Why Are My Scilla Leaves Curling?

Waterlogging

Waterlogging is the most common cause of scilla decline in UK gardens. The small bulbs are adapted to well-drained, humus-rich soil that dries out through summer, and persistently wet or waterlogged conditions cause the bulbs to rot rapidly. The narrow, strap-like leaves yellow and curl as the deteriorating bulb cannot supply adequate resources to the expanding growth. This is most likely in heavy clay soil during a wet autumn or winter, in low-lying positions that collect water, or in containers without adequate drainage. The very small size of scilla bulbs means they are more vulnerable to waterlogging damage than larger bulbs.

What to do

• Plant scilla in well-drained but moisture-retentive soil. The ideal conditions are leafy, humus-rich soil under deciduous trees that retains some moisture through the growing season but dries in summer under the tree canopy.
• In heavy clay, improve drainage significantly with coarse grit or plant in raised beds. The small scilla bulbs have very limited ability to resist wet-soil fungal pathogens.
• In containers, use a free-draining bulb compost and ensure excellent drainage holes. Raise containers off the ground in winter to allow free drainage.

Vine weevil

Vine weevil larvae are a serious pest of scilla, particularly in containers. The small, cream-coloured larvae feed on the tiny scilla bulbs from late summer through winter, and because scilla bulbs are so small, even limited larval feeding can destroy the bulb entirely. In spring, the damaged bulbs produce few, narrow, curling leaves that wilt rapidly. In severe infestations, no growth appears at all from planted bulbs. Container-grown scilla is far more at risk than border plants, as the larvae are concentrated in a limited volume of compost.

What to do

• Apply biological nematode control (Steinernema kraussei) to containers and susceptible areas in late summer when soil temperatures are above 5 degrees Celsius.
• When emptying scilla containers after the season, inspect the compost carefully for the characteristic c-shaped, cream larvae and destroy any found.
• Use a vine weevil killer drench in containers where the pest is a known problem: apply in autumn before the larvae cause significant bulb damage.
• Check for the adult vine weevil's characteristic notched leaf edges in summer as a warning sign that eggs are being laid nearby.

Bulb rot

Bulb rot, caused by fungal pathogens, affects scilla in wet or poorly drained conditions. The very small bulbs deteriorate rapidly once fungal rot takes hold, as they have limited reserves compared to larger bulbs. Rotting scilla bulbs produce curling, yellowing leaves in spring and may fail to produce any growth at all. Bulb rot is most common in bulbs planted in wet soil, stored damp before planting, or purchased in poor condition.

What to do

• Purchase firm, plump scilla bulbs from reputable suppliers and plant promptly in autumn. The very small bulbs are at risk of desiccation if stored for too long.
• Dust bulbs with sulphur powder before planting to provide protection against fungal rot.
• Inspect all bulbs before planting and discard any that are soft or show brown discolouration.
• Ensure excellent drainage: this is the most important preventive measure against scilla bulb rot.

Squirrel and mouse damage

Squirrels and mice dig up and eat scilla bulbs, though they are less preferentially targeted than larger, more nutritious bulbs such as crocus, tulip, and narcissus. In gardens with heavy squirrel or mouse pressure, scilla bulbs can still be significantly depleted. The signs are small excavation holes and the disappearance of expected growth in spring. Mice in particular may tunnel to scilla bulbs without disturbing the soil surface, leaving no visible disturbance until the expected growth fails to appear.

What to do

• Cover newly planted scilla areas with fine chicken wire mesh pegged to the ground until early spring. The very small size of scilla bulbs makes them harder to smell and locate than larger bulbs, providing some natural protection.
• Plant at the deeper end of the recommended range (8 cm): deeper bulbs are harder to detect and excavate.
• In containers, raised off the ground on pot feet, mice are less able to access the bulbs than in the open ground.

Overcrowding

Scilla increases readily by offset production and, in suitable conditions, by prolific self-seeding. Over many years, established colonies become very densely packed and individual bulbs begin to produce narrower, more curling leaves as they compete for space. Flowering declines in very overcrowded colonies as individual bulbs lack sufficient resources. This is usually a slow process in well-established naturalistic plantings, but in containers or restricted borders it can become a problem more quickly.

What to do

• Divide overly congested scilla in late summer or early autumn, when the bulbs are dormant and easy to handle. Separate into small groups and replant at 5 to 8 cm apart.
• In naturalistic woodland plantings, overcrowding of scilla is usually self-regulating as the colony spreads outward. Intervention is rarely needed unless flowering declines significantly.
• In containers, refresh the compost and divide bulbs every two to three years to maintain vigorous growth and flowering.

Frequently asked questions

Why are my scilla leaves curling?

Scilla leaves curl most often from waterlogging or vine weevil damage. Waterlogging in poorly drained soil causes the narrow, strap-like leaves to yellow and curl as the small bulbs deteriorate. Vine weevil larvae feed on the small bulbs from below, and in spring the damaged bulbs produce few, weak, curling leaves as they cannot support normal growth. Both problems are most common in containers.

What is the difference between scilla and muscari?

Scilla (squill) and muscari (grape hyacinth) are both small, blue-flowered spring bulbs that are often confused. Scilla produces open, star-shaped flowers in intense blue or violet, whereas muscari produces dense, urn-shaped flower clusters resembling tiny bunches of grapes. Scilla flowers are more open and visible individually, while muscari's appeal is the dense spike of tiny flowers. Both naturalise readily and are excellent front-of-border or underplanting bulbs.

When should I plant scilla bulbs?

Plant scilla bulbs in autumn from September to November, at a depth of 5 to 8 cm in well-drained but moisture-retentive soil. They perform best in full sun or light dappled shade and naturalise readily in short grass, at the base of hedges, and under deciduous trees. Plant in bold drifts of at least 20 to 30 bulbs for the best visual impact. Scilla siberica is the most commonly grown species in UK gardens.

Do scilla spread on their own?

Yes, scilla spreads both by producing offset bulbs and by self-seeding. In favourable conditions, the intense blue of Scilla siberica can spread to carpet large areas over successive seasons, particularly under deciduous trees where the open canopy in spring allows adequate light while the leaf litter creates ideal humus conditions. Scilla bifolia in particular is a prolific self-seeder and can naturalise very widely in a garden.