Why Are My Thunbergia Leaves Curling?

Aphid infestation

Aphids are the most common pest on thunbergia. They colonise the shoot tips, the twining growing points, and the undersides of the heart-shaped leaves, where their feeding causes the young leaves to curl, pucker, and distort. Green and black aphids are both found on thunbergia. Because the vine climbs rapidly and the shoot tips are always at the leading edge of growth, aphid infestations often go unnoticed until the upper portions of the plant are heavily colonised. Heavy aphid pressure reduces flowering and weakens the whole vine.

What to do

• Check the shoot tips and the undersides of young leaves regularly, working up the vine from the base.
• Spray with insecticidal soap every four to five days for two to three weeks, covering the undersides of leaves and the shoot tips thoroughly.
• A jet of water helps dislodge aphids from the larger leaves and stems as a first step before chemical control.
• Encourage natural predators by avoiding broad-spectrum insecticides that kill beneficial insects as well as aphids.

Spider mite

Spider mite is one of the most common and damaging problems on thunbergia, particularly on plants grown under glass or in warm, sheltered positions outdoors. The mites feed on the undersides of the leaves, causing the upper surface to develop fine stippling or bronzing, the leaf edges to curl, and the plant to lose its characteristic lush, green appearance. Fine webbing appears between the stems in more severe cases. The warm, dry conditions that favour thunbergia's flowering are also ideal for spider mite population growth.

What to do

• Examine the undersides of curling leaves with a magnifying glass for the tiny mites, their eggs, and webbing.
• Spray with insecticidal soap or neem oil every five to seven days for four to six weeks. Cover the leaf undersides thoroughly.
• Increase humidity around the plant: mist the foliage daily and improve ventilation. Spider mites thrive in hot, dry, still conditions.
• For plants under glass, the predatory mite Phytoseiulus persimilis provides highly effective biological control when introduced early.

Cold stress

Thunbergia is a warm-season plant from tropical Africa that requires temperatures above 12 degrees Celsius to grow and flower well. Cold nights, particularly in early summer after planting out or in late summer as temperatures begin to drop, cause the leaves to curl, pale, and develop a yellowish or bleached appearance. Plants exposed to temperatures below 5 degrees Celsius suffer significant damage and may not recover. Cold stress is the most common reason thunbergia fails to establish or perform well in exposed positions in the UK.

What to do

• Plant thunbergia in the warmest, most sheltered position available, ideally against a south or southwest-facing wall or fence.
• Do not plant out until late May or early June when nighttime temperatures are reliably above 12 degrees Celsius.
• Harden off plants thoroughly for at least two weeks before planting out.
• Cover with fleece on cold nights in early summer and remove when temperatures recover.

Drought stress

Thunbergia climbs and flowers rapidly during summer and has significant water demand, particularly when grown in containers. In dry conditions the leaves wilt and curl, and the plant may drop flower buds before they open. Container-grown thunbergia is especially vulnerable since the vigorous vine quickly outgrows the water-holding capacity of a small pot. Drought stress significantly reduces flowering and makes the plant more susceptible to spider mite infestation.

What to do

• Water thunbergia regularly throughout the growing season, never allowing the compost to dry out completely between waterings.
• Container-grown plants may need watering daily in warm weather. Use a large pot to increase the water reservoir available to the roots.
• Feed weekly with a balanced liquid fertiliser, switching to a potassium-rich feed once the plant is in flower to encourage continued blooming.

Overwatering

While thunbergia needs consistent moisture, it does not tolerate persistently waterlogged compost. In containers without adequate drainage or in cool, wet periods when the compost dries slowly, the roots deteriorate and the plant declines. Yellow, curling leaves despite wet compost, soft stems at the base, and a general loss of vigour indicate root damage from overwatering.

What to do

• Ensure containers have excellent drainage and use a free-draining, peat-free compost blended with perlite.
• Reduce watering frequency during cool or overcast periods. Adjust watering to actual soil moisture rather than a fixed schedule.
• If root rot is suspected, remove the plant from its container, trim away any mushy roots, and replant in fresh, well-aerated compost.

Frequently asked questions

Why are my thunbergia leaves curling?

Thunbergia leaves curl most often from aphid infestation or spider mite. Aphids cluster on the shoot tips and young leaves, causing them to curl and distort. Spider mite, particularly common on thunbergia grown under glass or in warm, dry conditions, causes the leaves to develop bronzing and curling as the mites feed on the leaf undersides.

Why has my thunbergia stopped flowering?

Thunbergia stops flowering when temperatures drop below 12 degrees Celsius, when it is under water stress, or when it has become severely pot-bound. In UK gardens, flowering naturally slows as temperatures cool in late summer. Ensure consistent moisture, adequate feeding with a potassium-rich fertiliser, and a warm sheltered position to extend the flowering season.

Can thunbergia grow outdoors in the UK?

Yes, Thunbergia alata grows well outdoors in the UK during summer, given a warm, sheltered position in full sun. It is frost-tender and must be started under glass in March or April and planted out after the last frost in late May or June. In cool or exposed positions it may flower only briefly before temperatures drop in late summer.

How do I overwinter thunbergia?

Thunbergia alata can be overwintered as a perennial if brought indoors before the first frost. Cut the plant back by about half and keep it in a frost-free, bright position at a minimum of 10 degrees Celsius with minimal watering through winter. It will regrow strongly in spring when temperatures rise and watering is gradually increased.