Plant problems

Why Are My Prostanthera Leaves Curling?

Q: Why are my prostanthera leaves curling?

Prostanthera (mint bush) leaves curl and drop most commonly in UK gardens because of cold damage in hard winters (the small, aromatic, evergreen leaves curl and brown after frost below about -5 to -8°C), waterlogging in wet or poorly drained conditions (prostanthera has no tolerance of saturated soil; the roots rot rapidly in persistently wet conditions), or drought stress (the small leaves curl in dry conditions in containers or in very freely draining soils). Prostanthera is a genus of around 90 species of aromatic evergreen shrubs and sub-shrubs from Australia; several species and cultivars are grown in UK gardens, primarily as conservatory or cool greenhouse plants in colder areas, and as outdoor wall shrubs or border plants in the mildest UK gardens. The most commonly grown species in UK outdoor gardens include Prostanthera cuneata (alpine mint bush), a compact, very small-leaved, mat-forming shrub from the mountains of south-eastern Australia that produces white flowers with purple spots in early summer and is the hardiest species (to approximately -10°C in freely draining conditions); Prostanthera rotundifolia (round-leaved mint bush), a larger, more upright shrub with small, round, very strongly mint-scented leaves and masses of purple to mauve flowers in spring (April to May), which is one of the most spectacular flowering shrubs in the genus but is significantly less hardy than P. cuneata; and Prostanthera ovalifolia (oval-leaved mint bush), with purple flowers and oval, aromatic leaves, sometimes seen as a conservatory or container plant in UK gardens. All prostanthera species produce intensely aromatic foliage when bruised or rubbed; the scent is a clean, fresh, spearmint-like fragrance that is one of the plant's most distinctive and attractive qualities.

Q: Is prostanthera hardy in the UK?

Prostanthera hardiness in the UK varies significantly between species; Prostanthera cuneata (alpine mint bush) is the hardiest, surviving to approximately -10°C in freely draining, sheltered conditions, and can be grown in the open garden throughout most of England and Wales in a suitable position (rock garden, raised bed, sheltered border in full sun). Prostanthera rotundifolia is less hardy, surviving to about -5 to -7°C in sheltered conditions, and is best restricted to the very mildest UK gardens (sheltered coastal gardens in the south-west, mild urban positions) or grown as a conservatory plant. Most other prostanthera species are tender and unsuitable for UK outdoor cultivation except in the very mildest gardens. The key to maximising prostanthera cold hardiness in the UK is the combination of excellent drainage, full sun, and shelter from cold north and east winds; these three factors, working together, allow P. cuneata and similar hardy forms to survive temperatures that would kill plants in wetter, shadier, more exposed conditions. Cold combined with wet is far more lethal to prostanthera than cold alone; a prostanthera in a rock garden in perfectly draining, gritty soil can survive -10°C while the same plant in a moist border would be killed at -5°C. Container-grown prostanthera can be overwintered in a frost-free conservatory or cold greenhouse in colder UK areas; the plants are sufficiently compact and ornamental to be attractive as cool conservatory specimens in winter.

Q: How do I grow prostanthera in the UK?

Prostanthera cuneata (the hardiest species for outdoor UK cultivation) grows best in full sun in freely draining, lean to moderately fertile, acid to neutral soil; it is well suited to rock gardens, raised beds, and the front of sunny, well-drained borders. It comes from the rocky, high-altitude heathlands of south-eastern Australia where drainage is excellent, rainfall is seasonal, and the soil is acid and low in fertility; replicating these conditions in a UK garden produces the best growth and the greatest cold tolerance. Add coarse grit generously to heavier soils; a raised bed with a gritty, slightly acid growing medium (with some ericaceous compost mixed in) provides ideal conditions. Full sun is important for prostanthera both for flowering and for cold hardiness; plants in shade are less cold-tolerant and produce fewer flowers. Do not overwater or overfeed; prostanthera in lean, dry conditions produces harder growth that is both more fragrant and more cold-tolerant than the lush growth of rich, moist soil. Prune after flowering (June to July): trim back the flowered shoots by about one third to maintain compact shape and encourage vigorous new growth; do not cut into old, bare wood. Take semi-ripe cuttings in July to August: the cuttings root readily in a gritty compost under a cold frame and provide insurance plants for winter losses. Prostanthera rotundifolia should be treated as a conservatory plant or grown in the very mildest UK gardens only.

Q: Why has my prostanthera died suddenly?

Sudden death or rapid collapse in prostanthera in UK gardens is most commonly caused by one of two factors: waterlogging and root rot (the roots of prostanthera rot extremely rapidly in saturated soil; a plant that appears healthy in autumn can collapse completely within a single wet winter), or an unexpectedly severe frost event that exceeded the plant's cold tolerance threshold (prostanthera collapses rapidly in hard frost and may appear dead within days of a cold event). A less common but significant cause is Phytophthora root rot (water mould), which spreads through saturated soil and kills prostanthera and related Australian plants rapidly; the foliage collapses and turns brown without the progressive yellowing that typically precedes die-back from cold or drought. To distinguish between these causes: if the soil was wet before the collapse, root rot (waterlogging or Phytophthora) is the most likely cause. If the soil was dry and the collapse followed a cold event, frost is the most likely cause. Inspect the roots: healthy roots are white and firm; rotted roots are brown, mushy, and often smell unpleasant. If some roots are still firm and white, the plant may recover if drainage is improved and it is kept drier; if all roots are rotted, recovery is very unlikely. Prevention is the key: excellent drainage before planting, a coarse grit mulch, and avoidance of overwatering in autumn and winter prevent the root rot that is the most common cause of sudden prostanthera death in UK gardens.

