Common hackberry (Celtis occidentalis) is one of the tougher shade trees you can plant, tolerating drought, wind, poor soil, and urban pollution that would stress most species. Yet that reputation for resilience can mask a handful of persistent problems that frustrate homeowners year after year. Nipple galls, witches’ broom, premature leaf drop, internal wood rot, sidewalk-heaving roots, and storm breakage are all part of owning a hackberry. Most of these issues are manageable once you understand what is actually happening inside or on the tree.
Nipple Gall Psyllids
If you have a hackberry, you almost certainly have galls. The tiny, wart-like bumps on the undersides of leaves are created by several species of psyllids, jumping insects related to aphids. The most common culprit is Pachypsylla celtidismamma, the hackberry nipple gall maker. In spring, adult psyllids emerge from leaf litter where they overwintered, fly to expanding hackberry leaves, and lay eggs. The feeding of the developing nymph triggers the leaf to form a hard, enclosed gall around it, where the insect matures in relative safety.
A heavily infested tree can have dozens of galls per leaf, and the sheer number alarms people. Research on hackberry galls in northeastern Kansas found that certain gall types, specifically petiole galls and leaf-lamina spine galls, reduced photosynthesis in infested leaves, likely by disrupting the leaf’s internal light-harvesting machinery rather than closing its stomata. Petiole galls may also interfere with water flow into the leaf blade.1BioOne Complete. Physiological Effects of Four Gall Types on Leaves of Hackberry (Celtis occidentalis) in Northeastern Kansas, U.S.A. The common nipple gall, however, causes relatively little physiological harm to a mature tree. An otherwise healthy hackberry can carry heavy gall loads year after year without declining.
Chemical control exists but is rarely justified on landscape trees. Trials in the early 1990s evaluated systemic insecticides like imidacloprid, applied in late May just before egg-laying, as well as acephate sprays.2Arthropod Management Tests. Control of Gall-Making Psyllids on Hackberry—A two Year Evaluation, 1993-1994 Soil-injected systemic treatments timed to bud break or to the spring flight of the budgall psyllid have also been tested.3Insecticide and Acaricide Tests. Control of Hackberry Psyllids with Soil-Injected Insecticides, Fort Collins, 1987 Timing is critical with any approach because once the gall has formed around the nymph, no spray can reach the insect inside. For most homeowners, the practical advice is to accept the galls. They do not kill the tree, and the psyllids become a bigger indoor nuisance than an outdoor one when adults seek shelter in homes during fall, crawling through window screens and caulking gaps. Sealing entry points around windows and doors does more for your quality of life than treating the tree.
Witches’ Broom
The dense, tangled clusters of twigs scattered through a hackberry’s canopy are called witches’ brooms. They develop when a powdery mildew fungus and an eriophyid mite team up at a bud site. The mite’s feeding combined with fungal infection causes the bud to produce a tight clump of stunted, shortened shoots instead of normal branching. Over years, these brooms grow into dark, bushy masses that are especially visible in winter after the leaves have dropped.
Witches’ brooms are the single most common aesthetic complaint about hackberry trees. A mature tree can carry dozens of them, giving the canopy a cluttered, unkempt look. The good news is that brooms rarely threaten the tree’s health. They divert some energy from normal growth, and heavily broomed branches may be slightly weaker, but the tree keeps growing around them. There is no practical chemical cure. Fungicide applications are not cost-effective on a large shade tree, and the mites are protected inside the bud tissue. The only removal method is pruning, which is straightforward: cut the broomed branch back to a healthy lateral branch or to the branch collar. On a large tree with brooms scattered throughout the crown, a certified arborist with a bucket truck can selectively remove the worst ones and noticeably improve the tree’s winter silhouette. You will not eliminate them permanently since the organisms are endemic in hackberry populations, but periodic pruning keeps them from dominating the canopy.
