Albizia julibrissin, the silk tree or Persian silk tree, is one of the most exotic-looking trees that can be grown in a sheltered UK garden. Its large, feathery bipinnate leaves give it a tropical presence unlike any other hardy tree, and from July to September it produces extraordinary fluffy pompom flowers made entirely of long pink stamens. When those spectacular leaves start curling or folding, the first thing to establish is whether you are looking at a natural biological response or a genuine problem.
This guide covers the full range of causes, starting with the most important distinction: thigmonastic leaf-folding (completely normal and not a cause for concern) versus genuine stress responses that need attention.
About Albizia julibrissin
The silk tree is a deciduous tree or large shrub in the subfamily Mimosoideae of the Fabaceae family. It is native to Asia from Iran across to China and Japan and in ideal conditions can reach 5 to 12 metres, though specimens in UK gardens are often considerably smaller. The leaves are very large and bipinnate, made up of pairs of pinnae each carrying many small oblong leaflets; they have a soft, almost fern-like texture that moves gently in even a light breeze. The flower clusters appearing from July to September consist of brush-like heads of long pink stamens and are quite unlike anything else available to UK gardeners.
Albizia julibrissin is not fully hardy throughout the UK. The form almost always sold in UK nurseries is A. julibrissin f. rosea, which is slightly hardier than the straight species and better suited to UK conditions. It thrives in the warmest, most sheltered parts of southern and western England, particularly against south or west-facing walls, in walled gardens, or in sheltered city courtyard gardens. The cultivar 'Summer Chocolate' has striking bronze-purple foliage and is becoming more widely available.
Cause 1: Natural thigmonastic leaf folding (not a problem)
The single most important thing to know about Albizia julibrissin is that its leaves fold up every evening, and also in rain, in cold weather, and when touched. This is completely normal. The bipinnate leaves respond to changes in temperature, light, touch, and rain by folding the individual leaflets together along each pinna, giving the appearance of a tree that has closed itself down. The response is called thigmonasty (movement triggered by touch) or nyctinasty (movement triggered by darkness), and it is the same mechanism as the famous movement of Mimosa pudica (the sensitive plant).
It is an adaptation that reduces water loss from the many small delicate leaflets and may also protect them from physical damage by rain. In warm, sunny weather, the leaves open again in the morning as temperatures rise and light levels increase. On cool or overcast days they may remain partly folded for longer.
If you notice your silk tree's leaves folding up at dusk, closing in the rain, or drooping slightly on a cold grey day, there is nothing wrong. This is a feature of the tree, not a fault. Many gardeners are unaware of it until they see it for the first time, and it consistently surprises and delights garden visitors. The only time folded or curled leaves on a silk tree warrant investigation is when they stay closed or collapsed during a warm, sunny morning when you would normally expect them to be fully open and displaying the full feathery canopy.
Cause 2: Cold damage and frost
Cold damage is the most common genuine cause of leaf curl and leaf collapse on silk trees in UK gardens. The large, thin bipinnate leaves are extremely sensitive to low temperatures and begin to show stress at temperatures approaching 10 degrees Celsius. A late spring frost catching the emerging foliage can cause the new leaves to brown, collapse, and curl within hours. Cold winds, even without frost, cause the leaflets to fold and flag in a way that resembles thigmonastic movement but does not recover quickly in the morning as natural folding does.
Young trees are considerably more vulnerable than established specimens. A tree that has been in the ground for five or more years will have a much stronger root system and more stored energy to recover from cold damage. A tree in its first or second year is at real risk of being killed outright by a hard frost or a prolonged cold wet spring.
Frost damage on emerging spring foliage is perhaps the most common disappointment for UK growers: the tree produces a flush of beautiful new bipinnate leaves in May or June and a late cold snap blackens and collapses the whole canopy overnight. In most cases, if the trunk and main branches are undamaged, the tree will produce a second flush of leaves as temperatures improve. However, repeated cold damage weakens the tree year on year.
