Hackberry is the common name for trees in the genus Celtis, which are deciduous trees related to elm (family Cannabaceae). Several species are relevant to UK growers: Celtis australis, the European or Mediterranean hackberry (also called the nettle tree), is native to southern Europe and is grown in the UK as a street tree and ornamental, particularly in southern England where the climate suits it; it is a medium to large tree with rough, nettle-textured leaves. Celtis occidentalis, the common hackberry, is native to North America and is grown as an ornamental and wildlife tree in UK gardens; it produces the distinctive, sweet, edible hackberries. Celtis sinensis, the Chinese hackberry, is also occasionally grown in the UK. Hackberry trees are highly valued wildlife trees: the small, sweet berries are eaten by many bird species including thrushes, waxwings, and woodpeckers; the trees are also host plants for many butterfly species in North America, including the hackberry emperor butterfly. The berries are edible for people as well: they are very small (about the size of a large pea), sweet, and date-like in flavour when ripe (dark purple-black); they were eaten by Native American peoples and by early European settlers in North America.

Plant problems

Why Are My Hackberry Leaves Curling?

Q: Why are my hackberry leaves curling?

Hackberry (Celtis occidentalis, North American hackberry; Celtis australis, European/Mediterranean hackberry; and related species) leaves curl most commonly because of woolly hackberry aphid infestation, hackberry nipple galls, or witch's broom disease complex. The hackberry woolly aphid (Shivaphis celti, also classified as Colobanthus celti) colonises the undersides of hackberry leaves, producing a conspicuous covering of white waxy wool; infested leaves curl and pucker as the aphids drain sap. The aphid's white, cotton-wool-like coating distinguishes it clearly from other aphids. Hackberry nipple galls (Pachypsylla celtidismamma) are cone-shaped, nipple-like protrusions on the upper surface of the leaves caused by tiny psyllid insects; these are not true leaf curl but the affected leaves may distort and lose their flat shape as multiple galls develop. Witch's broom of hackberry is caused by a combination of the powdery mildew fungus Sphaerotheca phytophila and the eriophyid mite Aceria celtis; it produces the characteristic dense clusters of abnormal, clustered, broom-like twigs visible on the branches in winter; leaves on these witch's broom clusters are small, crinkled, and distorted.

Q: What are hackberry nipple galls?

Hackberry nipple galls (caused by the psyllid insect Pachypsylla celtidismamma) are one of the most characteristic and striking features of hackberry trees in summer. Each gall appears as a small, cone-shaped or nipple-like protrusion on the upper surface of the leaf, typically 3 to 5 mm tall and green maturing to brown; the galls form on the leaf tissue as a response to the feeding activity of the tiny psyllid nymph inside. A single hackberry leaf can have dozens to hundreds of individual nipple galls, giving the leaf a heavily studded, almost decorative appearance. The galls cause leaf distortion as they develop but do not kill the leaf or cause serious harm to the tree; a mature hackberry tree can sustain heavy nipple gall infestation every year without any significant reduction in growth or vigour. Psyllid (also called jumping plant lice) are tiny, sap-sucking insects related to aphids; Pachypsylla species are specific to hackberry (Celtis species) and will not infect other tree species. There is no effective or necessary chemical control for hackberry nipple galls on established trees; the galls are an unusual and interesting feature of the tree rather than a damaging problem.

Q: How do I grow hackberry in the UK?

Hackberry (Celtis australis for southern England; Celtis occidentalis for wider UK use) grows in a wide range of UK soils and conditions, making it a versatile tree for UK gardens and urban planting. Both species tolerate chalk and alkaline soils, drought once established, pollution, and partial shade, though they prefer full sun. Celtis australis is particularly valued as a street tree because of its pollution tolerance and attractive corrugated bark; it has been planted in many southern English towns and cities. Celtis occidentalis is fully hardy throughout the UK and is available from some specialist UK tree nurseries, valued for its wildlife interest (berries for birds, larval host plant in North America though not for UK butterflies). Plant in a sunny position in well-drained to moderately moist soil; the trees establish well from container-grown or bare-root stock in autumn or early spring. Hackberry trees have an irregular, rounded crown and are attractive in autumn when the small berries ripen to dark purple-black and attract birds. They are relatively long-lived, low-maintenance trees with no significant pest or disease problems that require active management; the galls, witch's brooms, and aphid infestations that commonly affect hackberry are largely cosmetic.

Hackberry (Celtis australis, European hackberry or nettle tree; Celtis occidentalis, North American hackberry) is a versatile deciduous tree valued for its pollution tolerance, edible berries, and outstanding wildlife value. Related to elm, it tolerates chalk, drought, and urban conditions and is planted as a street tree in southern England. Hackberry has a suite of characteristic gall-causing insects and a witch's broom disease complex that cause distinctive leaf symptoms; true leaf curl from aphid infestation is also common.

