Corylus Leaves Curling

Hazel aphids

Hazel aphids (Myzocallis coryli) are the most common cause of leaf curling on corylus in UK gardens, establishing on the undersides of the large, softly hairy, rounded leaves from late spring and building to high population densities through summer. The aphids feed from the underside of the leaf, extracting sap and causing the leaf to cup and curl downward around the feeding colonies. The large, softly textured leaves of hazel are particularly hospitable to aphid populations: the prominent veins and hairy surface provide shelter, and the vigorous growth habit of hazel means there is always an abundance of fresh, nutritionally rich foliage available for colonisation. The honeydew produced by dense hazel aphid colonies drips onto the leaves below and onto anything beneath the plant, including garden furniture and paths, and supports sooty mould growth that further dulls and darkens the affected foliage. On ornamental corylus such as Corylus avellana Contorta, the aesthetic impact of heavy aphid infestations with their accompanying sooty mould can be considerable during summer, though the plant's health is rarely seriously threatened.

Natural predators provide the most effective and sustainable control of hazel aphids. Hoverfly larvae, ladybirds, lacewings, and parasitic wasps are abundant in UK gardens through summer and routinely reduce hazel aphid populations to insignificant levels without intervention, typically by midsummer. If the plant is in a position where honeydew drip is a nuisance or where aesthetic quality matters, a strong jet of water directed at the leaf undersides dislodges aphid colonies effectively. Insecticidal soap spray applied to the undersides of the leaves provides chemical control if natural predators are slow to establish. Avoid broad-spectrum systemic insecticides on hazel, which is an important wildlife plant and frequently visited by beneficial insects including the hazel-associated mining bees.

Powdery mildew

Powdery mildew (Phyllactinia guttata and related species) is common on corylus in late summer, producing a white or pale grey floury coating on the upper surfaces of the large, rounded leaves and causing the margins to curl upward and the affected tissue to turn pale and papery. Hazel powdery mildew develops most readily in late summer conditions of warm temperatures, moderate air humidity around the plant, and dry soil at the roots. Plants growing in positions with limited air circulation, particularly hazel grown as a dense multi-stemmed bush that has not been coppiced or thinned, are most susceptible. The softly hairy texture of corylus leaves makes mildew somewhat harder to distinguish in the early stages than on smooth-leaved plants, but the characteristic upward curl of the leaf margins and the powdery surface coating become clearly visible in moderate to heavy infections. Ornamental corylus cultivars including Corylus maxima Purpurea (purple-leaved filbert) are susceptible to powdery mildew, and the grey mildew coating contrasts conspicuously with the rich purple foliage, making infestations more visually noticeable than on the green-leaved common hazel.

Coppice hazel on a rotation of three to seven years, removing all stems to near ground level in late winter, to maintain vigorous new growth and an open structure with good air circulation. Regular coppicing is the traditional management of hazel in UK woodland and is the most effective long-term preventive for powdery mildew, as it removes the oldest, most disease-susceptible wood and creates a completely fresh growth framework. Where coppicing is not appropriate, remove one-third of the oldest stems at the base each year to achieve a similar effect gradually. Water during prolonged dry spells and apply a mulch to maintain soil moisture and reduce drought stress. Remove affected leaves where practical and apply a potassium bicarbonate fungicide at the first sign of white coating.

Leaf roller weevil

The hazel leaf roller weevil (Apoderus coryli) produces the distinctive, tightly rolled leaf cigars that appear on corylus from late spring and through summer. The female weevil, a small, bright red and black insect, selects a healthy hazel leaf and makes precise cuts through the leaf blade to create a flap that she rolls into a tight cylinder, laying an egg or two inside before sealing the roll. The rolled cylinders are clearly visible hanging from the hazel branches as papery brownish tubes of rolled leaf, and can appear in considerable numbers on a single plant. Unlike many leaf-rolling pests, the hazel leaf roller weevil causes minimal lasting damage to the plant because each roll uses only a small portion of one leaf, the rolled leaf eventually drops or is blown from the plant, and the beetle is associated with a healthy, vigorous plant growing in the right conditions rather than a stressed one. The rolled leaves are a curiosity of the garden rather than a problem requiring treatment.

No treatment is required or recommended for hazel leaf roller weevil. The insect causes insignificant damage to the plant and is part of the natural ecology of hazel woodland. Removing rolled leaf tubes reduces the breeding population slightly but is practically ineffective given the speed with which new rolls are created and the widespread distribution of the weevil in UK gardens with hazel. Tolerance is the appropriate response, and the presence of the characteristic leaf rolls is an interesting indicator of the ecological richness of the hazel in the garden.

