Black locust (Robinia pseudoacacia) is a tree that is both hated and loved. On one side of the fence, you will find people who say that black locust is a horrible invasive species. On the other, folks will say black locust is a miracle tree with endless uses and ecological services. In the middle, you will find black locust itself, as a post holding up the fence.
Originally black locust was native to central Appalachia and the Ozark Mountains. It has spread from these locations and is now found naturalized in every continental state, in several Canadian provinces, and in parts of Europe. Locust is an opportunist. The trees will find a home where destruction and disturbance occur. They love abandoned gravel pits, vacant lots, roadsides, and transitioning fields. Locusts grow fast and will spread into any well-drained sunny location with shocking speed. They spread by root runners, occasionally extending 20 feet in a single season. Give black locust some space and it will create a beautiful grove in the harshest of environments, or in the most fertile soil.
Black and Honey Locust
Black locust is not honey locust. The trees are very different. Honey locust is commonly planted as a male, thornless clone in yards and parks. It’s in a different genus from black—the genus Gleditsia, of which there are several members. Honey locusts spread by seed and only a little by root suckers—nothing like the way black locust does. The thorns on honey locust are big enough to kill a person; they can easily be 6 inches long, with branches of more thorns upon them. On most wild trees, the thorns of honey locust cover the trunk completely. I believe this may be a defense they developed to keep giant sloths from climbing them to reach their giant sweet pods. Which is yet another difference between Gleditsia and Robinia: Honey locust pods are around a foot long; black locust pods are just a few inches. Black locust thorns are also less than an inch long. The flowers of the two species are different. The wood is also different: Black locust has dark yellow wood, while honey locust is pink. They are different trees.
Growth
Black locust is an incredibly fast-growing tree. It is able to form relationships in the soil with certain bacteria that allow it to fix nitrogen right out of the atmosphere. This ability allows locust trees to grow in very poor soils, so long as they are out in the sunshine and the ground is not waterlogged. They can thrive in pure sand or on the most eroded clay hillside.
I have seen black locust trees grow 6 feet in their first year of life (though 2 to 3 feet is more typical for starting from seed). Established locusts that are cut down can put on as much as 10 feet of regrowth the following year. Within seven years black locust can be harvested for small-diameter firewood. Within a 20-year span, they are often big enough for lumber.
This is also a tree that will spread and form large colonies. Rarely do we see a single black locust trunk by itself. They send out runners and sprout up endlessly until they reach shade or a barrier (road, water, lawn, et cetera). Many black locust colonies are several acres large. They spread as vigorously into an open field as quaking aspens. Once they get established in a field they will be very competitive, even under heavy browse conditions.
Ecological Niche
Black locusts will stampede out into a field or old gravel pit, or anywhere that things have been opened up for them. They are a pioneer species that will not become established in a forest. Locusts are opportunistic, looking for places where the canopy has been cleared.
Black locust casts a very light shade. The leaves are made up of small round leaflets that allow a tremendous amount of light to pass through. The shade created by black locusts is so weak that undergrowth is always rampant underneath them. Most stands of black locust are tangles of honeysuckle and multiflora rose. Where exotic shrubs do not dominate the understory, hardwood tree seedlings find an excellent place to become established. The light shade of locust offers protection, while they improve the soil through their nitrogen fixation and easily compostable leaf litter.
Black locusts are short-lived trees. Because of their shallow root system, they typically start falling over by the time they reach 60-plus years of age. By this time an abundance of hardwood seedlings have become established in the understory. The most common tree to become established in a locust grove in the Northeast is sugar maple. Locust-maple woods paint a very clear picture of an abandoned pasture that evolved into a grove of locust and then the maple. Sugar maples have a much harder time establishing themselves in an open field.
Seed production of black locust begins early and can be heavy. The trees produce pea-shaped pods containing a row of small, hard seeds. These edible seeds, with their hard seed coat, can remain dormant in the soil for decades. Perhaps they are waiting for the next forest disturbance to sprout again. The seedpods flutter off in the wind, but they do not travel very far at all. Many trees and plants that rely on disturbed environments will make long-lived seeds that lie dormant until conditions are right. For example, lotus seeds can remain dormant for thousands of years—some have been sprouted from Egyptian tombs. I don’t know how long black locust seeds can last, but I bet it is at least several decades.
