Plant problems

Why Are My Kerria Leaves Curling?

Q: Why are my kerria leaves curling?

Kerria (Kerria japonica) leaves curl most commonly in the UK because of aphid infestation on the young shoot tips in spring, shoot dieback (where the growing tip dies and the associated leaves curl and wilt as they die), or leaf spot disease. Kerria is a vigorous, easy-going deciduous flowering shrub with bright golden-yellow flowers in April and May and distinctive bright green, arching canes that remain green through winter; it is one of the most trouble-free flowering shrubs in UK gardens and its leaf curl problems are usually minor. Aphids, typically the green rose aphid (Macrosiphum rosae) and related species, can colonise the tender young shoot tips of kerria in spring; the young leaves curl around the aphid colonies. Shoot dieback, in which the growing tip of a cane dies back for no obvious reason (possibly minor fungal infection, cold damage to the tip, or mechanical damage), causes the leaves at the tip to curl and wilt before browning; this is very common in kerria and can look alarming but is not usually a sign of a serious problem. Leaf spot diseases (minor fungal infections) can cause small brown spots that distort the leaf as they expand.

Q: What is the difference between single and double kerria?

Kerria japonica produces bright golden-yellow flowers that are available in two distinct forms: the single-flowered species (Kerria japonica species type) has simple, 5-petalled flowers resembling a small, bright yellow wild rose; and the double-flowered cultivar Kerria japonica 'Pleniflora' (also called 'Flore Pleno') has full, pompom-like, spherical double flowers with many petals arranged in a globe about 3 to 4 cm across. Both are widely grown in UK gardens. 'Pleniflora' is by far the more commonly grown in UK gardens and is the form most likely to be encountered in garden centres; its fully double, pompom flowers are showier and last slightly longer in bloom than the single flowers of the species. The single-flowered form has a more delicate, natural appearance that is preferred by some gardeners who find the double flowers too formal or heavy-looking; it also produces small, round, shiny black fruits after flowering (the double form is largely sterile and produces little or no fruit). Both forms are equally easy to grow and have similar habits; both spread freely by suckers. There are also variegated forms of kerria, the most notable being Kerria japonica 'Picta' (also sold as 'Variegata'), which has smaller, pale green leaves with a white margin; this form is less vigorous than the plain green-leaved forms and less commonly planted, but is attractive as a lighter, airier alternative.

Q: How do I prune kerria?

Kerria (Kerria japonica) should be pruned immediately after flowering, typically in late May or early June, following the same timing rule as most spring-flowering shrubs: prune after they flower, never before. The pruning method depends on what you want to achieve. For maximum flowering the following year: after all flowers have finished, cut all the flowered shoots back to young shoots or buds lower on the same cane; this redirects the plant's energy into the new canes that will carry next year's flowers. For renovation or controlling size: cut all the canes to the ground immediately after flowering; the plant will produce a mass of new, vigorous green canes from the base through summer and will flower the following spring. This hard pruning every 3 to 4 years keeps the plant compact and free-flowering. To control suckers: kerria spreads by underground suckers and can colonise a significant area over time; dig out suckers from around the plant in spring or autumn when they can be cleanly pulled or cut from the parent root. If you have a wall-trained specimen, tie in the young canes after pruning to fill gaps and maintain the coverage; the bright green canes are worth showing off on a wall for their winter interest.

Q: Why are my kerria canes turning brown and dying?

Kerria shoot dieback, where individual canes or shoot tips turn brown and die, is very common in kerria and is usually not a serious problem requiring treatment. The most common causes are: minor fungal infections entering through small wounds or natural lenticels on the green canes (the canes are moisture-rich and can be susceptible to fungal entry points); cold damage to the soft cane tips in a late frost after new growth has started; physical damage from garden tools, animals, or rubbing against structures; and simple age-related dieback (the oldest canes naturally die after 3 to 4 years). The brown dieback typically progresses from the tip downward; the cane is dead above the point where you can see it clearly brown and the cane changes from brown to green when cut. Remove dead canes by cutting back to healthy, green tissue or to the base if the whole cane is affected. To reduce dieback: maintain the plant by cutting out all canes that are more than 3 to 4 years old (these older canes are brown rather than bright green and are less productive); a plant maintained on young, vigorous green canes is much less prone to dieback than an old, crowded plant with many old brown canes. Very severe or rapid dieback of many canes simultaneously suggests a more serious problem (root disease, crown gall, or severe cold damage) and the plant should be investigated more thoroughly.

Kerria (Kerria japonica) is one of the most obliging flowering shrubs in UK gardens: cheerful golden-yellow pompom flowers in April and May (or delicate single flowers on the species), bright green arching canes that look attractive even in winter, tolerance of shade, and minimal pest and disease problems. In the Rosaceae family and native to China and Japan, it is very easy to grow. The leaves rarely cause concern; the most common issues are minor aphid damage on young spring growth and the very common shoot dieback that affects individual cane tips.

