Why Are My Nemesia Leaves Curling?

Overwatering and root rot

Nemesia is a small plant with a relatively modest root system, and it is easily overwatered, particularly in containers. When roots sit in wet, poorly draining compost, they rot and the plant can no longer take up water, causing the leaves to curl, yellow, and wilt even though the compost feels damp. The collapse happens quickly once root rot is established, and by the time the symptoms are obvious it may be too late to save the plant.

Container nemesia planted in heavy, moisture-retentive compost or in pots without drainage holes is particularly at risk, especially in cool, cloudy periods when the compost takes far longer to dry out than in warm sunny weather.

What to do

• Use a well-draining, peat-free compost with added perlite for container nemesia, and ensure the pot has drainage holes.
• Water only when the top centimetre of compost has dried out, reducing frequency significantly in cool or cloudy weather.
• If root rot is suspected, carefully remove the plant from its pot. If the roots are dark and mushy, trim away the damaged material, allow the roots to dry briefly, and replant in fresh, dry compost.
• Never leave nemesia sitting in a waterlogged saucer.

Excess heat

Nemesia originates from the cool, coastal regions of South Africa and is adapted to mild temperatures rather than prolonged summer heat. When temperatures regularly exceed 22 to 25 degrees Celsius, nemesia typically stops flowering, becomes leggy, and may develop curling, yellowing leaves as a heat-stress response. This is particularly common in a hot July or August in the UK. The plant is not dying: it is simply going semi-dormant until conditions become more favourable.

Container plants in full sun on a south-facing patio are most vulnerable, as the pot itself heats up and the roots experience higher temperatures than plants in the open ground.

What to do

• Move container plants to a slightly shadier position during the hottest weeks of summer, such as a spot that receives morning sun only.
• Cut the plant back by one third to one half in midsummer to remove tired, heat-stressed growth and encourage a fresh flush from the base.
• Keep watered during the semi-dormant period even though the plant is not actively growing, as the roots still need moisture.
• Once temperatures drop in late August and September, nemesia typically recovers rapidly and produces a second, often even better flowering period before the first frosts.

Underwatering

Although nemesia dislikes waterlogging, it also needs consistent moisture to perform well, particularly in containers. If the compost dries out completely between waterings, the small leaves wilt and curl along their length, and the plant may shed flower buds. Recovery from underwatering is usually quick once watered, but repeated cycles of drying out cause the plant to become progressively weaker.

What to do

• Water thoroughly when the top centimetre of compost feels dry, allowing water to drain freely from the base.
• In hot weather, container nemesia may need watering every day. Check by feeling the compost rather than relying on appearance, as the surface can look dry while the lower compost is still moist.
• A pebble tray beneath the pot can provide a modest buffer of humidity and slight moisture between waterings.

Aphid infestation

Aphids occasionally colonise nemesia shoot tips and leaf undersides, causing the leaves to pucker and curl. On nemesia, aphids are most often found on the tender young growth at the tips of shoots and in the buds, where their presence causes distorted, bunched growth that does not develop normally. Green or black aphids are the most common species.

What to do

• Pinch out and dispose of shoot tips with heavy aphid colonies.
• Knock remaining aphids off with a jet of water or rub them away with a gloved hand.
• For persistent infestations, spray with insecticidal soap every four to five days until the colony is eliminated.

Downy mildew

Downy mildew is a fungal-like disease that affects nemesia in cool, damp conditions with poor air circulation. Infected leaves develop yellowish or pale patches on the upper surface and a grey or purple downy coating on the underside. The affected areas eventually curl, collapse, and turn brown. Downy mildew is most common in spring and autumn when temperatures are cool and nights are humid.

What to do

• Improve air circulation around the plants by spacing more widely and removing any dead or overcrowded growth.
• Avoid overhead watering and water in the morning so that leaves dry off during the day.
• Remove and dispose of infected leaves promptly. In severe cases, remove the whole plant to prevent spread.
• A copper-based fungicide can help protect remaining healthy growth when applied at the first sign of infection.

Frequently asked questions

Why are my nemesia leaves curling?

Nemesia leaves curl most often from overwatering and root rot or from excess heat in summer. Nemesia is a cool-season plant that peaks in spring and autumn and struggles during hot spells. In high temperatures the plant may go semi-dormant, stop flowering, and develop curling, yellowing leaves.

Why has my nemesia stopped flowering and the leaves are curling?

Nemesia naturally stops flowering and may look bedraggled during the hottest part of summer. This is not a permanent problem: cut the plant back by about a third, keep it watered, and it will usually reflower vigorously in early autumn when temperatures drop. The curling during this period is a heat-stress response.

How often should I water nemesia?

Water when the top centimetre of compost has dried out. Nemesia dislikes both drought and waterlogging. In containers, this may mean watering every two to three days in warm weather. Ensure the pot drains freely and never leave nemesia sitting in standing water.

Can nemesia survive summer heat?

Nemesia tolerates mild UK summers but struggles in prolonged heat above 25 degrees Celsius. Moving container plants to a slightly shadier position in midsummer, cutting back the stems, and keeping watered usually carries the plant through until cooler autumn weather triggers a second flush of flowering.