Why Are My Tithonia Leaves Curling?
Tithonia rotundifolia, the Mexican sunflower, is a bold half-hardy annual that rewards UK gardeners who give it a warm, sunny spot with a long season of vivid orange, red-orange, or yellow daisy flowers on tall, branching stems. The large, velvety, three-lobed leaves are handsome in their own right and make the plant a dramatic presence at the back of a border. When the leaves curl or the plant looks unhealthy, a small number of identifiable causes are almost always responsible. This guide covers each one.
Drought stress
Tithonia originates from the dry, sunny conditions of Mexico and Central America and is more drought tolerant than many annuals once established in open ground. However, the large, velvety leaves have significant water demand during active growth, and both young transplants and container-grown plants are vulnerable to drying out. When the plant is under-watered, the large leaves wilt rapidly, curl inward from the edges, and the stems may droop. Recovery is usually good once watered, but repeated drought stress slows growth and reduces the long-season flowering display that tithonia is prized for.
What to do
- Water young plants and recent transplants regularly until established, particularly in warm, sunny weather.
- Once established in the ground, tithonia manages on typical UK summer rainfall but benefits from watering during prolonged dry spells.
- Container-grown tithonia needs watering every two to three days in warm weather. The large leaves transpire significant quantities of water in full sun.
- Mulch around border plants with 5 cm of garden compost to conserve soil moisture and suppress competing weeds.
Whitefly infestation
Whitefly is one of the most common pest problems on tithonia. The tiny white insects cluster in large numbers on the undersides of the large, velvety leaves, feeding on the sap and producing sticky honeydew that promotes sooty mould. Infested leaves pale, yellow, and curl. When the plant is disturbed, a cloud of tiny white insects rises briefly before settling back on the undersides of the leaves. The velvety surface of tithonia leaves makes whitefly particularly difficult to remove with water alone.
What to do
- Use yellow sticky traps near the plant to monitor and catch adult whiteflies.
- Spray the undersides of leaves thoroughly with insecticidal soap or a pyrethrin-based spray every five to seven days for four to six weeks. Multiple treatments are necessary as newly emerged adults from resistant pupal cases are not killed by a single application.
- For plants grown under cover, the parasitic wasp Encarsia formosa provides highly effective biological control at warm temperatures.
- Wipe sooty mould deposits from the large leaves with a damp cloth to improve their appearance and light absorption after treating the pest.
Cold damage
Tithonia is very frost-tender and should not be planted outside until late May or early June when night temperatures are reliably above 10 degrees Celsius. Young plants exposed to temperatures below 5 degrees develop water-soaked, translucent patches on the leaves, which then curl, collapse, and turn black. Cold damage is a common problem when gardeners are keen to get this spectacular plant established early in the season, particularly after a warm spell in April or May that is followed by a late cold snap.
What to do
- Harden off tithonia transplants thoroughly for at least two weeks before planting out, starting with sheltered daytime outdoor exposure.
- Plant out only after late May and only once nighttime temperatures are reliably above 10 degrees Celsius.
- Cover plants with double-layer fleece if a cold snap is forecast after planting.
- Cold-damaged leaves should be removed cleanly. Plants with healthy stem bases often regrow from below the damaged tissue once warm weather returns.
Aphid infestation
Aphids occasionally colonise tithonia, particularly on young plants in spring before natural predator populations have built up. They cluster on shoot tips and the undersides of young leaves, causing the leaves to curl and pucker as they feed on the sap. On the large leaves of tithonia, aphid damage is less visually severe than on smaller-leaved plants, but heavy infestations on shoot tips can stunt growth and delay flowering.
What to do
- Check shoot tips and the undersides of young leaves for aphid colonies.
- Knock aphids off with a strong jet of water or spray with insecticidal soap every four to five days for two to three weeks.
- Encourage natural predators: the bright orange flowers of tithonia are highly attractive to beneficial insects including hoverflies and parasitic wasps.
Overwatering
While tithonia benefits from regular watering, it does not tolerate persistently waterlogged soil. In heavy clay or in containers without adequate drainage, the roots deteriorate and the plant begins to decline: the large leaves yellow and curl even when the soil is wet, and the stems may soften at the base. Container-grown tithonia on a fixed watering schedule is most vulnerable during cool, overcast periods when the compost takes much longer to dry between waterings.
What to do
- Plant tithonia in free-draining soil. On clay, improve drainage with grit and organic matter before planting.
- In containers, ensure excellent drainage holes and use a free-draining compost blended with perlite.
- Reduce watering during cool or overcast periods. Tithonia does not need watering as frequently as most bedding plants and benefits from the soil drying out somewhat between waterings.
Frequently asked questions
Why are my tithonia leaves curling?
Tithonia leaves curl most often from drought stress or whitefly infestation. The large, velvety leaves wilt and curl quickly when the plant dries out, particularly in containers. Whitefly cluster on the undersides of the large leaves, causing them to yellow, pale, and curl as they feed.
How often should I water tithonia?
Tithonia is reasonably drought tolerant once established in the ground but needs consistent moisture during the growing season to perform well. Water border plants during dry spells when the top 5 cm of soil feels dry. Container-grown tithonia needs watering every two to three days in warm weather as the large leaves transpire heavily.
Why are tithonia leaves turning yellow and curling?
Yellow curling leaves on tithonia suggest whitefly infestation, nutrient deficiency, or overwatering. Check the undersides of leaves for clouds of tiny white insects that rise when the plant is disturbed. If no pests are present and the plant is in containers, it may need feeding with a balanced liquid fertiliser.
Why is my tithonia not flowering?
Tithonia flowers best in long, warm summers and may be slow to flower in a cool or cloudy UK summer. It also needs full sun to flower well: plants in partial shade produce more foliage but fewer flowers. Pinching out the growing tips when young encourages bushier plants with more flowering stems.