Why Are My Coriander Leaves Curling?

Bolting and heat stress

The central challenge of growing coriander in the UK. Coriandrum sativum is a cool-weather annual programmed to set seed rapidly in response to warmth, long days, and heat stress. The broad, rounded lower leaves yellow and are replaced by fine, feathery thread-like upper stem leaves completely different in appearance and flavour. Bolting can begin within two to three weeks of sowing in warm UK spring and summer conditions. It cannot be reversed once started.

What to do

• Successive sowing every three to four weeks from March to September is the most reliable strategy; by the time one sowing is bolting, the next provides fresh leaves; earlier sowings (March to April) and later sowings (August to September) typically stay leafy longer than mid-summer sowings; grow in a cooler, partially shaded position (morning sun, afternoon shade) to slow bolting; keep compost consistently moist as drought stress triggers bolting; sow directly in the final growing position as root disturbance from transplanting also triggers bolting; try slow-bolt varieties such as 'Slowbolt' or 'Santos'.

Aphid infestations

The willow-carrot aphid (Cavariella aegopodii) and other aphid species attack the young, tender shoot tips and developing flower stems of coriander; feeding causes the young leaves to curl, pucker, distort, and the shoot tips to become stunted. Colonies build up rapidly on the tender foliage, particularly in spring from April to June.

What to do

• Squash or rub off aphid colonies with fingers, or knock off with a jet of water; for culinary coriander, hand control is strongly preferred over insecticide sprays to avoid contaminating the leaves with spray residues; since coriander plants are short-lived, the practical option when aphids have severely damaged the plant is simply to remove the affected plant and immediately sow fresh seed for the next succession.

Drought stress in containers

Particularly acute in supermarket herb tray pots, which contain many more seedlings than the small pot can sustain; the compost volume is very limited and can dry out within 24 hours in warm weather. Drought-stressed coriander shows very rapid wilting, leaf curl, and an even faster tendency to bolt because drought stress is itself a bolting trigger.

What to do

• Divide supermarket pots immediately: tip out the root ball and gently tease into two to four clumps of seedlings; replant each clump in a separate, larger pot with fresh multipurpose compost; the larger soil volume per plant extends productive life significantly; water by standing the pot in a saucer of water for 30 minutes to thoroughly rehydrate the compost from the base; keep consistently moist; harvest leaves regularly.

Overwatering and root rot

Less common than drought or bolting but does occur in pots without adequate drainage or in cold, wet, poorly ventilated conditions. The lower leaves yellow and the plant loses vigour as roots succumb to Pythium rot in persistently waterlogged, compacted compost.

What to do

• Ensure pots have adequate drainage holes and do not sit in a constantly filled saucer; use a good-quality, well-structured peat-free multipurpose compost that drains freely; allow the compost surface to begin to dry between waterings rather than keeping the compost in a constantly waterlogged state; in very cold, wet UK autumn and winter conditions, coriander is best grown undercover in a cold frame, greenhouse, or on a windowsill where waterlogging can be controlled.

Frequently asked questions

Why are my coriander leaves curling?

Coriander leaves curl most commonly because of bolting and heat stress (the most common and most frustrating coriander problem in UK gardens; cool-weather annual programmed to bolt and set seed rapidly in response to warmth long days and heat stress; broad rounded lower leaves yellow and are replaced by fine feathery thread-like upper stem leaves; cannot be reversed once started; successive sowing every 3 to 4 weeks March to September; cooler partially shaded position; consistent moisture to avoid drought stress as a bolting trigger; sow directly in final position as root disturbance triggers bolting; slow-bolt varieties such as 'Slowbolt' or 'Santos'), aphid infestations (willow-carrot aphid and others attack young shoot tips; leaves curl pucker distort and shoot tips become stunted; squash or rub off by hand; hand control preferred for culinary coriander; if severely damaged remove plant and sow fresh seed for next succession), drought stress in containers (especially supermarket herb tray pots; compost dries out within 24 hours in warm weather; drought stress itself a bolting trigger; divide supermarket pots immediately into 2 to 4 clumps of seedlings in separate larger pots; water by standing in saucer; keep consistently moist), or overwatering and root rot (less common; pots without adequate drainage holes; ensure drainage; free-draining compost; allow compost surface to begin to dry between waterings).

How do I stop coriander from bolting?

Successive sowing: most important single strategy; sow coriander seed in small quantities every 3 to 4 weeks from March to September; each sowing provides a fresh flush of pre-bolting plants; by the time one sowing is bolting the next provides fresh leaves; earlier (March to April) and later (August to September) sowings tend to stay leafy longer than mid-summer sowings; this makes bolting irrelevant rather than trying to prevent it. Cool partially shaded growing position: stays leafy longer in a cooler partially shaded position than in a hot full-sun position; morning sun and afternoon shade significantly slows the rate of bolting in a UK summer. Avoid drought stress: drought stress is one of the most reliable bolting triggers; keep compost consistently moist; do not allow compost in a container to dry out completely. Sow directly where it is to grow: root disturbance of transplanting is itself a bolting trigger; sow seed directly in the final container or bed; thin to recommended spacing rather than transplanting. Slow-bolt varieties: 'Slowbolt' and 'Santos' are UK-available varieties described as less prone to premature bolting; they still bolt in very warm weather but may stay leafy a few weeks longer than standard varieties.

Why does my coriander have different shaped leaves?

Very common observation and completely normal. Lower (juvenile) leaves: broad rounded irregularly toothed palmate in outline; look somewhat like very small finely toothed flat-leaf parsley; characteristic fresh coriander leaves most typically shown in food photography; most widely used in fresh cooking; most familiar bright strongly citrusy-herbal coriander flavour and aroma; produced by the plant in its early vegetative growth stage. Upper (adult and bolting stem) leaves: very fine feathery thread-like much more finely divided than the lower leaves; look almost like very fine dill fronds; completely different in appearance from the lower juvenile leaves; slightly less pleasant weaker different coriander flavour and aroma; less useful culinarily; progressive transformation from lower juvenile leaves to upper fine leaves is one of the clearest signs the plant is bolting. What to do: entirely normal; indicator of the plant's developmental stage; once the plant has produced extensively finely divided upper leaves and a visible flower stalk it has bolted; harvest any remaining lower leaves promptly and begin a new succession sowing; allow some plants to set seed if a supply of coriander seeds for cooking or self-sowing is desired.

How do I grow coriander from a supermarket pot?

The problem: supermarket coriander herb trays contain very many seedlings in a very small pot volume; grown hydroponically or in tightly packed peat-based compost in controlled greenhouse conditions; when taken home the plants face completely different conditions; pot dries out within 24 hours in a warm kitchen; roots already root-bound; many seedlings compete intensively for limited compost nutrients and water; plants bolt and die rapidly. What to do immediately: place in a saucer of water for 30 minutes to rehydrate from the base; harvest any leaves needed immediately; within 1 to 2 days divide the pot: tip the entire pot out and gently tease the root ball into 2 to 4 equal clumps of seedlings; replant each clump in a separate pot of good-quality multipurpose compost; the larger soil volume per plant gives each clump space to continue growing; do not transplant individual seedlings as root disturbance is too severe; transplant clumps. Growing position: brightest lightest position available that avoids direct hot midday and afternoon sun; east-facing or north-east facing windowsill often ideal; maintain consistent moisture; successive sowing of new seed alongside divided pots maintains continuous supply.