Flowering
Strings of white and lavender pea-like flowers hang over the trees as if they are covered in robes of blossoms in late spring. Stands of black locust can be seen from very far away during this time of year. Driving down roads through urban and suburban areas, we can suddenly see just how abundant the locusts are: Bright white trees lie around every curve. The flowers are edible and delicious. I think they taste just like sugar snap peas. They are a special treat for only about a week every year. They have to be picked fresh for best flavor.
Honeybees as well as many other pollinating insects will flock to these fragrant blossoms in great numbers. Black locust is considered one of the biggest nectar flows of the season by beekeepers.
Several varieties of locust are cultivated for their blossoms and planted as ornamental trees. For the most part these are purple-flowered varieties or trees with interesting forms.
Black locust flowers.
Wood
This is where black locust really stands out and makes a name for itself. Despite its rapid growth rate, locust produces a super-dense wood. It is a world-class hardwood as valuable as most tropical hardwoods and is traded around the Earth as a premium product. Around my neighborhood typical softwood lumber is 50 cents a board foot, while hardwoods like maple and oak can range from $1 to $3, usually. The black locust mill down the road sells lumber for as much as $7 a board foot and cannot keep any in stock because demand is so high.
Locust is one of the most rot-resistant woods on the planet. It is used for boardwalks, bridges, piers, fence posts, furniture, musical instruments, decks, docks, playgrounds, raised beds, and terracing. It is an immensely useful wood, and also incredibly beautiful. Locust wood is easily identified by anyone who is familiar with it. The rings are porous with dark and light alternating bands of deep brown-gold. When it’s first cut, locust is bright yellow, and then it darkens with age and exposure to light. A touch of oil brings out the best in locust.
The rot resistance of black locust is better than that of oak, cedar, and pressure-treated lumber. Pressure-treated lumber is created by infusing wood with decay-resisting chemicals at high pressure. This lumber is widely used for picnic tables, decks, raised beds, and playgrounds. Unfortunately, it is created by using toxic chemicals that leach out of the wood over time. Black locust lasts much longer than pressure-treated lumber and is safe enough for kids to eat off.
The wood of locust is also excellent fuel. It burns hotter and longer than oak, maple, and just about every other hardwood. It is tied with hickory at 26.8 million BTUs per cord. Because these trees grow so rapidly and can be cut over and over, their value as a renewable fuel is significant. The word renewable literally explains consuming locust as a source of heat and energy. The trees are building ultra-dense wood from elements in the sky. Actual soil is not even required, as these nitrogen-fixing trees make their own.
Some Notes on Black Locust Rot Resistance
- The inner heartwood of black locust is rot-resistant, not the outer ring of white sapwood.
- Heart rots destroy the rot resistance of locust.
- Kiln-dried locust has almost no decay resistance compared with air-dried.
- In the ground, posts often last over 50 years.
- Traditionally, the Haudenosaunee (Iroquois) used black locust poles in building their longhouses. They charred the ends that were in contact with the soil to extend the life of the post.
Pests and Diseases
There are some pests of black locust to be aware of, most notably the locust borer, which is the biggest challenge to producing quality timber.
Locust Borer (Megacyllene robiniae)
This native insect can cause considerable damage to locusts. Adults deposit eggs under the bark. The eggs hatch into larvae that tunnel through the wood. These tunnels can weaken stems on younger trees, causing them to snap off in a strong wind. The borer tunnels also lessen the value of lumber. Goldenrod is an alternative host for the locust borer. Open-grown trees suffer more damage than trees with shaded trunks.
Locust borer tunneling was severe enough to cause this tree to fall over. The roots will send up vigorous sprouts in response.
Heart Rot (Phellinus robiniae)
This fungus primarily finds its way into locust trunks by way of tunnels left open by the locust borer.
