Honey locust trees (Gleditsia triacanthos) rank among the most widely planted urban shade trees in North America, prized for their light, filtered canopy and tolerance of poor soils. That popularity, however, comes with a well-documented set of headaches. Fungal canker, several aggressive insect pests, unusual sensitivity to common herbicides, and structural weak points all show up regularly in landscape and street plantings. Understanding which problems are cosmetic annoyances and which threaten the tree’s survival can save you years of frustration.
Canker Disease
The most serious fungal problem honey locusts face is canker caused by the mold Thyronectria austroamericana, a member of the Nectriaceae family and a well-known pathogen of Gleditsia triacanthos. The infection produces sunken, dead areas of bark on the trunk and major limbs. As it progresses, you may notice branch die-back, reduced or yellowing foliage, premature fall coloration, and early leaf drop well before neighboring trees change color.1PubMed Central. A Canker Barking at the Wrong Knee: Thyronectria austroamericana Septic Arthritis – Section: DISCUSSION
What makes canker especially frustrating is its relationship to stress. Trees already weakened by drought, compacted soil, root damage from construction, or repeated defoliation from insects are far more vulnerable. Once the fungus colonizes the bark, it restricts the flow of water and nutrients through the affected branch or trunk section. If the canker girdles a major limb, that limb dies. If it girdles the trunk, the tree dies. There is no reliable chemical cure once infection is established; management revolves around pruning out cankered wood well below the visible margin of dead bark and improving the tree’s overall vigor through proper watering and mulching.
Early detection matters. Watch for any area of bark that looks sunken or discolored compared to the surrounding tissue, especially on the trunk or the underside of major scaffold branches where moisture lingers. Peeling back the bark over a suspected canker usually reveals dead, brown cambium beneath. Prune infected branches during dry weather to minimize the chance of spreading spores, and sterilize your pruning tools between cuts.
Insect Pests
Honey locusts attract a handful of insect pests that can defoliate a tree or distort its growth. The most commonly encountered include the mimosa webworm, the honeylocust plant bug, spider mites, and the honey locust pod gall midge.
The mimosa webworm is perhaps the most visible. Larvae spin gray-brown webs that enclose clusters of leaflets, feeding inside the protective silk and skeletonizing the foliage. A heavy infestation turns the canopy into an unsightly mess of webbing and brown, curled leaves by midsummer. While a single season of defoliation rarely kills a mature tree, repeated years of heavy webworm pressure weaken it and open the door to secondary problems like canker.
The honeylocust plant bug feeds on expanding leaflets in spring, puncturing cells and causing stippled, distorted leaves that often drop early. Because the damage happens when the foliage is young and tender, the tree may push a second flush of growth, but that new growth is smaller and less vigorous. Spider mites cause a similar stippled appearance later in the season, especially during hot, dry stretches when their populations explode. Affected leaves turn bronze or yellow and drop prematurely.
The honey locust pod gall midge is a tiny fly whose larvae feed inside developing leaflets, causing them to fold and form small, pod-like galls. Heavy infestations distort large portions of the canopy. Research into chemical control of this pest has been conducted at sites like the University of Idaho campus, where insecticides were tested for both efficacy against the midge and potential phytotoxicity to the tree.2Insecticide and Acaricide Tests. Honey Locust POD Gall Midge Control and Phytotoxicity Evaluations of Two Insecticides, 1979 – Section: Abstract The gall midge is primarily a cosmetic issue, but on specimen trees or street plantings where appearance matters, it can be a persistent headache.
Managing Insect Problems Without Overreacting
A common mistake with honey locust pest management is treating too aggressively. Broad-spectrum insecticide applications kill not only the target pest but also the natural predators that keep spider mites and plant bugs in check. This can create a rebound effect where secondary pests flare up worse than the original problem. If you spray for webworm in June and wipe out predatory mites at the same time, you may face a far worse spider mite outbreak in August than you would have had otherwise.
