Plant problems

Why Are My Leptospermum Leaves Curling?

Q: Why are my leptospermum leaves curling?

Leptospermum (tea tree or manuka) leaves curl and drop most commonly in UK gardens because of cold damage in hard winters (leptospermum is of varying hardiness depending on species and is damaged by hard frost; the small, narrow, aromatic leaves curl and bronze before dropping), waterlogging in wet or heavy soils (most leptospermum species come from freely draining, acid, heath-like habitats and their roots rot in saturated conditions), or drought stress in containers or very freely draining soils in hot summer conditions. Leptospermum is a genus of around 80 to 90 species of aromatic evergreen shrubs and small trees, primarily from Australia and New Zealand (the common name 'tea tree' refers to the fact that Captain Cook's crew reputedly made tea from the leaves of New Zealand species). Several species and cultivars are grown in UK gardens; the most commonly grown are cultivars and hybrids derived primarily from Leptospermum scoparium (manuka, from New Zealand and south-eastern Australia), which produce small, single, five-petalled flowers in white, pink, or red (in single or double forms) in May to June over neat, wiry, aromatic foliage. The most widely available cultivars include 'Red Damask' (double, deep red flowers), 'Nichollsii' (single crimson flowers, purple-bronze foliage), 'Snow Flurry' (double white flowers), and many others. Leptospermum scoparium cultivars are marginally hardy in the UK; they are reliable outdoors in the mildest coastal and urban gardens and are increasingly grown as conservatory or patio container plants in colder areas.

Q: Is leptospermum hardy in the UK?

Leptospermum hardiness in the UK varies by species; Leptospermum scoparium cultivars are generally considered hardy to about -5 to -8°C in a sheltered position with freely draining, acid soil, making them reliable outdoor plants only in the mildest parts of the UK (sheltered coastal gardens in Cornwall, Devon, the Isles of Scilly, the Channel Islands, coastal western Ireland, and mild urban positions in London and similar cities). In most of the UK, L. scoparium cultivars are best grown as container plants that can be moved under glass or into a frost-free conservatory from October to May. Leptospermum lanigerum (woolly tea tree, from south-eastern Australia) is considered significantly hardier than L. scoparium, with some forms surviving to approximately -10°C or below in freely draining conditions; it has smaller, more silvery-grey leaves and small white flowers in June to July, and is worth trying as an outdoor shrub in sheltered positions in much of southern England and Wales. The combination of cold and wet is far more damaging than cold alone for all leptospermum species; a plant in freely draining, acid soil against a sheltered wall survives much lower temperatures than one in heavy, wet clay. In UK outdoor gardens, the most reliable positions for leptospermum are the same as for cistus and Californian lilac (ceanothus): south or west-facing walls or banks with freely draining, acid to neutral, lean soil in full sun.

Q: How do I grow leptospermum in the UK?

Leptospermum grows best in a sunny, sheltered position in full sun, in freely draining, acid to neutral (ideally pH 5.0 to 6.5), lean, moderately fertile soil; it does not grow well in alkaline or chalky soil and does not tolerate waterlogging. The plants are from heathland and scrub habitats in Australia and New Zealand where the soil is acid, freely draining, and low in fertility; this defines the ideal UK growing conditions precisely. Do not add lime or alkaline fertiliser; use an ericaceous fertiliser in spring if needed. Against a warm, sheltered, south or west-facing wall is the ideal outdoor position in the UK; the wall provides both frost amelioration and shelter from cold wind. Mulch with pine bark or ericaceous compost to maintain soil acidity and conserve moisture. Prune lightly after flowering (June to July) to maintain compact shape; leptospermum does not regenerate from old bare wood, so pruning must be gentle (take no more than one third of the stem length at any single pruning). Do not prune in autumn or winter. For container growing: use ericaceous (acid) compost; water with rainwater in hard-water areas; bring under glass from October to May in most UK areas; repot every two to three years in spring. Container-grown leptospermum flowers better when slightly pot-bound; do not over-pot. Feed container plants with a liquid ericaceous fertiliser monthly from April to August.

Q: Why is my leptospermum not flowering?

Leptospermum flowers on the previous year's short lateral shoots; the small, five-petalled flowers are produced abundantly from the leaf axils along the stems in May to June on healthy, well-grown plants. Common reasons for poor or absent flowering: the plant was pruned too hard or at the wrong time (pruning in autumn or cutting back hard in spring removes the developing flower buds that are forming on the previous year's laterals; prune only lightly and only immediately after flowering in June to July); soil pH too high (alkaline conditions weaken the plant progressively and reduce flowering; use ericaceous compost and rainwater); cold damage to the flowering shoots in winter (a hard frost kills the laterals that carry next season's flowers; the plant survives but produces fewer flowers); or the plant is in too much shade (leptospermum flowers poorly in shade and needs full sun for the most prolific display). In the right conditions (full sun, acid soil, freedom from frost), leptospermum in flower is spectacular: the small stems are so densely covered in tiny flowers that the foliage may be almost invisible beneath the massed blooms, producing a stunning effect in May and June. The flowering display typically lasts two to four weeks; in a warm, sheltered position it may extend into July.

