robinia zirkelii.³³

Several hybrid species have also been cultivated, such as the Idaho Locust (Robinia x ambigua),³⁴ a cross between the Black Locust and the Clammy Locust Tree.^23,28

Black Locust Tree Identification

Black Locust Tree identification is simpler than many other tree species as the Black Locust Tree has many distinct features.

Locust Plants, in general, are noted for their compound leaves and leguminous fruit.

However, the Black Locust has many more distinguishing qualities of its bark, leaves, fruit, and flowers.

Black Locust Tree Bark

Black Locust bark is noted for its deep furrows and dark color, particularly in larger, older specimens.

In older tree specimens, the orange hue of the inner bark may be apparent.

Young Locust Tree bark may exhibit spines or thorny projections.^5,22,24

Black Locust Leaf

The deciduous leaves of the Black Locust Tree appear late in the spring, displaying an attractive dark blue-green color which fades to yellow in the autumn.

The Locust leaf is compound, with a single leaf having up to 20 oval-shaped leaflets in pairs and a terminal leaflet at the end.

The alternating leaves can grow up to 14 inches long with leaflets up to 2 in. The leaflets have smooth (entire) margins.^22,24

Black Locust Seed Pods (Bean Tree)

The Black Locust Tree fruits in the late summer and fall, when the Black Locust seed pods appear. These flat, brown pods grow up to 4 in. long and contain between 4 and 8 reddish brown seeds.

The seeds ripen late in the fall and are a favorite food of rabbits. The seed pods often hang on throughout the winter after the leaves have fallen.^22,24

Black Locust Flowers

The white flowers of the Black Locust Tree bloom from mid-spring to early summer. The flowers themselves are small (<1 in.) but strongly scented.

Black Locust flowers emit a pleasant aroma that is attractive to many types of bees, such as honey bees and black and white bees. The 5 petals of the flower are fused, creating a lipped or pea-shaped blossom.

These flowers grow in abundance on drooping racemes up to 8 in. long. The flowers of the Black Locust Tree are the only part of the tree which is edible.

These blossoms are used in recipes for fritters and marmalade.^22,28

Honey Locust vs Black Locust

Knowing how to distinguish the Honey Locust vs. Black Locust Trees is a useful skill to have, but in reality, neither of these trees are suitable for residential planting.

Keep reading to find out how to tell these trees apart and why they shouldn’t be added to the backyard anytime soon.

The Honey Locust Tree (Gleditsia triacanthos)³² is not a member of the Robinia³⁶ genus. Rather, it hails from the semi-closely related Gleditsia³⁸ genus, home of the Honey Locusts.

(Image: Jamain³⁹)

The Honey Locust Tree shares many characteristics with the Black Locust Tree, and thus, these two species are often confused.

Some Similarities Between the Two Species Include:

• Their scientific classifications: The Honey Locust and Black Locust Trees are both members of the Fabaceae (bean, pea) family. Thus, the trees are both leguminous.¹⁵
• Their rates of growth: Both are moderate to fast growers, with the Honey Locust somewhat slower.²⁵
• Their native areas: Eastern to Central United States
• Their wood: Both trees have strong and durable hardwood which can have various uses, with black locust wood being slightly superior.^15,25
• Their leaves: The leaves of the two trees are among their most similar features. Black Locust and Honey Locust both have beautiful, compound leaves with many leaflets.
• Their thorns: Although the thorns of the two trees are very different, both species may grow and display thorns.
• Their growth habits: Both trees are notorious for sending up suckers from their root system, allowing the organism to spread and take over when left unchecked.

Notable Differences Between Honey and Black Locust

• Their sizes: The Honey Locust can grow to be significantly larger than the Black Locust Tree and may reach heights exceeding 100 ft.²⁰
• Their crowns: The Black Locust generally develops a narrow, irregular crown while the Honey Locust Tree often has a broad and flatter crown.
• Their leaves: While the layout of the leaves are very similar, Honey Locust leaves are more likely to grow in a bipinnate (twice-compound) arrangement.²⁰
• Their leaflets: The leaflets of the Honey Locust are more narrow and may be subtly toothed.

(Image: Syrio⁴⁰)

• Their bark: The young Honey Locust Tree has smooth bark which transitions into thin, scaled plates with age. It often has a profusion of large, sharp, thorny projections.

In contrast, the bark of the Black Locust Tree rarely has thorns. With age, the bark becomes deeply furrowed.^15,27

• Their thorns: The thorns of the two species are drastically different, and this is explored in more detail in the very next section.
• Their flowers: While the Black Locust displays showy white flowers in large clusters in the spring, the later blooming inflorescence of the Honey Locust is very small by comparison. Honey locust flowers are tiny, greenish-yellow, and not showy at all.

