Are Locust Trees Invasive?
Yes, some locust trees are invasive, especially black locust (Robinia pseudoacacia). This fast-growing tree spreads aggressively through root suckers and seeds, often outcompeting native plants. Honey locust (Gleditsia triacanthos) is native to parts of North America but can also become weedy in disturbed areas. Understanding the difference between these two common locust species helps you decide whether to plant, manage, or remove them.
Which Locust Trees Are Considered Invasive?
Black locust is widely considered invasive outside its original range in the eastern United States. It has been introduced to nearly every U.S. state and many countries, where it forms dense thickets that crowd out native vegetation. The tree's ability to fix nitrogen actually changes soil chemistry, favoring other non-native weeds.
Honey locust, by contrast, is native to the central and eastern U.S. but has been planted widely as an ornamental. Its thornless, seedless cultivars (like 'Sunburst' and 'Skyline') are well-behaved in landscapes. However, the wild type produces large seed pods and sharp thorns, and it can sprout vigorously after disturbance. While honey locust is rarely labeled invasive, it can naturalize in prairies and open woodlands.
Other locust species like bristly locust (Robinia hispida) and clammy locust (Robinia viscosa) are less common but also have invasive tendencies. Always check with your local extension office before planting any locust.
How Can You Identify Invasive Locust Trees?
Identifying your locust tree is the first step to understanding its invasive potential. Here are the key features to look for:
- Black locust: Dark gray, deeply furrowed bark; compound leaves with 7–19 oval leaflets; paired thorns at leaf bases; white, fragrant flower clusters in late spring; flat brown seed pods (2–4 inches long).
- Honey locust: Smooth, reddish-brown bark that becomes scaly with age; twice-compound leaves with many small leaflets; long, branched thorns (up to 6 inches) on wild types; greenish-yellow flowers; curved, twisted seed pods (6–18 inches long).
- Suckering habit: Both black and honey locust can produce large numbers of root suckers, especially after cutting or injury. A single tree can create a whole colony within a few years.
If you see a locust tree sprouting aggressively along fence lines, roadsides, or in your yard, check for the bark and thorn pattern. Black locust is far more likely to be a problem.
Why Are Black Locust Trees Problematic?
Black locust earns its invasive label for several reasons beyond just spreading quickly.
First, it produces allelopathic chemicals in its leaves, bark, and roots. These compounds suppress the growth of other plants nearby, reducing biodiversity. The fallen leaves create a toxic layer that discourages native seedlings.
Second, its root system is aggressive. Black locust spreads through rhizomes (underground stems) that can extend 50 feet or more from the parent tree. Each sucker can become a new tree, forming dense monocultures.
Third, the wood is brittle and weak. Branches break easily during storms, creating hazards. The tree also has a shallow root system that can damage sidewalks, driveways, and septic systems.
Finally, black locust is a host for the locust borer (Megacyllene robiniae), an insect that weakens the tree further. Infested trees may drop limbs or die prematurely.
How Do Locust Trees Spread?
Understanding the spread mechanisms helps you plan control methods. Locust trees spread in two main ways:
1. Seeds. Both black and honey locust produce abundant seed pods. Black locust pods open in winter, scattering seeds on the ground. Honey locust pods are eaten by deer, cattle, and birds, which distribute the seeds far away. Seeds can remain viable in soil for years.
2. Root suckers. This is the most problematic method. Any damage to the tree — cutting, pruning, or even a broken branch — can trigger sucker growth. Mowing or weed-whacking around locust trees often makes the problem worse by stimulating more suckers.
Common mistakes people make include trying to remove the tree by cutting it down without treating the stump. This almost always results in dozens of new suckers sprouting from the roots.
What Should You Do If You Have Invasive Locust Trees?
If you want to remove invasive locust trees, follow these steps for effective control. Do not simply cut the tree to ground level — that encourages vigorous regrowth.
- Identify the species to confirm it is invasive black locust or a problematic wild honey locust.
- Cut the tree as close to the ground as possible using a chainsaw or handsaw. Remove all branches and the trunk.
- Apply herbicide immediately (within 15 minutes) to the fresh stump. Use a concentrated glyphosate or triclopyr product. Paint it on the outer ring of the stump where the cambium layer is active.