Leptospermum (tea tree or manuka) is a genus of aromatic Australian and New Zealand evergreen shrubs grown for their masses of small five-petalled flowers in May to June; the most popular UK cultivars are derived from Leptospermum scoparium and come in white, pink, and red. Most forms are marginally hardy and best grown as container plants in cool conservatories in most UK areas. Cold damage and waterlogging are the most common outdoor causes of leaf curl; excellent drainage and acid soil are non-negotiable.

Cold damage

Cold damage is the most common cause of leaf curl and plant death in leptospermum in UK gardens; Leptospermum scoparium cultivars are damaged by frost below about -5 to -8°C and can be killed entirely in hard UK winters. The small, narrow leaves curl, bronze, and drop progressively after hard frost; the stems may blacken and collapse. Even plants that appear dead may resprout from the base if the rootstock has survived; do not remove apparently dead material until May. Leptospermum lanigerum is significantly hardier than L. scoparium and is a better choice for outdoor UK gardens in less mild areas.

What to do

Move container-grown leptospermum under glass or into a frost-free conservatory from October to May; this is the most reliable approach in most of the UK. For outdoor specimens, choose the warmest, most sheltered south or west-facing wall position; mulch the root zone in autumn; wrap young plants in fleece from November to March. Take semi-ripe cuttings in July to August annually as insurance. Do not prune winter-damaged growth until May.

Waterlogging

Waterlogging causes rapid root rot in leptospermum; the plants come from freely draining, acid heathland habitats and have no tolerance for persistently wet, saturated soil. Symptoms include progressive yellowing, leaf curl, and sudden collapse. In heavy UK soils the problem is most acute in winter when the soil remains wet for prolonged periods. Container-grown leptospermum is also vulnerable if the compost is consistently wet; use a very free-draining ericaceous mix and ensure the pot drains freely.

What to do

Plant in freely draining, acid soil; add generous quantities of coarse grit to heavy soils. In containers, use free-draining ericaceous compost; add 20 to 30% perlite or coarse grit to standard ericaceous compost to improve drainage. Elevate containers on pot feet to ensure free drainage. Reduce watering to a minimum in autumn and winter; the plants need far less moisture when dormant than in active growth.

Drought stress

Leptospermum can suffer drought stress in containers or in very freely draining, sandy soils during hot UK summers; the small leaves curl and the plant looks stressed in very dry conditions. Container-grown plants are most vulnerable, as the restricted root volume dries out quickly in warm weather. The fine, wiry stems and small leaves of leptospermum give the plant some drought tolerance, but consistent moisture in the root zone is needed for the best growth and flower production.

What to do

Water container-grown plants regularly in summer; use rainwater in hard-water areas (tap water in alkaline areas gradually raises pH and harms the plant). Check containers every 1 to 2 days in warm weather; the compost should be moist but not wet. In the ground, mulch with pine bark to conserve moisture. The plants recover quickly from moderate drought stress when watered.

Frequently asked questions

Why are my leptospermum leaves curling?

Leptospermum leaves curl most commonly because of cold damage (the small leaves bronze and curl below -5 to -8°C; move containers under glass from October; take cuttings as annual insurance), waterlogging in wet conditions (roots rot rapidly in saturated soil; use very free-draining ericaceous compost and reduce winter watering), or drought stress in containers or dry positions (water regularly with rainwater; recover quickly when watered). Cold combined with wet is the most lethal combination.

Is leptospermum hardy in the UK?

Most popular L. scoparium cultivars are hardy to only -5 to -8°C and are best grown in containers that can be brought under glass from October to May in most UK areas. Leptospermum lanigerum (woolly tea tree) is significantly hardier, surviving to about -10°C in freely draining conditions. For outdoor UK gardens outside the very mildest areas, L. lanigerum or other hardier species are the most reliable choices.

How do I grow leptospermum in the UK?

In containers: use free-draining ericaceous compost; water with rainwater; feed monthly with liquid ericaceous fertiliser from April to August; bring under glass from October to May. Outdoors (mild gardens only): south or west-facing wall, freely draining, acid soil, full sun; mulch with pine bark; prune lightly after flowering in June to July (never hard; does not regenerate from old wood). Alkaline soil or tap water progressively harms the plant.

Why is my leptospermum not flowering?

Common causes: pruned at the wrong time (only prune lightly in June to July after flowering; any other pruning removes developing flower buds); soil or compost pH too high (alkaline conditions weaken the plant; use ericaceous compost and rainwater); cold damage to flowering shoots in winter; or too much shade (full sun is needed for the most prolific display). In the right conditions, leptospermum in flower covers every stem tip with small five-petalled blooms in a spectacular two to four week display.