Acacia dealbata, the silver wattle, is one of the most spectacular trees you can grow in a mild British garden. From January to April its branches are smothered in small golden-yellow pompom flowers that fill the air with a sweet, powdery scent. Garden writers love to reach for superlatives, and with this plant they are usually justified. But the feathery, silvery-blue bipinnate foliage that makes it so attractive is also the first part of the plant to signal distress. When the leaves curl, the tree is telling you something is wrong.
Most of the time, in the UK, that something is cold.
Understanding Acacia dealbata
Native to south-eastern Australia and Tasmania, Acacia dealbata is a fast-growing evergreen tree that reaches 5 to 15 metres in cultivation. It belongs to the subfamily Mimosoideae within the family Fabaceae, and the fine, finely divided bipinnate leaves give it a delicate, almost fern-like texture quite unlike most trees grown in Britain. In its native habitat it grows in well-drained, often rocky soils on hillsides, which tells you something important: it expects good drainage and resents sitting in wet clay.
In the UK it is mostly confined to mild coastal gardens in Cornwall, Devon, the Isles of Scilly, Pembrokeshire, western Scotland, and the western coast of Ireland. The urban heat island of London and some other south-eastern cities allows it to thrive in sheltered spots there too. It is simply not reliably hardy enough for cold inland gardens without serious protection. A severe frost is the number-one killer, and curling leaves are almost always the first visible sign of the damage it causes.
The cut stems are sold by florists as "mimosa", particularly in late winter and early spring. Despite the name, Acacia dealbata is not a true Mimosa (that is a different genus entirely) but the florist trade has used the name for so long that it has become universal. Grow it in your garden and you can cut your own.
Cause 1: Cold Damage and Frost Injury
By some distance, cold damage is the most common reason for leaf curl on Acacia dealbata in UK gardens. The foliage is killed by temperatures below roughly -5 to -8°C, depending on the duration of the cold spell and how established the plant is. When frost strikes, the feathery leaflets curl, then brown, then die. In a moderately cold spell the damage may be confined to the growing tips. In a hard winter the entire plant above ground can be killed.
Young plants are far more vulnerable than established ones. A tree that has had five or six growing seasons to sink deep roots and develop a substantial trunk may shrug off a night at -6°C that completely destroys a plant in its first or second year. If you are growing mimosa in a marginal location, the first two or three winters are the most dangerous.
The roots are hardier than the shoots. Even after a severe winter that kills all visible growth, the rootstock often survives and throws up new shoots in spring. These reshoot vigorously and can reach flowering size remarkably quickly, because the root system is already fully developed. Do not be too quick to write the tree off.
The most important rule after a frost is patience. Do not prune frost-damaged growth until late spring, when new growth is clearly visible. Then cut into the damaged stems and look for a ring of green tissue just beneath the bark (the cambium). Any stem showing green cambium is alive, even if the leaves are entirely dead. Remove only the genuinely dead wood, cutting back to living tissue. If you prune in February you risk cutting off shoots that would have recovered, and you also expose fresh cuts to any remaining frost.
To reduce the risk of frost damage, plant in the most sheltered position available: ideally against a south or south-west-facing wall, where the structure stores daytime heat and deflects cold winds. Protect young plants during cold spells with a double layer of horticultural fleece. The cultivar 'Gaulois Astier' has a reputation for being somewhat hardier than the straight species, though no form of Acacia dealbata is reliably frost-proof in the UK.
In autumn, apply a thick mulch of garden compost or bark around the base of the tree. This insulates the root zone and markedly improves the chances of a plant reshooting from the base if the top growth is killed.
Cause 2: Drought Stress
The thin, feathery leaflets of Acacia dealbata lose water quickly, and the tree's exceptionally fast growth rate (1 to 2 metres per year in good conditions) means it has a high water demand. In free-draining coastal soils, or in any garden during a prolonged dry summer, the bipinnate leaves begin to curl at their tips and margins as the plant struggles to manage water loss. If drought stress is severe or prolonged, the curl extends to whole fronds and the tree drops foliage.
Young trees are the most vulnerable, because they have not yet developed the deep root system that allows established plants to access moisture from well below the surface. Water young trees thoroughly and regularly throughout the first two or three growing seasons, especially during dry spells. A good soak once a week is better than shallow watering every day: deep watering encourages the roots to follow moisture down into the soil rather than concentrating near the surface.