Big bud mite

Big bud mite (Phytoptus avellanae) is a microscopic gall mite that lives within the dormant buds of hazel and causes them to swell abnormally, producing the enlarged, rounded buds that are clearly visible on bare hazel stems in winter compared to the normal, pointed, tightly scaled dormant buds. Affected buds fail to open in spring or open to produce distorted, stunted shoots that do not develop normally, and the mite population moves to infest new buds as the season progresses. Big bud mite is a vector of hazel big bud virus, which causes mosaic patterns and distortion in the leaves of infected plants and can reduce nut production significantly, though the virus is rarely debilitating in ornamental corylus. The mites spread between plants on the wind, on garden tools, and through contact with infested plant material. Corylus avellana cultivars grown for nut production are more seriously affected than ornamental cultivars, as reduced nut set is an economically significant consequence of heavy infestations.

Remove and destroy infested buds and affected shoots in early spring before the mites migrate to new growth. There are no chemical controls available to amateur gardeners that are effective against big bud mite inside the buds. Tolerating a moderate level of big bud mite is realistic in most ornamental corylus situations, as the plant's overall vigour is maintained by the large number of unaffected buds and stems. In hazel grown for nut production, replacing severely infested plants with resistant varieties such as Corylus avellana Cosford is the most effective long-term approach if infestations are heavy.

Drought

Drought stress causes corylus leaves to curl inward, lose their characteristic fresh green colour and become noticeably paler and yellow-green, and develop a wilted, limp texture. Hazel is remarkably adaptable to a range of soil conditions in the UK, growing naturally on chalk, clay, loam, and sandy soils, and established corylus in average garden conditions manages typical UK summer conditions without supplementary watering. However, newly planted specimens in their first one to two seasons before the root system is established, and corylus in very free-draining, sandy soils or in positions of full exposure without any surrounding shelter, can show drought stress in prolonged hot, dry conditions. The large, hairy leaves of hazel have a relatively high transpiration rate, which makes the plant more responsive to changes in soil moisture than smaller or more waxy-leaved shrubs, and the visible stress response is often the first indication that the plant needs watering. Drought stress on corylus reduces the vigour of the new growth that will carry the following winter's catkins and the following autumn's nut crop.

Water newly planted corylus during its first two growing seasons in dry conditions. Once established, hazel requires no supplementary watering in typical UK gardens. Apply a mulch of organic material at the base annually to retain soil moisture and improve soil structure over time. In very free-draining soils, incorporating organic matter at planting significantly improves the water-holding capacity of the root zone. Established hazel is one of the most drought-resilient native shrubs once its extensive root system is fully developed, and visible stress in a mature plant is unusual unless the drought is prolonged or the soil conditions are exceptionally free-draining.

Frequently asked questions

Why are my hazel leaves curling?

Hazel aphids (Myzocallis coryli) are the most common cause of leaf curling on corylus in summer, colonising the undersides of the large, softly hairy leaves and causing them to curl downward and cup around the feeding colonies. Powdery mildew produces a white floury coating on the upper leaf surfaces and causes the margins to curl upward in late summer. Hazel leaf roller weevil (Apoderus coryli) creates the distinctive cigar-shaped leaf rolls seen from late spring.

What is wrong with my hazel tree?

The most common problems on hazel in UK gardens are aphid infestations in summer, powdery mildew in late summer, and big bud mite (Phytoptus avellanae) which causes the dormant buds to swell abnormally and fail to open properly in spring. Big bud mite also carries hazel big bud virus. None of these problems significantly threaten the life or vigour of a well-established corylus, which is one of the most resilient native shrubs in UK conditions.

Does hazel get aphids?

Yes, hazel is commonly colonised by hazel aphids (Myzocallis coryli) on the leaf undersides and by hazel-grass aphid (Cavariella archangelicae) on the shoot tips in spring and early summer. The large, hairy leaves of corylus harbour particularly dense aphid colonies. Natural predators including ladybirds, hoverfly larvae, and parasitic wasps typically control hazel aphid populations without treatment by midsummer, and the robust growth of established hazel means that the impact on plant health is minimal.

Is corylus easy to grow?

Yes, corylus avellana (common hazel) is one of the most adaptable and undemanding native shrubs for UK gardens. It is fully hardy, grows on a wide range of soils including chalk and clay, tolerates partial shade, and is rarely troubled by serious problems. The main management task is regular coppicing if a compact, multi-stemmed bush is wanted, as hazel naturally grows into a large multi-stemmed shrub or small tree without cutting. Corylus avellana Contorta (corkscrew hazel) is slower-growing and more ornamental but equally easy to establish.