Gleditsia Leaves Curling

Spider mites

Two-spotted spider mite (Tetranychus urticae) is a significant pest of gleditsia in hot, dry UK summers, causing the small individual leaflets of the elegant pinnate leaves to develop pale, silvery stippling on the upper surface before progressing to an overall bronzed, scorched appearance as the infestation intensifies. The mites are microscopic and invisible to the naked eye but feed on the undersides of the leaflets, piercing individual cells and removing their contents, leaving the cell walls intact as pale, empty husks that create the characteristic silvery stippling pattern. As the infestation builds in hot, dry conditions, the stippling becomes denser and the overall leaf colour changes from the normal green or gold of healthy gleditsia to a dull, bronzed or greyish tone that signals serious mite pressure. The affected leaflets lose their normal flat posture and curl slightly around the undersides where mite feeding is most intense, and in severe infestations the leaflets drop prematurely, reducing the canopy significantly before autumn. Fine, silken webbing on the undersides of the leaflets and between adjacent leaflets is the most reliable confirmatory sign of heavy mite infestation. Gleditsia triacanthos Sunburst, the golden-leaved cultivar that is the most popular garden form, shows spider mite symptoms particularly clearly because the pale golden background colour makes the stippling highly visible even at relatively low infestation levels.

Increase humidity around the canopy by misting the foliage with water in the evening during hot, dry periods, as spider mites are most active in hot, dry conditions and populations are suppressed by humidity and direct contact with water. A strong jet of water directed at the undersides of the leaflets dislodges mites mechanically and increases humidity simultaneously. Predatory mites, particularly Phytoseiulus persimilis, provide effective biological control and can be introduced to the canopy of ornamental trees as a commercial product; they are most effective when applied early before the spider mite population reaches damaging levels. Fatty acid or plant oil sprays applied to the undersides of the foliage provide contact chemical control by coating and suffocating the mites. Avoid applying broad-spectrum insecticides to gleditsia during the growing season, as these kill the natural predatory mite populations that otherwise provide effective regulation of spider mite numbers and can trigger population explosions of the pest following application.

Gleditsia aphid

Gleditsia aphid (Therioaphis luteola and related species) colonises the shoot tips and the undersides of the leaflets of honey locust in spring and early summer, forming colonies of pale yellow-green aphids that cause the developing new leaflets to curl tightly around the feeding insects as the shoot tips extend. The pinnate leaves of gleditsia, which are composed of many small oval leaflets arranged along a central stalk, curl and twist around aphid colonies at the shoot tips, producing a characteristic contorted appearance on the affected new growth. The honeydew produced by the colonies drips onto the leaflets below, coating the delicate leaflets with a sticky deposit that is particularly visible on the golden foliage of Gleditsia Sunburst, where the transparent honeydew droplets catch the light on the pale yellow-green leaflet surface. Sooty mould developing on the honeydew further detracts from the ornamental foliage. The infestation typically peaks in late spring to early summer and declines rapidly as natural predator populations build and the leaflets harden, but the distorted shoot tips produced during the peak infestation remain visible on the tree through the season.

Natural predators including ladybirds, hoverfly larvae, lacewing larvae, and parasitic wasps establish rapidly on gleditsia aphid colonies and provide effective biological control in most garden situations without the need for intervention. A strong jet of water directed at the curled shoot tips in early spring before the leaflets have closed tightly around the colony provides effective mechanical control. Fatty acid spray applied to the new growth as soon as colonies are noticed, before the leaflets curl to enclose the insects, provides contact chemical control. The very fine, pinnate structure of gleditsia foliage makes thorough spray coverage of the individual leaflets somewhat more challenging than on broad-leaved trees, and early application before the leaves are fully developed is more effective than spraying after the leaflets have hardened. The vigorous growth of gleditsia means that the distorted spring shoot tips are quickly supplemented by new, undistorted growth and the overall summer appearance of the tree is rarely significantly compromised.

Powdery mildew

Powdery mildew on gleditsia produces a white or pale grey floury coating on the upper surfaces of the small leaflets and causes the leaflet margins to curl upward as the infection develops in warm, dry late summer conditions. Erysiphe species cause powdery mildew on gleditsia and related leguminous trees, developing most actively on the vigorous new growth at the shoot tips in the warm, dry periods that often follow summer pruning or in the final flush of growth before leaf fall. The individually small leaflets of the pinnate gleditsia leaf display the mildew coating clearly despite their small size, and heavily infected shoot tips take on a noticeably white or grey appearance that contrasts with the clean golden or green foliage elsewhere in the canopy. Powdery mildew on gleditsia is typically a late-season phenomenon and in most garden situations does not significantly threaten the health of the tree, developing when the leaves are approaching their natural senescence and autumn colour season in any case.

Maintain adequate soil moisture during dry periods in late summer to prevent the drought stress that amplifies powdery mildew susceptibility. Apply a potassium bicarbonate or sulphur fungicide to the new shoot growth from late July on trees where mildew has been a persistent problem in previous seasons. Remove and destroy mildewed leaves where practical. Ensuring adequate air circulation around the tree by avoiding dense surrounding plantings reduces the microhabitat that favours mildew spore germination and spread. For gleditsia grown in sheltered courtyard or enclosed garden positions, powdery mildew in late summer is a regular management consideration; for trees in open, well-ventilated positions, the disease is generally less severe and less consistent in its occurrence.

