Plant problems

Why Are My Convolvulus Leaves Curling?

Q: Why are my Convolvulus cneorum leaves curling?

Convolvulus cneorum (silverbush or silver morning glory) leaves curl most commonly in the UK because of waterlogging and winter wet, cold damage from hard frost, drought stress in containers, or vine weevil attacking the roots. Convolvulus cneorum is a compact, dome-forming, evergreen Mediterranean sub-shrub with distinctive, narrow, intensely silver-silky leaves and funnel-shaped white flowers with a yellow centre produced from May to August; the contrast of bright white flowers against the gleaming silver foliage is one of the most effective combinations in the Mediterranean-style garden. The plant is native to rocky, dry, sun-baked hillsides around the western Mediterranean and requires excellent drainage, full sun, and alkaline to neutral soil; in UK conditions the most common causes of failure are wet, poorly draining soil in winter (the silver leaves brown and curl as root rot develops) and cold damage from temperatures below about -5 to -8°C, which is within the range of a typical hard UK winter. Drought stress in containers causes the silver leaves to curl inward; the plant is highly drought-tolerant in freely draining ground conditions but can dry out rapidly in a container in summer. Vine weevil larvae attack the roots, particularly in containers, causing sudden wilting and collapse of individual shoots.

Q: Is Convolvulus cneorum hardy in the UK?

Convolvulus cneorum is marginally hardy in the UK; it survives most UK winters in ideal conditions (very well-drained soil, sheltered south or west-facing position, mild climate) but can be killed or severely damaged by temperatures below about -5 to -8°C combined with wet soil, which can occur in a normal UK winter in many parts of the country. The hardiness of Convolvulus cneorum in the UK depends critically on drainage: in bone-dry, freely draining soil (gravel garden, raised bed, dry-stone wall, rock crevice), the plant can tolerate considerably lower temperatures than in average UK garden soil that holds moisture through winter. In ideal, well-drained conditions in southern England and coastal Wales and Ireland, Convolvulus cneorum typically survives most UK winters, though the leaves may be damaged or lost in severe frosts; it regrows from the woody crown in spring. In heavier or wetter soils, or in more northerly or exposed UK positions, the plant should be considered half-hardy and either grown in containers (brought inside or into a cold greenhouse for winter) or treated as a plant that may need replacing after a hard winter. A position against a south or west-facing wall in very free-draining gritty soil or in a gravel garden provides the best conditions for reliable UK outdoor survival. Protect the crown with a thick layer of grit or coarse gravel mulch in autumn, which improves drainage around the crown and provides some frost insulation without retaining moisture.

Q: How do I grow Convolvulus cneorum in the UK?

Convolvulus cneorum requires full sun, very well-drained alkaline to neutral soil, and shelter from cold north and east winds to thrive in UK conditions. The ideal growing position is a sunny bank, gravel garden, raised bed with gritty soil, or dry-stone wall top where drainage is naturally excellent and the position is sheltered from cold winds; a position against a south or west-facing wall is ideal in less mild UK gardens. The soil must be very free-draining; add substantial coarse grit or gravel to any soil that does not drain rapidly, or build a raised bed with a gritty, sharply draining growing medium. Convolvulus cneorum actually prefers poor, alkaline soil over rich, fertile, moisture-retentive soil; in rich conditions it grows more lushly but is more susceptible to root rot and produces fewer flowers. Unlike many UK garden plants, it does not benefit from added fertiliser or organic matter; a lean, gritty, sun-baked position produces the tightest, most silvery, most floriferous specimens. In containers: use a gritty loam-based compost (John Innes No. 2 with added grit at a ratio of 1:1 by volume) in a terracotta pot with a generous layer of crocks over the drainage hole; ensure the pot drains freely and does not sit in a saucer of water. Feed container plants with a liquid tomato fertiliser (high potassium, low nitrogen) monthly in summer to encourage flowering without excessive leafy growth. Take semi-ripe cuttings in July or August to ensure a supply of replacement plants; the plant can be short-lived, particularly in less than ideal drainage.

Q: How do I prune Convolvulus cneorum?

Convolvulus cneorum benefits from a light trim after the main flowering period in June or July to keep it compact and bushy; over-long, straggly stems can be shortened by about one third after flowering. Do not cut into old, bare wood as the plant does not regenerate well from completely leafless stems; always trim to where leafy growth is visible. In spring (March to April), cut back any frost-damaged or dead stems to healthy, live tissue; wait until new growth is clearly visible before pruning, as the extent of winter damage can be hard to assess on the silver-leaved stems before the season's growth begins. If the plant has become very woody and open at the centre after several years without pruning, it is usually better to replace it with a fresh plant raised from semi-ripe cuttings than to attempt a hard renovation; Convolvulus cneorum does not respond well to severe pruning. The plant naturally maintains a fairly compact, dome-shaped habit if given the right conditions (full sun, poor drainage, stress); in too-fertile or too-shaded conditions it becomes rangy and requires more frequent trimming to maintain a good appearance. The silver foliage is attractive throughout the year and the plant does not need to be cut back for winter; leave it untouched until spring.

Convolvulus cneorum (silverbush) is a compact, dome-shaped, evergreen Mediterranean sub-shrub with gleaming silver-silky leaves and white funnel flowers from May to August; one of the most effective plants for a sunny gravel garden or dry bank in the UK. It needs full sun, very well-drained soil, and alkaline to neutral conditions; the main threats in UK gardens are waterlogging in winter and cold damage from hard frost, both of which brown and curl the silver leaves.