Once established, Acacia dealbata is reasonably drought-tolerant. The fast growth that makes it so exciting also works in its favour once the roots are established: the tree can find water from a wide area. But in an exceptionally hot dry summer even a large established tree can show leaf curl at the tips, which is generally not cause for alarm.
Other Causes
Aphids occasionally colonise the soft new shoot tips of Acacia dealbata in spring and early summer. The feathery leaflets curl around the feeding colonies, which are most visible if you unfurl the curled tip. A strong jet of water will dislodge most colonies. Insecticidal soap is effective but should only be used outside the flowering season, as the flowers attract beneficial insects and pollinators in quantity.
Root rot caused by waterlogging is the main reason Acacia dealbata fails in inland UK gardens. The tree evolved in well-drained Australian woodland soils and rots quickly in heavy clay or poorly-drained ground. If leaves yellow and curl and the soil stays wet for extended periods, poor drainage is almost certainly the cause. Always plant in very free-draining soil. On a heavy site, raise the planting level by mounding the soil or building a low raised bed, and incorporate plenty of grit into the planting hole.
Scale insects, particularly brown soft scale (Coccus hesperidum), can infest the stems and cause leaves to yellow and curl as the insects drain sap and excrete honeydew. Inspect the stems closely: an infestation looks like small brown lumps encrusting the bark. Treat with horticultural oil applied in late spring before the crawlers hatch.
On very alkaline soils, Acacia dealbata can develop chlorosis: leaves yellow between the veins because the plant cannot absorb sufficient iron and manganese. The tree prefers neutral to slightly acid soil. On chalk or limestone, planting in a large raised bed filled with ericaceous compost and adding a chelated iron feed can help, but heavy alkalinity is difficult to correct long-term.
Prevention Summary
Plant in the warmest, most sheltered position available, ideally against a south-facing wall with overhead protection from a roof overhang or established canopy. Ensure the soil drains freely before planting: dig in grit, raise the bed if necessary, or choose a naturally sloping site. Water young trees deeply throughout the first two to three growing seasons, especially in warm dry spells. Protect plants in their first few winters with horticultural fleece during cold spells and a thick mulch around the base in autumn. If you are in a borderline climate, choose 'Gaulois Astier' over the straight species.
The rewards for getting the conditions right are considerable. Mimosa growing in a suitable UK garden is one of the most spectacular plants you can own: flowering in the depths of winter when almost nothing else is in bloom, growing at a pace that feels almost tropical, and providing cut stems that fill the house with scent. In the right microclimate, it is not a difficult tree at all.
Frequently Asked Questions
Where in the UK can you grow mimosa (Acacia dealbata)?
Acacia dealbata grows reliably in mild coastal gardens in Cornwall, Devon, the Isles of Scilly, Pembrokeshire, western Scotland, and the west coast of Ireland. Sheltered city gardens in London and the south-east also work well thanks to the urban heat-island effect. Cold inland gardens without significant shelter are generally too risky, as a hard frost below about -8°C can kill the plant outright.
What should I do after my mimosa has been hit by frost?
Do not prune frost-damaged growth immediately. Wait until late spring, when you can clearly see which stems are alive. Cut into a stem and look for green cambium just beneath the bark. Remove only dead wood. Even plants that appear entirely dead above ground will often reshoot vigorously from the base and can flower the following winter.
Is the mimosa sold by florists the same plant?
Yes. The sprays of small yellow pompom flowers sold by florists as "mimosa", especially around early March, are cut stems of Acacia dealbata. The plant is not a true Mimosa (which is a separate genus) but the florist trade name has stuck. If you grow Acacia dealbata in your garden you can cut your own stems when it flowers in January to April.
Can mimosa recover if it dies back completely in winter?
Often, yes. The roots of an established Acacia dealbata are hardier than the top growth and can survive temperatures that kill all the visible stems. New shoots frequently emerge from the base or from low on the main trunk in spring. These grow extremely fast, and in a mild-climate garden the plant can reach flowering size again within a year or two.
Why are the leaves on my potted mimosa curling?
Container-grown mimosas are far more vulnerable to both drought and frost than garden-planted trees, because the root system is confined and the pot offers no insulation. In summer, curl at the leaf tips usually means the compost dried out too quickly. In winter, the whole plant may collapse after a single cold night. Move pots to a sheltered position against a south-facing wall, water regularly in warm weather, and wrap the pot in fleece or bubble wrap during cold spells.