Why Are My Carthamus Leaves Curling?

Aphid infestation

Aphids are the most common pest on carthamus, particularly on young plants before the spiny leaf margins develop fully. Green aphids and cotton aphid both colonise safflower, clustering on the shoot tips and the undersides of the young leaves where the spines are not yet fully formed. Aphid feeding causes the leaves to curl and the shoot tips to distort. Heavy infestations on young plants can delay flowering significantly. As the leaf spines develop with maturity, aphid access to the plant becomes more restricted.

What to do

• Check the shoot tips and the undersides of young leaves from late spring when aphid populations begin to build.
• Spray with insecticidal soap every four to five days for two to three weeks. Take care with the spiny leaf margins when applying spray.
• A jet of water dislodges aphids from the young leaves effectively before the spines have fully developed.
• Established plants with fully spiny leaves are much less susceptible to aphid colonisation than young seedlings.

Rust disease

Rust is the most serious disease problem on carthamus in UK gardens. The fungal disease Puccinia carthami produces characteristic orange-brown powdery pustules on the undersides of the leaves, which then appear on the upper surfaces as the infection spreads. Infected leaves curl, yellow at the margins, and drop early. Rust spreads rapidly in warm, humid conditions and can significantly reduce the vigour of the plant and the quality of the cut stems. It is most damaging in warm, wet summers with poor air circulation around the plants.

What to do

• Remove and bin affected leaves immediately at the first signs of rust pustules. Do not compost rust-infected material.
• Spray with a sulphur-based fungicide or copper-based fungicide every ten to fourteen days during susceptible periods.
• Space plants at least 30 cm apart to improve air circulation, which reduces the humidity that favours rust development.
• Avoid overhead watering which wets the foliage and promotes rust spore germination and spread.
• Clear all plant debris at the end of the season and rotate carthamus to a fresh area each year to reduce soil-borne rust inoculum.

Drought stress

Carthamus is a plant of dry, sunny climates and is substantially more drought tolerant than most annual cut flowers once established in the ground. However, young plants in their first few weeks of growth and container-grown plants can suffer from drought during warm, dry spells. The grey-green leaves curl inward and the plant's growth slows. Drought stress also makes carthamus more susceptible to pest attack, particularly aphids on young plants.

What to do

• Water young plants regularly for the first four to six weeks after sowing or transplanting until they are established.
• Established border carthamus is largely self-sufficient during a typical UK summer and needs additional watering only during prolonged dry spells.
• Container-grown plants need more frequent watering: check the compost every two to three days in warm weather.

Overwatering

Carthamus is adapted to dry conditions and performs poorly in persistently wet soil. In heavy clay or in containers without adequate drainage, the tap root deteriorates and the plant declines: the leaves yellow and curl, and the whole plant loses its characteristic upright vigour. Overwatering is most likely during a wet UK summer or when plants are grown in containers on a fixed watering schedule during cool, overcast periods.

What to do

• Plant carthamus in well-drained soil. It performs best in relatively poor, free-draining conditions rather than in a heavily enriched, moisture-retentive bed.
• In containers, use a gritty, free-draining compost and ensure excellent drainage holes. Reduce watering during cool or wet periods.
• Avoid waterlogging the root zone: carthamus tap roots deteriorate rapidly in saturated soil.

Powdery mildew

Powdery mildew occasionally affects carthamus in warm, dry conditions or when plants are crowded. The grey-green leaves develop a white powdery coating and curl slightly at the margins. It is less common on carthamus than rust disease but can become significant in sheltered positions with poor air circulation.

What to do

• Thin direct-sown seedlings to at least 30 cm apart to improve air circulation.
• Remove and bin affected leaves. Spray remaining healthy growth with a sulphur-based fungicide.
• Ensure adequate watering: drought-stressed plants are more susceptible to powdery mildew than well-watered ones.

Frequently asked questions

Why are my carthamus leaves curling?

Carthamus leaves curl most often from aphid infestation or rust disease. Aphids cluster on the shoot tips and the undersides of the spiny-edged leaves, causing them to curl and distort. Rust causes orange or brown pustules on the leaf undersides and the leaves curl and yellow as the infection spreads.

Why are there orange spots on my carthamus leaves?

Orange or rust-coloured spots on carthamus leaves are almost always caused by Puccinia rust disease. The pustules appear first on the leaf undersides and then on the upper surface. Affected leaves curl, yellow, and drop early. Remove affected leaves immediately and avoid overhead watering. Spray with a sulphur-based fungicide at the first signs of infection.

How do I dry carthamus flowers?

Cut carthamus stems when the orange flowers are beginning to open but before they are fully mature: the flowers and spiny bracts continue to develop after cutting. Remove the lower leaves and hang upside down in small bunches in a warm, dry, well-ventilated space. The orange flowers dry to a warm golden-orange and hold their colour well, making carthamus a valuable component in dried arrangements.

When should I sow carthamus?

Sow carthamus direct in its final position in March or April. It germinates readily in cool soil and dislikes root disturbance, so direct sowing is preferable to raising under glass and transplanting. Thin to 30 cm apart once the seedlings are large enough to handle. Carthamus also grows well from an autumn sowing where conditions allow.