Why Are My Agrostemma Leaves Curling?
Agrostemma githago, corncockle, is a tall, graceful hardy annual that was once a common wildflower of arable fields across the UK, now rare in the wild but widely grown in cottage gardens and cutting gardens for its large, five-petalled flowers in shades of deep pink, lilac, and white, held on long slender stems above the narrow, silky-hairy foliage. The long, narrow, grey-green leaves covered with soft white hairs give the whole plant a delicate, silvery quality. Agrostemma is an undemanding annual in most respects but has a small number of predictable vulnerabilities. This guide covers the most common reasons the leaves curl.
Aphid infestation
Aphids are the most common pest on agrostemma. They colonise the shoot tips and the undersides of the long, narrow, hairy leaves, causing them to curl and the developing flower buds to distort. Green aphids and black bean aphid are both found on corncockle. The soft white hairs on the leaves can make aphid colonies harder to spot against the pale, silvery foliage. Heavy infestations on young plants delay flowering and can significantly reduce the height and vigour of the characteristically tall stems.
What to do
- Check the shoot tips and the undersides of the narrow leaves from early spring onwards, paying particular attention to the soft hairy surfaces where aphid colonies can be hidden.
- Spray with insecticidal soap every four to five days for two to three weeks, covering all surfaces of the hairy leaves carefully.
- A jet of water dislodges aphids from the hairy leaves, though the soft hairs mean aphids are not as easily removed as from smooth-leaved plants.
- Avoid high-nitrogen feeding: agrostemma grows well in poor to moderate soil and excess nitrogen produces lush growth attractive to aphids.
Drought stress
Drought stress causes agrostemma leaves to curl inward and the slender stems to wilt. As a plant of arable fields and open, well-drained ground, agrostemma tolerates moderate drought once established in the border, but extended dry spells during the flowering period reduce flower size and quality and cause the plant to set seed prematurely. Container-grown agrostemma is particularly vulnerable to drying out. In very dry conditions, the characteristically tall stems become shorter and the plants flower more rapidly without reaching their full potential height.
What to do
- Water agrostemma during prolonged dry spells in late spring and early summer during the main flowering period.
- Container-grown plants need regular monitoring: water when the top 3 cm of compost has dried.
- Mulch around border plants with organic material to retain soil moisture during dry periods.
Overwatering
Agrostemma does not tolerate persistently wet or waterlogged soil. In heavy clay or in containers without adequate drainage, the roots deteriorate and the narrow leaves yellow and curl. Overwatering is most likely during a wet UK spring or when plants are kept in containers on a fixed watering schedule during cool, overcast periods when the compost dries slowly. Established border plants rarely need additional watering and are more at risk from poor drainage than from drought.
What to do
- Plant agrostemma in well-drained soil. It is best suited to light to medium soils rather than heavy clay.
- In containers, use a free-draining compost and ensure excellent drainage holes. Allow the compost to dry slightly between waterings.
- Improve drainage in heavy clay beds with coarse grit or raised planting before sowing agrostemma.
Powdery mildew
Powdery mildew can affect agrostemma in warm, dry conditions, particularly on plants that are drought-stressed or crowded. The narrow, hairy leaves develop a white powdery coating and curl at the margins. It is most common on plants in their second half of the season when they are beginning to age naturally. The soft white hairs on agrostemma leaves can initially make it difficult to distinguish between the plant's natural hairiness and a powdery mildew infection: look for a powdery coating on the upper leaf surface rather than the natural felt-like texture underneath.
What to do
- Thin direct-sown seedlings to 20 to 30 cm to improve air circulation between the tall stems.
- Remove and bin affected leaves. Spray healthy growth with a sulphur-based fungicide at the first signs of infection.
- Ensure adequate watering: drought stress significantly increases susceptibility to powdery mildew.
Rust disease
Rust disease occasionally affects agrostemma, producing orange-brown pustules on the undersides of the narrow hairy leaves and causing the leaves to curl and yellow. It is less common on agrostemma than powdery mildew but can occur in warm, humid conditions with poor air circulation. Rust spreads rapidly from plant to plant in damp weather and can significantly weaken the plants and reduce flowering.
What to do
- Remove and bin affected leaves immediately at the first signs of orange rust pustules. Do not compost rust-infected material.
- Spray with a sulphur or copper-based fungicide every ten to fourteen days during periods of susceptibility.
- Space plants adequately and avoid overhead watering, which promotes rust spore spread. Water at the base of the plant rather than over the foliage.
Frequently asked questions
Why are my agrostemma leaves curling?
Agrostemma leaves curl most often from aphid infestation or drought stress. Aphids colonise the shoot tips and the undersides of the long, narrow, silky-hairy leaves, causing them to curl and distort. Drought causes the slender, hairy leaves to curl inward as the plant conserves moisture. Agrostemma is adapted to dry arable conditions and performs poorly in waterlogged soil but can still suffer in extreme drought.
Is agrostemma poisonous?
Yes, all parts of agrostemma (corncockle) are toxic if ingested, particularly the seeds, which contain githagin and agrostemmic acid. It should not be grown where children or pets might eat the seeds. As a cut flower, it is safe to handle normally. The plant was once a serious agricultural weed in grain fields because the toxic seeds contaminated grain stores.
When should I sow agrostemma?
Sow agrostemma direct in its final position in autumn for flowering the following summer, or in early spring from February onwards. It germinates readily in cool conditions and strongly dislikes root disturbance, so direct sowing in its final position is essential. Thin to 20 to 30 cm apart once the seedlings are established. Autumn-sown plants produce the tallest, most floriferous results.
Does agrostemma need staking?
Yes, agrostemma usually needs some support as the slender stems can reach 60 to 90 cm and are susceptible to wind damage and lodging. Insert twiggy sticks between the plants at an early stage and let the stems grow through them naturally. In a sheltered position, agrostemma may stand without support, but in exposed gardens staking is strongly recommended.