Waterlogging

Waterlogging is the most common cause of failure and leaf curl in Convolvulus cneorum in UK gardens; the plant's roots and crown are highly intolerant of saturated, poorly aerated soil and rot rapidly in wet winter conditions, causing the silver leaves to brown from the tips inward and curl before the whole plant collapses. Plants in heavy clay soils, in low-lying positions, or in containers sitting in saucers of water are at highest risk. The symptoms can appear suddenly in winter or early spring: a plant that appeared healthy in autumn may be dead or dying by February after a wet winter on an unsuitable site.

What to do

Plant exclusively in very well-drained soil; a sunny gravel garden, raised bed with gritty compost, dry-stone wall top, or rocky bank is ideal. Add substantial coarse grit (at least 50% by volume) to any soil that does not drain freely and quickly. Avoid low-lying positions. Top-dress the soil surface around the plant with coarse grit to divert rainwater away from the crown. In containers, use a very gritty compost and ensure drainage holes are clear; never allow the pot to sit in standing water.

Cold damage

Convolvulus cneorum is marginally hardy, surviving to about -5 to -8°C in ideal, bone-dry, well-drained conditions; below this threshold the silver leaves blacken, curl, and may die, and the crown can be killed by a combination of cold and wet. The combination of frost and wet soil is far more damaging than frost in dry soil: in a dry gravel garden, the plant can survive temperatures that would kill it in a waterlogged bed. Cold damage is most visible in late winter (February to March) after a hard frost; affected shoots may appear dead but can sometimes recover if the crown is alive.

What to do

Choose the warmest, most sheltered position available (south or west-facing wall or bank) for the best winter survival. Apply a thick layer of coarse grit (5 to 8 cm) over the crown in November to insulate it from frost while improving drainage around the most vulnerable tissues. Do not cut back frost-damaged shoots until April when new growth reveals the extent of damage. Take semi-ripe cuttings in July or August as insurance against winter losses; these root readily and provide replacement plants if the parent plant is killed.

Drought stress

Convolvulus cneorum is highly drought-tolerant in the ground in freely draining soil; drought stress is primarily a problem for container-grown specimens, where the restricted root volume can dry out rapidly in warm summer weather. The silver, silky leaves curl inward when the plant is severely drought-stressed; though the plant recovers well from moderate drying out, repeated severe drought stress in a container weakens the plant over time. In the ground, even in the driest UK summers, established Convolvulus cneorum rarely needs watering.

What to do

In containers, check moisture every 2 to 3 days in warm weather and water when the top 5 cm of compost is completely dry; allow the compost to dry substantially between waterings, as the plant is adapted to dry-wet cycles rather than constant moisture. In the ground, no irrigation is needed once established. Use a terracotta pot rather than plastic; terracotta breathes and the compost dries more quickly, reducing the risk of overwatering which is more damaging than underwatering for this plant.

Vine weevil

Vine weevil (Otiorhynchus sulcatus) larvae feed on the roots of Convolvulus cneorum, particularly plants grown in containers; root damage causes sudden wilting and collapse of shoots that were previously healthy. Adults notch the margins of the silver leaves with characteristic irregular U-shaped cuts at night in summer. Container-grown plants are much more vulnerable than ground-planted specimens because vine weevils lay eggs preferentially in the compost of container plants and the larvae have no other food source than the plant's roots.

What to do

Apply the biological control nematode Steinernema kraussei to container compost in August or September when soil temperatures are above 5°C; drench the compost thoroughly and repeat in spring. Check the roots of any wilting container plant; cream, C-shaped larvae confirm vine weevil. Remove and destroy larvae and repot in fresh compost. For ground-planted specimens, vine weevil is rarely a serious problem; the population of natural predators in garden soil keeps larval numbers in check.

Frequently asked questions

Why are my Convolvulus cneorum leaves curling?

Convolvulus cneorum leaves curl most commonly because of waterlogging (the most serious threat; roots rot in wet winter soil and the plant collapses, with silver leaves browning and curling), cold damage from hard frost (the marginally hardy plant suffers below about -5°C, especially in wet conditions), drought stress in containers (the silver leaves curl inward when pot compost dries out completely), or vine weevil larvae in container compost. In perfect drainage and full sun, this is a trouble-free plant.

Is Convolvulus cneorum hardy in the UK?

Convolvulus cneorum is marginally hardy, surviving to about -5 to -8°C in very well-drained soil against a sheltered south or west-facing position. In wet, poorly draining soil, even milder frosts can be fatal. In most of southern England in a free-draining gravel garden or raised bed, the plant is reliably perennial. In heavier soils, colder gardens, or container culture, treat it as potentially needing replacement after a hard winter; keep rooted cuttings as insurance.

How do I grow Convolvulus cneorum in the UK?

Grow in full sun, very well-drained alkaline to neutral soil (gravel garden, raised bed, dry-stone wall, sunny bank); add at least 50% coarse grit to any soil that does not drain freely. Poor, lean soil produces the tightest, most silvery, most floriferous plants; avoid fertile, moisture-retentive soil and fertilisers. Top-dress with coarse grit around the crown. In containers, use gritty John Innes No. 2 in a terracotta pot; water sparingly and never allow to sit in standing water.

How do I prune Convolvulus cneorum?

Trim lightly by about one third after the main flowering flush in June or July, cutting to where leafy growth is visible; never cut into bare old wood. In spring, remove frost-damaged or dead stems to healthy tissue once new growth is visible. Replace old, very woody, bare-centred plants with fresh specimens raised from semi-ripe cuttings taken in July or August rather than attempting hard renovation pruning.