Prostanthera (mint bush) is a genus of intensely aromatic Australian evergreen shrubs with small, spearmint-scented leaves and masses of tubular flowers in spring. The hardiest species, Prostanthera cuneata (alpine mint bush), survives to about -10°C in perfectly draining, gritty conditions and suits the rock garden or raised bed. Cold damage and waterlogging are the most common causes of leaf curl and plant loss; sudden collapse in wet conditions is often root rot.

Cold damage

Cold damage kills or damages the evergreen leaves of prostanthera in hard UK winters; even Prostanthera cuneata, the hardiest species, suffers leaf curl and browning below about -10°C in exposed conditions. Prostanthera rotundifolia and other less hardy species are damaged by any frost below about -5°C. The leaves curl inward and turn brown rapidly in hard frost; the stems may collapse. The combination of cold and wet is the most damaging scenario; in perfectly draining conditions the same plant can survive significantly lower temperatures.

What to do

Ensure excellent drainage before winter; this is the most important factor in frost survival. Position in full sun and shelter from north and east winds. Wrap young or borderline-hardy species in fleece from November to March. Do not cut back until May; surviving buds on apparently dead stems confirm the plant is alive. Take semi-ripe cuttings in July to August every year as insurance; prostanthera does not regenerate reliably from very old wood after severe die-back.

Waterlogging

Waterlogging causes rapid root rot and sudden plant death in prostanthera; the collapse can be dramatic and fast, with a previously healthy-looking plant turning completely brown within days of becoming waterlogged. Phytophthora root rot (a water mould) can spread through saturated soil and attacks prostanthera and related Australian plants particularly aggressively. The problem is most acute in heavy clay soils, in low-lying positions, in wet UK winters, and in containers without drainage holes or drainage material.

What to do

Plant only in perfectly draining soil; add 30% coarse grit by volume to heavier soils. Apply a coarse grit mulch around the base to prevent crown rot. In containers, use very gritty compost and ensure the pot drains freely; elevate the pot on pot feet to improve drainage. Reduce watering to an absolute minimum in autumn and winter. If Phytophthora is suspected, remove and destroy the affected plant; do not replant a susceptible species in the same soil without significant improvement to drainage and soil aeration.

Drought stress

Prostanthera is moderately drought-tolerant once established in freely draining soil; it is adapted to seasonal drought in its native Australian mountain habitats. The small leaves curl slightly at the margins in prolonged hot, dry conditions, but the plant typically recovers when watered or when temperatures cool. Container-grown plants are more vulnerable to drought stress and can dry out within a day in warm weather. Newly planted prostanthera in its first summer needs regular watering before the root system is established.

What to do

Water container-grown plants when the top 2 to 3 cm of compost is dry; do not allow to dry out completely in active growth. Established in-ground plants need watering only during prolonged dry spells in summer. Always water in the morning to allow the foliage to dry before nightfall; wet foliage overnight increases disease risk in prostanthera.

Frequently asked questions

Why are my prostanthera leaves curling?

Prostanthera leaves curl most commonly because of cold damage (the small evergreen leaves brown and curl below -5 to -10°C depending on species; drainage is the most important factor in cold tolerance), waterlogging causing rapid root rot (sudden collapse in wet conditions is often root rot or Phytophthora; plant in gritty, perfectly draining soil), or drought stress in containers or very dry positions (the leaves curl at the margins; water and they recover). Cold combined with wet is the most lethal combination.

Is prostanthera hardy in the UK?

Prostanthera cuneata (alpine mint bush) is the hardiest species, surviving to about -10°C in perfectly draining, gritty conditions; it can be grown in rock gardens or raised beds in most of England and Wales. Prostanthera rotundifolia is far less hardy (-5°C) and is best grown as a conservatory plant or in the very mildest UK gardens. Excellent drainage and full sun are the keys to maximising cold tolerance in any prostanthera species.

How do I grow prostanthera in the UK?

Grow P. cuneata in full sun in freely draining, lean, acid to neutral, gritty soil; the rock garden or raised bed is ideal. Do not add compost or feed heavily; lean soil produces harder, more fragrant, more cold-tolerant growth. Prune after flowering (June to July) by one third to maintain compact shape. Take semi-ripe cuttings in July to August annually as insurance. P. rotundifolia: conservatory or cool greenhouse in most UK areas; outdoor only in the mildest coastal gardens.

Why has my prostanthera died suddenly?

Sudden collapse is most commonly waterlogging or Phytophthora root rot, both preventable with excellent drainage. Check the roots: white and firm means the plant may recover if drainage is improved; brown and mushy means it is beyond saving. If the collapse followed a frost event in dry conditions, cold is the cause. Prevention is everything: perfectly draining gritty soil, a coarse grit mulch, and minimal autumn and winter watering are the most reliable safeguards.