Drought

Drought stress on gleditsia causes the pinnate leaves to droop from their normally horizontal or slightly ascending angle and the individual small leaflets to curl slightly upward along their midribs as the plant responds to moisture depletion by reducing its effective transpiration surface. Gleditsia triacanthos is native to the central and eastern United States where it grows naturally in a wide range of conditions including dry, poor soils and rocky slopes, and established gleditsia in UK gardens is among the more drought-tolerant ornamental trees available, managing typical UK summer dry periods without supplementary irrigation in the majority of sites. The situations where drought stress is most visible on gleditsia are in newly planted specimens in their first two seasons before root establishment, in very free-draining sandy or chalk soils where soil moisture reserves are minimal, and in particularly hot, enclosed south-facing positions where the soil temperature rises high enough to inhibit root activity even when some moisture is available. The fine, pinnate foliage of gleditsia shows the first signs of drought stress as a loss of the normal glossy, fresh appearance and a slight drooping of the leaf stalks before the leaflets begin to curl.

Water newly planted gleditsia during dry periods through the first two growing seasons. Once established, gleditsia in UK conditions rarely requires supplementary irrigation and performs best in the well-drained, sun-exposed positions that allow the soil to warm adequately and dry out between rain events. Gleditsia planted in heavy, persistently wet clay soils suffers more from waterlogging over winter than from summer drought, and the combination of good drainage and modest summer irrigation represents the ideal management for this tree. Apply a mulch over the root zone to retain moisture and moderate soil temperature. The drought tolerance of established gleditsia is one of its most valuable characteristics for urban garden use, where dry, compacted soils and heat-island effects stress less resilient trees.

Canker

Gleditsia canker, caused by opportunistic fungal pathogens including Nectria and related species, produces sunken, discoloured canker lesions on the bark of gleditsia branches that girdle the affected stem over one or more seasons, eventually cutting off water and nutrient supply to the growth above. The leaves on branches above a girdling canker wilt, curl, and discolour as the transport of water and nutrients to the affected shoot is progressively disrupted, producing a pattern of decline that begins at the shoot tip and progresses downward toward the main stem. Cankers on gleditsia typically enter through wounds, pruning cuts, frost damage, or areas of bark damage caused by mechanical injury, and the stressed plants in poor growing conditions are most susceptible. The canker itself is visible as a sunken, dark, often cracked area of bark, sometimes with orange or pink fruiting bodies of the fungus visible on the dead bark surface. Gleditsia in UK gardens is generally not severely affected by canker in the absence of predisposing stress or injury, and maintaining the tree in vigorous health by appropriate site selection and cultural care is the most effective preventive measure.

Remove all canker-infected branches by cutting into apparently healthy wood well below the visible canker margin, sterilising cutting tools between cuts. Avoid leaving stubs of dead wood on gleditsia, as these provide an entry point for canker-causing fungi into the main framework. Make all pruning cuts cleanly at an angle that sheds water and promotes rapid callusing. Prune gleditsia in dry summer weather rather than in wet conditions when the risk of fungal infection through fresh wounds is highest. Maintain gleditsia in vigorous health through appropriate site selection, ensuring adequate drainage and full sun exposure; trees in sheltered, shaded, or waterlogged positions are more susceptible to canker than those in the open, well-drained, sunny positions where the species naturally thrives. No fungicide treatment is effective against established canker in gleditsia; prevention through cultural care and prompt removal of dead wood is the only reliable management approach.

Frequently asked questions

Why are my gleditsia leaves curling?

Spider mites (Tetranychus urticae) are a common cause of gleditsia leaf distortion in hot, dry summers, feeding on the undersides of the small leaflets and causing pale stippling that progresses to bronzing, curling, and early leaf drop. Gleditsia aphid colonises the shoot tips in spring and early summer, causing the new leaflets to curl tightly around the dense colonies. Drought stress causes the pinnate leaves to droop and the individual leaflets to curl upward as the plant reduces moisture loss in dry conditions. Powdery mildew produces a white coating on the leaves and causes the leaflet margins to curl upward in late summer.

What is wrong with my Gleditsia Sunburst?

Gleditsia triacanthos Sunburst, the golden-leaved honey locust, is susceptible to spider mite attack in hot, dry summers, which causes the characteristic golden leaflets to become pale, stippled, and bronzed before curling and dropping prematurely. Aphid colonies on the new growth cause the shoot tips to curl and distort in spring. In very wet soils over winter, gleditsia may suffer root damage that manifests as poor leaf expansion and general dieback in the following growing season. Sunburst is generally a resilient garden tree, but the golden foliage colour makes pest and disease symptoms more visible against the bright yellow-green background.

Does gleditsia get spider mites?

Yes, gleditsia is susceptible to two-spotted spider mite (Tetranychus urticae) in hot, dry summers, with the small individual leaflets of the pinnate leaves developing pale, silvery stippling on the upper surface as the mites feed on the underside cells and remove their contents. In heavy infestations, the stippling progresses to an overall bronzed or scorched appearance, and the affected leaflets curl and drop prematurely. Fine webbing on the underside of the foliage confirms mite infestation. Increasing humidity by misting the canopy with water reduces mite populations, as spider mites thrive in hot, dry conditions and are suppressed by moisture.

Is gleditsia a good UK garden tree?

Yes, gleditsia triacanthos Sunburst is one of the most popular ornamental garden trees in the UK, valued for its elegant, feathery pinnate foliage that emerges golden yellow in spring before maturing to lime green through summer and turning clear yellow in autumn. The tree is fully hardy, tolerant of urban pollution, relatively drought-tolerant once established, and well-suited to sunny, well-drained positions. The thornless nature of Sunburst and most other garden cultivars, unlike the thorny straight species, makes it practical to plant near paths and in family gardens. It performs best in full sun in free-draining, preferably alkaline soil.