For mimosa webworm, targeted treatments with Bacillus thuringiensis (Bt) applied when larvae are young and actively feeding can reduce populations without hammering beneficial insects. For spider mites, a strong blast of water from a hose knocks populations down on smaller trees, and horticultural oil applied during the dormant season can reduce overwintering eggs. The pod gall midge is harder to control because larvae are protected inside the galled tissue, so timing matters: any treatment needs to target the adult flies before they lay eggs, which requires monitoring for the brief window when adults are active in spring.
On larger, established trees, tolerating moderate insect damage is often the smartest approach. A healthy honey locust can lose a significant portion of its leaf area and recover without lasting harm. The stress becomes problematic only when defoliation happens repeatedly, year after year, or when the tree is already compromised by drought, poor soil, or root disturbance.
Herbicide Sensitivity
Honey locusts have a surprisingly low tolerance for certain herbicides that are routinely applied to lawns and rights-of-way near street trees. This catches many property owners and municipal crews off guard, because other common street trees handle the same chemicals without complaint. A multi-year field study found that honey locust developed moderate to severe injury from aminocyclopyrachlor, a broadleaf herbicide, when applications were made within about 7 meters (23 feet) of the trunk. Green ash trees exposed to the same treatments showed high tolerance, while the honey locusts displayed leaf curling, stunting, and branch die-back. Recovery over time was minimal, and the cosmetic damage would not be acceptable to most landowners.3Journal of Environmental Horticulture. Field Response of Green Ash (Fraxinus pennsylvanica) and Honey Locust (Gleditsia triacanthos) to Aminocyclopyrachlor – Section: Abstract
Timing played a role: fall applications in October and November caused the least injury compared to treatments applied during the growing season, presumably because the tree’s root uptake slows as it enters dormancy. But even with fall timing, the injury was not eliminated. The practical takeaway is straightforward. If you have honey locust trees in your yard or along your street, you need to know what herbicides your lawn care company or municipal weed-control crew is using, and you need a buffer zone. Aminocyclopyrachlor is not the only problem chemical; honey locusts can also be sensitive to other growth-regulator herbicides in the same broad class. When in doubt, request that applicators avoid treating within the drip line of the tree.
Symptoms of herbicide injury on honey locust can mimic other problems. Leaf curling, cupping, elongated or strap-shaped leaflets, and stunted new growth may look like a nutrient deficiency or viral infection. The giveaway is often the pattern: if damage appears first on the side of the tree closest to a recently treated lawn or sidewalk strip, herbicide drift or root uptake is a strong suspect. Damage that appears uniformly across the canopy is more likely disease or an environmental stress.
Structural Weaknesses and Codominant Stems
Honey locusts have a growth habit that frequently produces codominant stems, where two or more main leaders grow at nearly the same size from the same point on the trunk rather than a single dominant trunk with subordinate side branches. This creates a structural weak point. The narrow angle where codominant stems meet traps bark between them (called included bark), and that bark acts as a wedge pushing the stems apart over time rather than fusing them together. The result is a junction far more prone to splitting, especially under wind load, ice, or the weight of a full canopy.
Research using digital image correlation to measure how strain propagates through branch connections has found a close relationship between the aspect ratio of a junction and how it fails. From a mechanical perspective, a stem becomes codominant at an aspect ratio of around 0.8, meaning the side branch is at least 80 percent the diameter of the main stem.4Semantic Scholar. Examining Strain Propagation in the Lateral and Codominant Branch Attachment Using Digital Image Correlation At that point, the attachment behaves less like a branch on a trunk and more like two trunks fighting for the same space.
For homeowners, this means structural pruning when a honey locust is young pays enormous dividends. Selecting a single central leader and reducing or removing competing stems while the tree is small creates a much stronger framework. Once a codominant junction reaches six or eight inches in diameter, corrective pruning becomes difficult or impossible without leaving a massive wound. At that stage, cabling or bracing hardware installed by a certified arborist can reduce the risk of catastrophic failure, but the hardware needs periodic inspection and eventual replacement. If you are planting a new honey locust, look for nursery stock that has been trained to a single leader rather than the cheapest multi-stemmed specimen in the row.