To minimise cold damage, grow the silk tree in the warmest and most sheltered position you have available. A south or west-facing wall provides reflected heat from the masonry and shelter from the cold winds that cause most damage. Never plant in a frost pocket. In colder areas of the UK, the tree may die back to ground level in a hard winter but reshoot from the base in spring. This behaviour is worth planning for: the tree treated as a coppiced large shrub can still be spectacular, and the vigorous new growth from the base each year often produces particularly luxuriant foliage. In very cold winters, a deep mulch applied over the root zone in autumn helps protect the roots from freezing.
Other causes of leaf curl
Aphid infestation
Aphid colonies on the soft new shoot tips of Albizia julibrissin are a common problem in spring and early summer. The small, soft-bodied insects cluster at the growing points and the leaflets around them curl inward, partly as a natural plant response to the feeding damage and partly because the aphids shelter among the folded leaves. The infested tips look bunched and the new leaflets do not open properly. Sticky honeydew produced by the aphids attracts ants and encourages sooty mould to develop on the leaf surfaces below the colony.
Treat aphids on silk tree by directing a strong jet of water from a hose at the infested shoot tips to dislodge the colony. Repeat after a few days. Where water jetting is not practical, insecticidal soap spray applied directly to the aphids is effective. Natural predators including ladybirds and hoverfly larvae will usually arrive and reduce numbers once the colony is established. Avoid systemic insecticides on a tree that will flower from July onward, as pollinating insects visit the flowers in numbers. If ants are tending the aphid colony, banding the trunk with a sticky barrier prevents them from protecting the aphids from predators.
Drought stress in young trees
Albizia julibrissin prefers a warm, reasonably dry summer climate and, once established, is tolerant of dry conditions. However, young trees on very free-draining soil, particularly sandy or gravelly soil, can experience drought stress in a prolonged hot dry period. The leaflets curl inward and the leaf pinnae droop when the tree cannot replace water through the roots quickly enough. Unlike natural thigmonastic folding, drought-related drooping tends to persist through the middle of the day rather than recovering by mid-morning.
Water young trees regularly in the first two to three years after planting, particularly during dry spells in the growing season. Once the root system is established (typically after three to four years in a good position), supplementary watering is rarely necessary. A mulch of bark or well-rotted organic matter around the base, kept clear of the trunk, helps the soil retain moisture around a young tree's roots.
Waterlogging and root rot
Albizia julibrissin is particularly intolerant of waterlogged soil. Roots sitting in poorly drained clay soil, especially in a wet winter, quickly develop root rot. The above-ground symptoms are a progressive decline in leaf quality, with the bipinnate leaves becoming sparse, yellowed, and curled throughout the canopy rather than in isolated areas. A tree suffering from waterlogging will often look generally unwell rather than showing a specific pattern of damage.
Always plant silk tree in free-draining soil. On heavy clay soils, improve drainage by incorporating grit and organic matter into a wide planting area before planting, or by raising the planting position slightly to ensure the root zone is above the water table in wet weather. Once a tree is suffering from root rot in waterlogged soil, it is very difficult to save. Improving drainage around the root zone and applying a root-stimulating treatment can sometimes stabilise a mildly affected tree, but prevention is far more reliable than cure.
Spider mites in hot dry conditions
In very hot, dry summers, red spider mite (Tetranychus urticae) can colonise the undersides of the leaflets on Albizia julibrissin. The fine speckling on the upper surface of the leaflets, progressive bronzing, and the development of fine webbing on the undersides and between leaflets distinguish spider mite damage from the other causes described here. Spider mite thrives in hot and dry conditions; maintain adequate soil moisture and, if possible, mist the leaf undersides during prolonged hot dry spells. The predatory mite Phytoseiulus persimilis can be used as a biological control where temperatures are consistently above 16 degrees Celsius.