Woolly hackberry aphid

The hackberry woolly aphid (Shivaphis celti) is the most damaging pest of hackberry in the UK, colonising the undersides of the leaves from late spring; the colonies are immediately recognisable by the conspicuous white, cottony wool that covers the insects and their egg masses. Infested leaves curl and pucker as the aphids drain sap, and the white waxy material can accumulate on the leaf surfaces and on surfaces below the tree. On young trees, heavy infestations can slow growth significantly; on established trees the infestation is more of an aesthetic problem than a serious health threat.

What to do

Encourage natural predators (ladybirds, lacewings, parasitoid wasps); they are the most effective long-term control. Apply insecticidal soap spray to the leaf undersides when colonies are first detected, before the waxy coating builds up (the wool makes later spraying less effective). On small or ornamental trees, a strong jet of water can dislodge aphid colonies. Avoid broad-spectrum insecticides. On established large trees, no treatment is usually necessary.

Hackberry nipple galls

Hackberry nipple galls (caused by the psyllid Pachypsylla celtidismamma) produce small, cone-shaped protrusions on the upper leaf surfaces; a single leaf can carry dozens of galls, making the leaf heavily studded and causing some distortion. The galls are caused by tiny psyllid nymphs inside, which manipulate the plant tissue to form a protective chamber around themselves. Despite appearing alarming, nipple galls cause no serious harm to the tree; established hackberry tolerates heavy annual galling without any decline in vigour.

What to do

No action required on established trees. Hackberry nipple galls are a natural and harmless feature of hackberry trees; chemical control is not effective and not warranted. Enjoy them as a curiosity. Young trees with very heavy gall infestations can be protected in future years by applying a systemic insecticide soil drench in early spring before psyllid egg-laying begins, though this is rarely necessary.

Witch's broom

Hackberry witch's broom is caused by a combination of the powdery mildew fungus Sphaerotheca phytophila and the eriophyid mite Aceria celtis; both agents together produce the characteristic dense, broom-like clusters of stunted, proliferating twigs visible on the branches, particularly prominent in winter when the tree is bare. The leaves on witch's broom clusters are small, crinkled, and distorted, not expanding fully in spring; the brooms are perennial structures that grow slightly larger each year. Witch's brooms are unsightly but rarely cause serious harm to a mature hackberry.

What to do

Prune out individual witch's broom clusters in winter if they are unsightly, cutting back to the main branch below the broom; this reduces but does not eliminate witch's broom recurrence because both the mite and the fungus may still be present on other parts of the tree. On young trees where witch's broom causes significant distortion of the growth habit, more aggressive removal is worthwhile. There is no practical chemical control for the broom complex in garden conditions.

Powdery mildew

Powdery mildew (Erysiphe celtis and other species) can affect hackberry foliage independently of the witch's broom complex, producing a white powdery coating on the upper surfaces of the young leaves and causing them to curl and distort. Powdery mildew on hackberry is most severe in warm, dry summers with poor air circulation around the crown; established, vigorous trees rarely suffer serious damage.

What to do

On young trees, apply a potassium bicarbonate or sulphur-based fungicide at the first sign of infection. On established trees, powdery mildew rarely requires treatment; the tree outgrows the infection. Avoid high-nitrogen fertilising that promotes soft, susceptible new growth. Ensure good air circulation around the crown by removing crossing or congested branches.

Frequently asked questions

Why are my hackberry leaves curling?

Hackberry (Celtis species) leaves curl most commonly because of woolly hackberry aphid (white cottony colonies on leaf undersides cause curling and puckering), hackberry nipple galls (cone-shaped protrusions causing leaf distortion, cosmetic only), or witch's broom complex (crinkled, distorted leaves on dense broom clusters). All three conditions are more cosmetic than harmful on established trees. Powdery mildew can also cause leaf curl on young growth in dry summers.

What is hackberry?

Hackberry is the common name for trees in the genus Celtis, related to elm (family Cannabaceae). Celtis australis (European hackberry, nettle tree) is grown as a pollution-tolerant street tree in southern England. Celtis occidentalis (North American hackberry) is grown as an ornamental and wildlife tree. Both produce small, sweet, edible dark berries eaten by birds; both tolerate chalk, drought, and urban conditions.

What are hackberry nipple galls?

Nipple galls are small, cone-shaped protrusions on hackberry leaves caused by the psyllid insect Pachypsylla celtidismamma; each gall houses a tiny nymph inside. A single leaf can have dozens to hundreds of galls. Nipple galls cause leaf distortion but no serious harm to established trees. There is no effective or necessary chemical control; they are a natural, harmless feature of hackberry trees.

How do I grow hackberry in the UK?

Hackberry grows in a wide range of UK soils including chalk and alkaline conditions; tolerates drought, pollution, and partial shade. Full sun preferred. Celtis australis suits southern England best; Celtis occidentalis is hardy throughout the UK. Low-maintenance once established; no pruning required beyond removing witch's broom clusters. Available from specialist UK tree nurseries.