Thorns on Seedling and Wild Trees
The species name triacanthos literally means “three-thorned,” and wild-type honey locusts live up to it. The trunk and major branches can be covered in clusters of stout, branching thorns that reach three to four inches long. These thorns are a genuine hazard: they can puncture shoes, tires, and skin. Fallen thorn clusters on the ground around the tree are particularly treacherous for anyone walking barefoot or for pets.
Nearly all honey locusts sold for landscape use are the thornless variety, Gleditsia triacanthos var. inermis, or cultivars derived from it, such as ‘Shademaster,’ ‘Skyline,’ and ‘Sunburst.’ These produce no thorns on above-ground growth. However, two situations still create thorn problems. First, if a thornless cultivar is grafted onto a thorned rootstock (which some nurseries do to take advantage of the rootstock’s vigor), suckers that sprout from below the graft union will carry thorns. If those suckers are not promptly removed, you end up with thorny growth emerging from the base while the upper canopy remains thornless. Second, volunteer seedlings from a thornless tree may revert to the thorned type, because the thornless trait can be carried as a single gene that segregates in the next generation. Any honey locust seedling growing uninvited in your yard should be assumed thorny until proven otherwise.
Leaf and Pod Litter
Honey locusts are deciduous, and their tiny compound leaflets create a distinctive litter issue. On the positive side, the leaflets are small enough that they often sift down through lawn grass and decompose quickly without needing raking. This is one of the reasons the tree is so popular for residential shade. But the flipside is that those tiny leaflets also blow into gutters, pool filters, and window wells where they form a compacted mat that is difficult to clean out.
The bigger litter nuisance comes from the long, twisted seed pods that some cultivars produce in abundance. The pods are thick, leathery, and up to 18 inches long. They drop in fall and persist on the ground through winter, becoming slippery when wet and attracting insects as the sweet pulp inside ferments. Pod-bearing trees planted near sidewalks, patios, or driveways create a sustained cleanup chore from October through spring. Many of the popular thornless cultivars are also marketed as “podless” or low-pod, but truly pod-free performance depends on whether another honey locust (including a wild one) is close enough to provide pollen. A cultivar sold as podless can produce a full crop of pods if it gets cross-pollinated.
Iron Chlorosis and Alkaline Soils
Honey locusts are generally tolerant of a wide range of soils, which is part of their appeal for urban planting. But in areas with strongly alkaline soil (pH above about 7.5), they can develop interveinal chlorosis, where the tissue between the leaf veins turns yellow while the veins themselves stay green. This is typically an iron-availability problem: the iron is present in the soil but chemically locked up in a form the roots cannot absorb at high pH.
Mild chlorosis is cosmetic and does not seriously harm the tree. Severe, chronic chlorosis reduces the tree’s ability to photosynthesize, weakens it over time, and makes it more vulnerable to pests and diseases. Soil amendments like elemental sulfur can lower pH in the root zone, but the effect is slow and temporary unless repeated. Trunk injections of chelated iron give a quicker green-up but need to be repeated every year or two and create wounds that invite decay. In regions where alkaline soils are the norm, choosing a honey locust cultivar with known tolerance to high pH is a better long-term strategy than trying to chemically modify the soil after planting.
When Stress Compounds
The most important thing to understand about honey locust problems is that they rarely arrive alone. A tree weakened by drought becomes more attractive to spider mites. Spider mite defoliation stresses the tree further, making it more susceptible to canker. A canker infection reduces the tree’s ability to recover from the next round of insect feeding. Herbicide damage layered on top of any of these stresses can push a tree past the point of recovery.
This compounding effect explains why some honey locusts seem to decline suddenly after years of apparent health. The tree was absorbing low-level stresses without visible symptoms until one additional insult tipped the balance. Municipal arborists sometimes call this “urban tree decline,” and honey locust is one of the species most prone to it because it is so often planted in challenging sites: narrow parking strips, compacted clay, reflected heat from pavement, and restricted root zones.
If you are managing a honey locust that has started showing signs of trouble, prioritize water and root-zone health first. Mulch out to the drip line (keeping mulch a few inches away from the trunk), water deeply during dry spells, and protect roots from compaction and construction damage. A tree with a functioning root system can tolerate a surprising amount of above-ground adversity. A tree with compromised roots cannot recover from any of it.