- Monitor for suckers for at least one growing season. Remove any sprouts by hand or spot-treat them with herbicide.
- Dig out small trees and suckers if the soil is loose. For larger stumps, consider grinding them or using a stump removal product.
Best timing is late summer or early fall when the tree is moving sugars to its roots. This makes herbicide applications more effective. Spring cuts often lead to heavy stump sprouting.
What Are Good Alternatives to Invasive Locust Trees?
Instead of planting black locust, consider native trees that provide similar benefits without the invasive risk. The table below compares a few options.
| Alternative Tree | Benefits | Drawbacks |
|---|---|---|
| Eastern redbud (Cercis canadensis) | Pink spring flowers, small size (20–30 ft), native | Short-lived, not a nitrogen fixer |
| Honey locust cultivar (thornless, seedless) | Dappled shade, tough, no messy pods | Not native; some still produce rootsuckers |
| Serviceberry (Amelanchier) | White flowers, edible berries, brilliant fall color | Small tree (15–25 ft), multiple stems |
| Kentucky coffeetree (Gymnocladus dioicus) | Large leaves, tough wood, native | Slow to establish, large seed pods |
| Oaks (Quercus spp.) | Long-lived, supports wildlife | Slow growing, acorns can be messy |
For quick shade and nitrogen fixation in restoration projects, some ecologists use black locust as a nurse tree but plan to remove it later. For home landscapes, it is best to avoid it entirely.
Can You Grow Locust Trees Without Them Becoming Invasive?
It is risky, but possible under certain conditions. Thornless, seedless honey locust cultivars are sterile or produce few viable seeds. They rarely sucker aggressively, making them safer choices for urban yards. Examples include 'Sunburst' (golden foliage), 'Skyline' (upright form), and 'Imperial' (compact size).
Black locust cultivars like 'Purple Robe' and 'Frisia' are often marketed as less invasive, but they still produce suckers and can escape cultivation. Many experts advise against any black locust planting outside its native range.
If you already have a locust tree and want to keep it, regular maintenance is essential. Remove all seed pods before they mature in fall. Cut back suckers as soon as they appear. Keep the tree pruned to reduce weight on weak branches.
How to Remove Locust Tree Suckers and Seedlings
Suckers and seedlings are the main way locust trees spread beyond the parent tree. Here is how to tackle them effectively.
- For young seedlings: Pull them when the soil is moist, ensuring you get the entire root. A weeding tool with a forked end works well for this.
- For established suckers: Cut them off at ground level and apply a small dab of concentrated herbicide to the cut surface. Use a herbicide brush for precision.
- For large clumps: Use a pruning saw to cut the main stems, then treat the stumps. Expect to repeat this process for at least a year.
Never mow over locust sprouts. Mowing only spreads the roots and stimulates more suckers. Instead, dig or treat each sprout individually.
What Tools Help Control Locust Tree Spread?
Having the right tools makes the job easier and safer. Here are the essentials for managing invasive locust trees:
- Chainsaw or handsaw – For cutting larger trunks. A chainsaw saves time on multiple trees.
- Herbicide sprayer – For spot-treating foliage on regrowth. Use a handheld sprayer that delivers a fine mist.
- Stump grinder rental – For removing stumps from large trees, though it is expensive.
- Protective gear – Thorns on locust trees are sharp and can cause infection. Wear thick gloves, long sleeves, and safety glasses.
If you manage frequent suckers, a glyphosate concentrate and a small paintbrush are more economical than sprayers for spot treatments.
Managing Locust Trees Responsibly
Locust trees are not inherently evil — they provide fast shade, erosion control, and nectar for bees. But their invasive potential demands careful management. Always confirm the species before deciding to plant or remove. If you have black locust on your property, take action early to prevent it from spreading to neighboring lands. Use the removal steps outlined here and follow up with persistent sucker control.
For those planning new landscapes, choose native alternatives or sterile honey locust cultivars. By understanding the behavior of locust trees, you can enjoy their benefits without contributing to a larger invasive problem. Stay vigilant, and your landscape will remain healthy and diverse.