Leaf Spot and Premature Leaf Drop
Hackberry leaves are prone to several fungal leaf-spot diseases, most of them caused by Cercospora or Phyllosticta species. The symptoms are small brown or tan spots, sometimes with darker borders, that may merge into larger dead patches by midsummer. In wet years, the infection can become heavy enough to trigger early defoliation, with the tree dropping a significant portion of its leaves by August.
Premature leaf drop looks dramatic and worries homeowners, but for the tree it is less dire than it appears. Hackberries leaf out early and have a long growing season, so even a tree that loses half its leaves by late summer has usually stored enough energy to survive winter and flush new growth the following spring. Repeated years of heavy defoliation can weaken a tree over time, but a single bad year is not cause for alarm.
Cultural practices help more than fungicides here. Raking and removing fallen leaves in autumn reduces the fungal spore load available to re-infect the tree the following spring. Thinning the canopy through selective pruning improves air circulation and lets leaves dry faster after rain, which makes infection less likely. Avoid overhead irrigation that wets the foliage. On younger or smaller trees where fungicide application is feasible, a copper-based spray applied at bud break and repeated once or twice during wet weather can reduce leaf spot severity, but spraying a 50-foot mature hackberry is impractical for most homeowners.
Wood Decay and Heart Rot
Hackberry wood is relatively soft compared to oaks or maples, and it is susceptible to internal decay fungi that enter through pruning wounds, storm damage, or natural branch dieback. Heart rot progresses silently inside the trunk or major limbs. By the time you see external signs, such as shelf-like fungal conks growing from the bark, the decay may be extensive.
A particularly aggressive pathogen documented on hackberry species in urban settings is Inonotus rickii, a fungus that causes white heart rot. An assessment of European hackberry (Celtis australis) street trees in Lisbon found that out of 374 trees evaluated, roughly three-quarters showed symptoms of decay caused by this fungus. Many were classified as having a probable or imminent likelihood of structural failure.4Acta Horticulturae. Risk assessment of Celtis australis affected by Inonotus rickii in Lisbon town The same fungus has been detected causing cankers and decay in hackberry and other broadleaf species across southern Europe.5Forest Pathology. Decay and canker caused by Inonotus rickii spreading on more urban tree species While those studies focus on the European species, Celtis occidentalis faces its own suite of wood-decay organisms, and the broader point holds: hackberry wood breaks down relatively easily once fungi gain entry.
Prevention centers on proper pruning. Make clean cuts just outside the branch collar rather than leaving stubs, which die back and become entry points for decay. Avoid topping the tree or making large heading cuts that expose big cross-sections of wood. If you suspect internal decay in a mature tree near a house or walkway, hire an arborist who can perform a resistograph or sonic tomography assessment. These tools map the density of wood inside the trunk without cutting it open. A tree with severe heart rot in the lower trunk is a failure risk and may need removal regardless of how healthy the canopy looks from the ground.
Ice Storm and Wind Damage
Hackberry’s branch architecture makes it more vulnerable to ice loading than you might expect from such a tough species. A study of ice storm damage to urban trees found that common hackberry was among the parkway species sustaining considerable ice damage. Fine branching patterns, structural weakness at attachment points, and higher degrees of lateral branching were all associated with greater damage. The research also noted that wood density alone did not predict which species broke; branch geometry mattered more than raw strength.6Academia.edu. Ice Storm Damage to Urban Trees
Hackberry’s tendency to form co-dominant stems, where two stems of roughly equal size share a narrow V-shaped crotch, compounds the problem. These included-bark unions are structurally weak and frequently split during storms. You can reduce this risk significantly by structural pruning when the tree is young. Selecting a single central leader and removing or subordinating competing leaders in the first ten to fifteen years of growth produces a tree that is far more storm-resistant as it matures. On an established tree, a qualified arborist can install steel cables between co-dominant stems to reduce the likelihood of splitting, though this is a management strategy, not a cure.