Powdery mildew
Powdery mildew occasionally affects Albizia julibrissin, producing a white or grey powdery coating on the leaflet surfaces. Affected leaflets may curl slightly. Powdery mildew is most common in warm dry summers with cool nights. Improve airflow around the plant and avoid overhead watering. In most cases the tree tolerates mild mildew without serious harm; severely affected shoots can be removed.
Prevention and long-term care
The foundation of a healthy silk tree in a UK garden is position. Plant in the warmest and most sheltered spot available: a south or west-facing wall in the south or southwest of England is ideal. Walled gardens and sheltered city gardens can also provide the extra warmth and protection the tree needs. Avoid frost pockets entirely.
Ensure free-draining soil from the outset; Albizia julibrissin will not tolerate waterlogged roots. Water young trees regularly during the first two to three growing seasons but reduce watering as the tree establishes. Apply a deep mulch of bark or compost over the root zone in autumn to insulate the roots against hard frosts; keep the mulch clear of the trunk to prevent collar rot.
Check shoot tips for aphid colonies in spring and treat early with a water jet before populations build. In colder areas, accept that the tree may be cut to the ground in a hard winter and plan to take advantage of the vigorous regrowth that usually follows. The natural leaf-folding at night and in rain requires no intervention and is best enjoyed as one of the most remarkable features of this extraordinary tree.
Frequently asked questions
Why do silk tree leaves fold up at night?
Silk tree leaves fold up every evening as a completely normal biological response called thigmonasty (or nyctinasty at night). The large bipinnate leaves are made up of dozens of small leaflets that fold together when the plant detects darkness, falling temperatures, rain, or physical touch. This is the same movement seen in mimosa (Mimosa pudica) and is an adaptation that reduces water loss from the delicate leaflets. The leaves open again in warm morning sunlight. Folding at dusk, in rain, or on cool overcast days is never a sign of disease or distress; it is one of the most striking and talked-about features of the silk tree.
Why are my silk tree leaves curling and not opening?
If your Albizia julibrissin leaves stay folded or curled during warm sunny weather rather than opening as they normally would, the most likely causes are cold damage to the foliage, aphid infestation on the soft new shoot tips, or drought stress on a young tree in free-draining soil. Cold damage is the most common cause in the UK: the thin bipinnate leaves are sensitive to temperatures below about 10 degrees Celsius and may fold, flag, or collapse after a cold night or late spring frost. Check the shoot tips for aphid colonies and check the soil moisture around the roots.
Can silk trees grow in the UK?
Yes, Albizia julibrissin can be grown in the UK but it needs a very sheltered, warm position to thrive and flower. It is not fully hardy throughout the UK and is best treated as a wall shrub against a south or west-facing wall, or grown in a walled garden or sheltered courtyard in the south and west of England. The form sold in most UK nurseries is A. julibrissin f. rosea, which is slightly hardier than the straight species. In colder areas it may be cut back to the ground in a hard winter but will often reshoot from the base. The cultivar 'Summer Chocolate', with bronze-purple foliage, is also worth seeking out.
How do I treat aphids on a silk tree?
Direct a strong jet of water from a hose at the infested shoot tips to dislodge the colony; this is effective, non-toxic, and safe for the tree. Repeat after a few days if numbers rebuild. Insecticidal soap spray applied directly to the aphid colony is a low-impact alternative. Encourage natural predators such as ladybirds and hoverfly larvae. Avoid systemic insecticides, which persist into the flowering period from July to September when pollinating insects visit the flowers. If ants are tending the colony, a sticky trunk band prevents them from protecting the aphids from predators.
When does a silk tree flower in the UK and what do the flowers look like?
Albizia julibrissin flowers from July to September in the UK. The flowers are produced in clusters at the branch tips and consist almost entirely of long, thread-like pink stamens, giving the overall appearance of fluffy pompoms or soft paintbrushes. Each flowerhead catches the light and moves in a breeze, producing an effect quite unlike any other tree available to UK gardeners. A well-positioned mature silk tree in flower in late summer, paired with its feathery bipinnate foliage that folds at nightfall, is genuinely one of the most spectacular sights in an English garden.