Locust Leaf Miner (Odontota dorsalis)
This insect eats leaves from the inside out, leaving behind a brown skeletonized leaf. Sometimes the damage is so severe that it can be seen from far distances across valleys. The trees usually recover fine on their own and will leaf back out after an infestation.
Wildlife and Livestock
Black locust supports wildlife populations in several ways. Numerous species of insects feed on its leaves and nectar. The foliage is often heavily browsed by deer, and rabbits will chew on the young bark in winter. Stands of black locust offer excellent cover because of the thick understory that is usually present. I might be in the minority, but I think black locusts add biodiversity. They transform landscapes from piles of bare sand to living biomes teeming with life. Many forests owe their rich soils and species composition to the work of these remarkable trees.
Black locust also has a lot of potential as a supplemental livestock feed. Branches cut in the summer have been fed to cows, sheep, and goats. The leaves are high in nitrogen. For a larger discussion on this use of black locust, check out Steve Gabriel’s book Silvopasture.
Propagation
Black locust is very satisfying to propagate. Most efforts are rewarded with huge trees after one season. Seed and root cuttings are the primary methods.
Seed
The seeds have a very tough coat. This allows them to remain dormant in the soil for several years until conditions are ripe for growth (usually after a massive disturbance). This coat must be weakened for the seeds to sprout. You can either abrade each seed with a file or use a hot-water treatment. To use the hot-water method, bring a pot of water to a boil and then take it off the heat. Let the water cool a few degrees. Drop the seeds into the hot water and let them soak for 12 to 24 hours. They will swell up and look quite different. Plant immediately after this treatment. They usually sprout within a couple of weeks, quicker if the soil is warm.
Black locust seedling nursery bed. These trees were seeds just a few months prior to this picture.
Seedlings are not frost-hardy. One night of light frost can kill newly sprouted locusts. I wait to plant mine until danger of frost has passed and cover them if the need arises.
Seedlings transplant very easily, even if roots are broken when digging. I have successfully transplanted locusts by ripping them out of the ground with a weed-wrench or chopping them out with a mattock. In some cases 10 percent or less of their roots were left intact. They still survived fine, and often put on several feet of growth.
If seedlings are dug out of a spot and some roots are left behind, the leftover roots will often turn into new trees the next season. Several times, I have dug up a locust only to find a ring of new trees around where the roots were severed.
Root Cuttings
This is the easiest method for cloning black locust other than transplanting suckers. To find root cuttings on larger trees, start at the base of the trunk. Find a root flare and follow it as far as you can, digging around the root as you go. The best material is as thick as a finger, but you can use fatter or skinnier roots, too. Two- or 3-inch sections work well. Plant them near the soil surface in spring. They can sometimes take a long time to sprout—often until midsummer. Growth can be phenomenal, reaching 6 to 9 feet the first year.
Arrow-straight black locusts towering above the surrounding forest in Danby, New York.
Varieties
There are many ornamental varieties of black locust selected for their attractive flowers or interesting form. However, there are also many selections of locust that have been made for timber. Most of the timber breeding work has taken place in Hungary, where huge plantations of black locust are grown. They have done such a good job at improving locust’s timber qualities that the trees look completely different. Instead of gnarly, twisted trees, they look like telephone poles.
The USDA has also made some selections of black locust for timber, but these do not compare to the Hungarian trees to date.
Many wild populations of locust have outstanding timber quality. You can find these if you keep an eye out for them. Carl Albers is a friend with a passion for finding trees with excellent qualities. Along with selecting many great nut trees, Carl has found some of the best stands of locust that I have ever seen. Anyone who pays attention can find stands the way he does. As Carl says, “Once your eyes are keyed in, you will find them.” These wild stands of superior trees offer us a path forward toward cultivating this remarkable tree.
Is Black Locust Native or Invasive?
If the definition of invasive is “a plant that is able to naturalize outside its native range,” then yes, black locust is certainly invasive. However, if the definition includes terms like degradation or crowding out of native species, we see a different story. We can look at the ecological effects of black locust rather than just maps of where it grows.