Wind damage follows a similar pattern. Hackberry crowns are dense and leafy, catching wind like a sail. Regular thinning of the canopy, removing crossing branches and reducing end weight on long horizontal limbs, lets wind pass through rather than pushing against a solid surface. A well-pruned hackberry holds up respectably in storms. A neglected one does not.
Root Spread and Sidewalk Lifting
Hackberry roots are aggressive and shallow, which makes the tree excellent at stabilizing soil and surviving drought but problematic near sidewalks, driveways, and foundations. As roots grow in diameter over years, they push up pavement slabs and crack curbing. This is not unique to hackberry, but hackberry’s adaptability means it is planted in tight urban spaces where root conflicts are almost inevitable.
The financial scale of the problem is significant. In California alone, repairing sidewalk damage from urban tree roots was estimated to exceed $62 million annually.7Arboriculture & Urban Forestry. Root Barrier and Extension Casing Effects on Chinese Hackberry Research on a related species, Chinese hackberry (Celtis sinensis), evaluated root barriers and extension casings designed to redirect root growth downward and away from pavement. These physical barriers can help when installed at planting time, but retrofitting them around an established tree is expensive and risks severing major roots, which both weakens the tree and opens wounds to decay fungi.
If you are planting a new hackberry, site selection is the single most effective mitigation. Keep the tree at least eight to ten feet from sidewalks and further from foundations. Where space is tight, consider structural soil or suspended-pavement systems that give roots room to grow beneath the walkway rather than lifting it. For existing trees already heaving pavement, options include ramping the sidewalk over the root with asphalt, grinding the raised slab, or in some municipalities, routing the sidewalk around the tree. Cutting large surface roots to save a sidewalk is a last resort since it destabilizes the tree and invites rot.
Berry Mess and Volunteer Seedlings
Hackberry produces small, dark purple berries in fall that are eaten enthusiastically by birds, especially cedar waxwings and robins. The berries that birds miss drop onto sidewalks, cars, and patios, leaving stains. And the berries that birds do eat get deposited across the landscape as seedlings. Hackberry is a prolific self-seeder, and volunteer seedlings pop up in flower beds, fence lines, gutters, and any neglected corner of the yard. In some regions, hackberry seedlings are the most common woody volunteer in urban gardens.
There is no way to prevent a mature hackberry from fruiting. Growth regulators that suppress flowering in some tree species have not been widely adopted for hackberry, and spraying a large tree is impractical anyway. Managing the mess comes down to regular cleanup: sweeping or power-washing stained hardscapes in fall and pulling volunteer seedlings while they are still small. Young hackberry seedlings pull easily when the soil is moist. If you let them grow for a season or two, they develop a surprisingly deep taproot for their size, and pulling becomes digging.
Some homeowners choose to plant male hackberry cultivars to avoid fruiting altogether, but hackberry flowers are polygamous, meaning individual trees carry a mix of male, female, and perfect flowers, so purely fruitless selections are rare in the nursery trade. If berry mess is a dealbreaker, this is worth discussing with a nursery that knows its source material, though you should set expectations realistically. Most hackberry cultivars were selected for form or disease resistance, not for reduced fruiting.
When to Call an Arborist Versus When to Relax
Hackberry owners tend to fall into two camps: those who panic over every gall and broom, and those who ignore a leaning, decayed trunk until it falls on something. The problems worth professional attention are structural. Co-dominant stems with included bark, large fungal conks on the trunk, a pronounced lean that developed recently, or a cavity you can fit your fist into all warrant an arborist’s assessment. These are failure risks, and a 60-foot hackberry falling on a house is not a cosmetic issue.
The problems you can safely live with are the cosmetic and foliar ones. Nipple galls, witches’ brooms, leaf spot, and berry mess are annoying but not dangerous to the tree or to you. A hackberry covered in galls and sporting a few brooms is usually just as structurally sound as a pristine one. Spending money on insecticides or fungicides to make a mature hackberry look slightly tidier almost never provides a meaningful return. Direct that energy toward proper pruning on a three- to five-year cycle, which addresses storm safety, canopy health, and aesthetics all at once.