Aphids

Aphids, typically green rose aphid (Macrosiphum rosae) and related species, can colonise the tender young shoot tips of kerria in spring; the young leaves curl around the aphid colonies and may become distorted. Aphid infestations on kerria are usually light and short-lived; the vigorous growth of the plant rapidly outpaces the aphid damage, and natural predators (ladybirds, lacewings) arrive quickly to reduce colony numbers. The curled leaves at the shoot tips may be obvious in April or May but are quickly covered by healthy new growth and rarely leave a lasting mark on the plant.

What to do

On most occasions, no treatment is needed; natural predators control the infestation within a few weeks and the plant outgrows the damage. Pinch off and discard the most heavily infested shoot tips. Apply insecticidal soap spray to the shoot tips if the infestation is heavy and the plant is small. Encouraging natural predators with companion planting is more effective and sustainable than spraying.

Shoot dieback

Shoot dieback is probably the most common problem noticed on kerria: individual cane tips, or sometimes whole canes, turn brown from the tip downward; the leaves at the tip curl and wilt as the cane dies. This can look alarming, especially when several canes show the problem simultaneously, but it is usually caused by minor fungal infections entering the cane through small wounds, cold damage to soft growing tips in a late frost, physical damage, or simply the natural senescence of older canes. Kerria canes are productive for about 3 to 4 years; older canes (which become brown-barked rather than bright green) naturally die and should be removed.

What to do

Cut each affected cane back to a healthy, green bud or to the base if the whole cane is dead; use clean, sharp secateurs to make a clean cut. Dispose of dead canes; do not compost them if fungal infection is suspected. Maintain the plant by removing all canes older than 3 to 4 years (those with brown rather than bright green bark) each year after flowering; this reduces the pool of older canes susceptible to dieback and keeps the plant productive. Very widespread and rapid dieback of multiple canes suggests a more serious root or crown problem; investigate the root zone.

Leaf spot

Minor fungal leaf spot diseases can cause small, circular, brown to black spots on kerria leaves; as spots enlarge and coalesce, the surrounding tissue distorts and the leaf may curl. Leaf spot on kerria is rarely severe enough to require treatment and rarely affects the plant's overall health. It is most common in wet seasons when spores spread freely from infected fallen leaves.

What to do

Rake up and dispose of fallen leaves in autumn; they carry fungal overwintering structures and are the primary source of next season's infection. On plants showing severe leaf spot, apply a copper-based fungicide in early spring as new growth begins. Improve air circulation by thinning crowded growth. No treatment is usually necessary on otherwise healthy, vigorous kerria.

Crown gall

Crown gall (Agrobacterium tumefaciens) can occasionally affect kerria, producing rough, rounded, woody galls at the stem base at or just below soil level; the galls interfere with the transport of water and nutrients and can cause general weakening, dieback, and leaf curl on the canes above the gall. Crown gall is most likely to enter through wounds (pruning cuts close to the base, mechanical damage from garden tools). There is no cure for crown gall once established.

What to do

Remove and destroy severely affected plants; do not replant with any member of the Rosaceae family (rose, kerria, pyracantha, chaenomeles, cotoneaster) in the same position for several years. On lightly affected plants, remove the galled tissue carefully, cutting into healthy tissue, and treat the wound with a copper-based bactericide. Avoid wounding the stem base during pruning or cultivation. Do not propagate from infected plants.

Frequently asked questions

Why are my kerria leaves curling?

Kerria (Kerria japonica) leaves curl most commonly because of aphids on young spring shoot tips (minor, short-lived infestation), shoot dieback (individual cane tips die and leaves curl as the tip browns), leaf spot, or occasionally crown gall at the stem base. Kerria is one of the most robust and trouble-free flowering shrubs in UK gardens; leaf problems are rarely serious and the plant typically recovers quickly from any minor damage.

What is the difference between single and double kerria?

Single-flowered Kerria japonica (the species) has simple 5-petalled bright yellow flowers resembling a small wild rose and produces small black fruits; double-flowered Kerria japonica 'Pleniflora' has full, globe-like pompom flowers about 3 to 4 cm across, is largely sterile, and is the form most commonly sold in UK garden centres. Both are equally easy to grow. 'Picta' (Variegata) is a variegated form with cream-margined leaves; less vigorous.

How do I prune kerria?

Prune immediately after flowering (late May to early June). Cut all flowered shoots back to young shoots or buds lower on the cane; or cut all canes to the ground every 3 to 4 years for renovation. Dig out suckers in spring or autumn to control spread. On wall-trained specimens, tie in young canes after pruning to fill gaps; the bright green canes are decorative through winter.

Why are my kerria canes turning brown and dying?

Shoot dieback of individual canes is very common and usually caused by minor fungal infection, cold damage to soft tips, or natural age-related dieback (canes are productive for 3 to 4 years). Remove dead canes by cutting back to healthy green tissue or to the base. Maintain the plant by removing all old brown-barked canes after flowering each year. Very widespread sudden dieback of many canes simultaneously suggests a more serious root or crown problem.