For the most part, invasive plants have negative effects on the environment for two reasons. They crowd out native species, thereby limiting diversity, and they are not fed on by native insects, creating biological dead zones. Black locust fits into neither of these roles.
It does not crowd out other native trees the way Norway maple or tree of heaven does. In fact, black locust actually improves conditions for native hardwoods to grow, in a similar fashion to another pioneer species, quaking aspen.
Black locust leaves are fed upon by a lot of insects; the flowers are used by many native pollinators; even the wood is bored into by the native black locust borer.
Here in upstate New York, black locust hasn’t traveled far to find a home. Other species that are regularly considered native here from the same home range as locust include eastern redbud, Carolina silver bell, pawpaw, persimmon, vernal witch hazel, fringe tree, and sourwood. It does not seem right to list these trees as native and black locust as not if they share the same place of origin. However, that is exactly what has happened. New York State has joined a host of other states that list black locust as an invasive species and limit its propagation and planting. There are groups of well-meaning people who will cut down locusts and paint the stumps with herbicide. This tree has been demonized by native-plant enthusiasts. It grows where native plants struggle, and then improves conditions for them to appear.
Rather than simply looking at the native range of a plant to determine if it is beneficial or harmful, I prefer to look at the actual impact of a species. From what I have seen, black locust is a wonderful tree able to grow in the most abused landscapes. It is useful to wildlife, native-plant communities, and people.
If we are to pursue ecological solutions to the problems we face, then black locust can be a key player. It is an outstanding soil improver, a biomass producer, and a source of nectar and high-quality, renewable timber. Black locust belongs in the hedgerow of every farm that uses fence posts, beams, or firewood. It can be cut again and again. Black locust will never complain about abuse, either to itself or to the land base; it will always respond with rapid growth and curtains of white blossoms. We can label black locust as an invasive plant, or we can recognize it as an ally on our path to a healthier world.
Commercial Possibilities
The commercial potential of black locust is much higher than most people would expect. Letting a field turn into a grove of locust can provide a huge portion of a family’s income. The grove can be put on a rotation, where a certain percentage is clear-cut every year. The clear-cuts will grow back fast and strong, yielding high-quality wood. There is a big demand for locust posts and lumber for several reasons. Many lakes and rivers do not allow for pressure-treated lumber to be used in docks because of chemical contamination. This leaves the options of tropical hardwoods or black locust.
Another reason the demand is high is because organic certification does not allow for fences or trellises to be installed with pressure-treated lumber.
High-density apple orchard trellised with locust posts at Eve’s Cidery. As demand for organic food grows, so does the demand for locust posts. Photo courtesy of Carl Albers.
As understanding of the dangers of pressure-treated wood spreads, so does the demand for black locust. Gardeners want it for their raised beds and tomato stakes; parents want it for picnic tables; schools and parks want it for playgrounds; conservationists want it for boardwalks across wetlands. At this point, there is such a demand for black locust that a supplier can be all but guaranteed to sell every piece of wood. I have even seen even the branches sold as natural pea trellises. In addition to all this, people will pay extra money for black locust firewood.
Nursery stock is another good option for harvesting income. Most state nurseries have stopped selling locust seedlings because of their invasive reputation. At the same time, demand for this tree has skyrocketed. Awareness of this tree’s amazing abilities and offerings are spreading as fast as its roots do. There are few nurseries that do not sell out of black locust seedlings well before spring.
Governments and native-plant purists fight this species. It is banned by law. Trees are cut and the stumps painted with poison to kill the roots. I believe these people are doing what they think is right to support native-plant communities. But if they looked closely at this species in the wild—not in any literature, but just seeing this tree as it is in the world—then a different story would unfold. Black locust supports native-plant communities by healing the scars of industrialization. It creates an environment for other plants to flourish and then it leaves. If left on its own, black locust will shade out its own suckers and seedlings. It will live for six to eight decades and leave behind a black soil filled with the roots of native trees.
Black locust may appear a rampant, selfish species at first glance, but it is really a servant of older ecosystems. This tree freely offers abundant honey, wood, soil, and land reclamation. It is a wonderful gift to an abused world.