Both flower types emit pleasing aromas, but the orange citrus smell of the Black Locust contrasts sharply with the ambrosia smell of the Honey Locust.^15,20,27

• Their seed pods: Both trees grow leguminous seed pods, but while the Black Locust fruits are typically less than 4” and flat, Honey Locust seed pods can grow up to 18” and often twist into a spiral.²⁰
• Their roots: Unlike many other Locust Plants, leguminous members of the Robinia³⁶ genus, the Black Locust in particular, the Honey Locusts typically lack the root nodules required for nitrogen fixation.²⁵
• Their habitats: These two trees can often be seen in the same environments and locations, but the Honey Locust Tree prefers richer soils while the Black Locust Tree seems to thrive in drier woods and fields.²⁰
• Their toxicity: All parts of the Black Locust Tree, except the flowers, are poisonous and can be very problematic for grazing livestock and wildlife. The Honey Locust, however, is non-toxic and a great tree for animals who usually love sweet legumes.

Honey Locust fruit was also used by Native American cultures as a sweetener.^15,20

Although each of these trees certainly has its merit, neither of them is a good choice for landscaping, for three reasons.

Firstly, the trees’ habits of spreading through vigorous root suckers make these trees invasive in most landscape areas. The competitive and aggressive spreading may smother out other native plant life, particularly in the case of Black Locusts.

Secondly, the trees are thorny. Especially in the case of the Honey Locust, the thorns can be messy, unattractive, and painful.

Finally, both tree species have a relatively short lifespan and are susceptible to various pests.

Note that thornless varieties of both species have been cultivated over time, but the suckering habit can still be problematic.

Black Locust vs Honey Locust Thorns

These two species are even more often confused because they are among the few native North American tree species to grow thorns.

However, careful observation of Black Locust vs. Honey Locust thorns should clarify any remaining uncertainty about the different species.

On the Black Locust Tree, thorns may be inconspicuous and are often found in pairs at the leaf nodes. Thorns (spines) are typically short and stiff and can be quite sharp.

They are rarely found along the main trunk of the tree but are common at the bases of branches and leaves.²⁰

Honey Locust thorns, in contrast, are entirely conspicuous, obvious, and recognizable. The Honey Locust Tree sports long, spindly thorns along its trunk and branches.

The stout thorns are three-pointed and can grow to be several inches long. They often grow in clusters.⁵

Black Locust Tree Pests and Diseases

The Black Locust Tree is praised for the hardiness of the wood and its resistance to rot. The tree can be susceptible to diseases such as canker and powdery mildew, but by far its greatest weaknesses are insect pests.²²

(Image: May_hokkaido⁴¹)

The Black Locust Tree is highly susceptible to the Locust Borer (Megacyllene robiniae)³⁰ which attacks Robinia pseudoacacia²² exclusively.

The Locust Borer is so named because the larvae of this beetle species tunnel through the trunk and branches of the Black Locust Tree, significantly weakening the wood and causing excessive damage to individual branches and the overall health of the tree.

The Locust Borer is often fatal to the Black Locust Tree, rendering the wood virtually unusable.²⁸ Ash Tree in the eastern U.S. are currently experiencing a similar fate from the Emerald Ash Borer.⁹

Another serious insect pest of the Black Locust Tree is the Locust Leaf Miner (Odontota dorsalis).¹⁶

Although the leaf miner will sometimes attack other trees such as the Birch Tree and Oak Tree, it is named for its favorite host tree.

Attacks of this small beetle species are not usually fatal but typically result in brown, scorched-looking foliage.

Invasion of the Black Locust Tree (Michigan, California, Maine)

The Black Locust Tree was originally found in the southern Appalachians and the Ozarks. It grows best in USDA zones 3a through 8b (See the USDA Plant Hardiness Zone Map),²⁹ but has been found to thrive in a variety of soil types.

Overtime, the range of this tree species was extended throughout the continental U.S. for utilitarian reasons (mining, railways, soil reclamation) and internationally for aesthetic reasons, becoming naturalized in many distinct areas.

However, it has since become apparent that the many benefits of Robinia pseudoacacia²² are outweighed by its competitive nature.

The abundant root suckers sent up by the Black Locust Tree are the most troublesome, as they are weedy, aggressive, and make this tree nigh impossible to contain or remove. But the root suckers are not the only issue.

The root system in and of itself is very thick and tends to smother out native flora as fast as the increased nitrogen from the Black Locust can fertilize it.

For these reasons, the Black Locust Tree has been added to the invasive species watch lists or restricted lists in several states across the U.S., particularly in the New England and Midwest regions. It is also considered invasive in many Central European countries, including Austria, Germany, Hungary, and Switzerland.¹⁴

To anyone interested in growing a Black Locust Tree, Michigan, Minnesota, Maine, Connecticut, and California are among some of the states where this tree species is not recommended.^{11,12,13,17,18}

The Black Locust Tree was once treasured for its hardy wood, its beautiful, fragrant flowers, its attractive foliage, and its ability to grow in and replenish depleted soil.

Awareness of the species’ aggressive spreading through root suckers and it’s poisonous nature has made this once cherished tree a species to beware.

By being able to identify the Black Locust tree from other varieties can help prevent unintentional poisoning. 

Frequently Asked Questions About Black Locust Tree