Why Are My Colchicum Leaves Curling?

Waterlogging

Waterlogging is a common cause of colchicum corm deterioration. The large corms are adapted to Mediterranean climates with dry summers and relatively free-draining soils, and persistently wet, anaerobic UK clay causes them to deteriorate. The large spring leaves curl and yellow as the rotting corm base fails to supply adequate resources. Waterlogging during the summer dormant period, when the corms are leafless and cannot resist fungal infection through active growth, is particularly damaging. Heavy clay soil in low-lying positions is the highest-risk environment.

What to do

• Plant colchicum in well-drained soil. In heavy clay, work generous amounts of coarse grit and organic matter into the planting area before planting, or install the corms in a raised bed.
• Avoid low-lying positions where water collects in summer. The summer dormant period is when waterlogging is most damaging, as the corm has no active growth to resist fungal attack.
• A position that dries out in summer, such as at the base of a south-facing wall or under the canopy of a deciduous tree, provides natural protection from summer waterlogging.

Smoulder disease

Smoulder (Botrytis colchicorum) is a specific fungal disease of colchicum that causes the emerging spring leaves to be covered in a grey, fuzzy fungal mould and to curl, distort, and collapse. The disease spreads rapidly through dense plantings in cool, damp spring weather with poor air circulation. Smoulder also infects the corms, producing hard, black resting bodies (sclerotia) that persist in the soil and reinfect plants in subsequent years. Smoulder is distinct from the general grey mould (Botrytis cinerea) that affects many other plants.

What to do

• Remove and bin all affected plant material at the first signs of smoulder. Do not compost infected leaves or corms, as the sclerotia survive the composting process.
• Improve air circulation around dense plantings by spacing corms more widely when replanting. Good air flow is the most important preventive measure against smoulder in established plantings.
• Apply a copper-based fungicide at the first signs of infection in spring, when the leaves are emerging. This provides some protection against spreading smoulder in the current season.
• If smoulder is persistent in a planting, lift the corms, inspect them for sclerotia on the outer scales, remove any affected corms, and replant in a different position.

Corm rot

Fungal corm rot causes colchicum to produce curling, yellowing spring leaves or to fail to emerge at all. The large corms deteriorate from the base plate upward, and when cut through show brown, rotted tissue rather than the healthy cream-white of a sound corm. Corm rot is most common in waterlogged conditions, in corms that were damaged mechanically during lifting or planting, and in corms purchased in poor condition.

What to do

• Purchase firm, heavy, plump colchicum corms and plant promptly in late summer. Inspect before planting and discard any that are soft, lightweight, or show brown discolouration on the basal plate.
• Handle corms carefully during lifting and replanting to avoid mechanical damage that creates entry points for fungal pathogens.
• Dust corms with sulphur powder before planting to reduce rot risk, particularly in any soil that does not drain very freely.

Slug damage

Slugs feed on the large colchicum leaves in spring, leaving characteristic irregular holes and slime trails. Heavy slug feeding causes the leaves to curl as the tissue is lost and the structural integrity of the leaf is compromised. Colchicum leaves are large and soft, making them an attractive target for slugs in wet spring weather. The autumn flowers are also susceptible to slug damage as they emerge at ground level in September and October.

What to do

• Apply biological nematode slug control (Phasmarhabditis hermaphrodita) around colchicum plantings in spring when soil temperatures are above 5 degrees Celsius. This is effective against soil-dwelling slugs that are hardest to control by other means.
• Apply a layer of sharp grit around the base of emerging colchicum leaves. Slugs are deterred by sharp surfaces and this significantly reduces leaf damage.
• Hand-collect slugs from colchicum in the evening with a torch, particularly in wet spring weather. Colchicum leaves are large enough to make slug damage very conspicuous.

Cutting leaves too early

Cutting or tying back colchicum leaves before they have died back naturally is one of the most common causes of declining flowering vigour. The large spring leaves are the only source of energy for the corm for the entire year, and removing them prematurely prevents the corm from building up the reserves it needs to produce its autumn flowers. After several years of early leaf removal, the corm becomes exhausted and the autumn flowers become progressively smaller and fewer, or cease entirely. The leaves must be left to die back completely on their own, even though they are large and untidy.

What to do

• Allow colchicum leaves to die back completely and naturally. This typically takes until early June in UK gardens. Only remove the leaves once they are fully yellow and completely dry.
• Plant colchicum among ground-cover plants such as low-growing geraniums, ajuga, or grass, where the dying colchicum leaves are partially hidden by the surrounding growth without being cut.
• Never tie colchicum leaves into a knot, as is sometimes suggested. This dramatically reduces photosynthesis and is equivalent to cutting the leaves early.

Frequently asked questions

Why are my colchicum leaves curling?

Colchicum leaves curl most often from waterlogging or smoulder disease (Botrytis colchicorum). The large, glossy, strap-like leaves that emerge in spring curl and yellow when the corm is deteriorating in wet, anaerobic soil. Smoulder disease causes the emerging leaves to be covered in grey fungal mould and to curl, distort, and collapse, particularly in cool, damp springs. Both problems are common in UK gardens where spring is wet.

Why does colchicum flower without leaves?

Colchicum flowers appear in autumn, entirely without leaves, which is why they are called naked ladies or naked boys. The flowers emerge directly from the corm in September and October and are pollinated before any foliage appears. The large, broad leaves emerge separately in late winter and spring, persist through to early summer, and then die back completely. The leaves must be left to die back naturally, as they manufacture the energy the corm needs to produce the following autumn's flowers.

When should I plant colchicum corms?

Plant colchicum corms in late summer, from July to September, at a depth of 8 to 12 cm. The corms flower very shortly after planting, so even newly planted corms will often produce flowers in their first autumn. Plant in well-drained, fertile soil in full sun or light dappled shade. The key requirement is that the soil must drain freely: colchicum corms are large and deteriorate rapidly in waterlogged conditions.

Is colchicum (autumn crocus) poisonous?

Yes, all parts of colchicum are highly toxic, containing colchicine and related alkaloids. The plant is far more toxic than true autumn crocus (Crocus speciosus) and is one of the most poisonous plants in UK gardens. Colchicine poisoning causes severe gastrointestinal symptoms and can be fatal. Keep the plant away from children and pets. When handling colchicum corms or foliage, wear gloves as the alkaloids can be absorbed through skin. Never confuse colchicum leaves with wild garlic (Allium ursinum), with which they can look superficially similar.