Why Are My Iberis Leaves Curling?

Aphid infestation

Aphids are the most common pest on iberis. Both the annual species and the perennial Iberis sempervirens are susceptible. Cabbage aphid, mealy cabbage aphid, and green peach aphid all colonise iberis, clustering on the shoot tips and the undersides of the narrow, dark green leaves. Aphid feeding causes the leaves to curl, pucker, and develop a characteristic blue-grey, distorted appearance. On annual iberis, heavy aphid pressure delays flowering significantly. On the perennial, aphids most often colonise the new growth that emerges after the main spring flush of flowers.

What to do

• Check the shoot tips and the undersides of leaves regularly. The waxy, slightly grey surface of cabbage aphid colonies can be difficult to spot against the dark green leaves initially.
• Spray with insecticidal soap every four to five days for two to three weeks, covering all undersides of leaves and shoot tips thoroughly.
• A jet of water dislodges aphids effectively. For the perennial, a spray immediately after flowering and cutting back removes aphids before they colonise the fresh new growth.
• Avoid high-nitrogen feeding which produces the soft, lush growth most attractive to aphids.

Clubroot

Clubroot is the most serious disease problem on iberis. Because iberis belongs to the Brassicaceae family (the same family as cabbage, kale, and wallflowers), it is susceptible to the soil-borne pathogen Plasmodiophora brassicae that causes clubroot. Infected plants develop swollen, distorted, club-like roots that are unable to absorb water and nutrients properly. The leaves wilt, yellow, and curl during warm weather even when the soil is moist, and the plants fail to flower normally. In severe cases, the plants collapse. Clubroot persists in the soil for up to twenty years once established and there is no chemical treatment available to gardeners.

What to do

• Do not grow iberis in soil known to be infected with clubroot. Rotate brassica family plants to a fresh area each year to reduce the risk of clubroot build-up.
• Improve soil drainage: clubroot thrives in wet, acidic conditions. Lime the soil to raise the pH above 7.0, which significantly reduces clubroot severity.
• Raise annual iberis in pots under glass and plant out with a large root ball of clean compost, which delays clubroot infection and may allow a reasonable display before symptoms appear.
• Remove and bin all infected plant material, including roots. Do not compost clubroot-affected plants as the spores survive composting.

Drought stress

Iberis tolerates moderate drought, particularly the perennial Iberis sempervirens which is a plant of dry, rocky, well-drained habitats. However, annual iberis in containers and young plants in their first season can suffer from drought in warm, dry conditions. The narrow leaves curl inward and the plant may wilt during the hottest part of the day. Container plants are most vulnerable as their limited compost volume dries rapidly.

What to do

• Water annual iberis in containers regularly, particularly during warm spring weather when it is at peak flowering.
• Established perennial Iberis sempervirens in the open ground rarely needs additional watering in UK conditions.
• Mulch around border plants to retain soil moisture. Gravel mulch is particularly appropriate for iberis as it replicates the plant's natural rocky habitat conditions.

Overwatering

Iberis does not tolerate persistently waterlogged soil, and the perennial Iberis sempervirens is particularly susceptible to root rot in wet, poorly drained conditions. Overwatered plants develop yellowing, curling leaves and may die back in sections. Overwatering of the perennial is most damaging during winter, when the plant is in its less active period and the soil dries very slowly. Annual iberis in containers without drainage holes is similarly at risk.

What to do

• Plant perennial Iberis sempervirens in well-drained, gritty soil. It performs excellently in rockeries, gravel gardens, and at the tops of dry stone walls where drainage is always excellent.
• In containers, use a free-draining compost with added perlite or grit and ensure excellent drainage holes.
• Avoid planting iberis in low-lying positions that collect water, particularly in winter when root rot risk is highest.

Flea beetle

Flea beetle is a common problem on annual iberis seedlings. The tiny, shiny black or bronze beetles jump when disturbed and are most active in warm, dry conditions. They eat small, round holes in the leaves of young seedlings, causing the leaves to look pitted and, in severe cases, to curl and wither. Annual iberis seedlings are most vulnerable in their first few weeks after germination. Established plants with tougher leaves are much less susceptible to serious flea beetle damage.

What to do

• Water seedlings regularly during dry spells: flea beetle activity is highest in dry conditions and well-watered seedlings suffer less damage.
• Cover direct-sown seedlings with fine insect mesh immediately after sowing to exclude flea beetles during the vulnerable seedling stage.
• Once the seedlings are established and developing their third and fourth true leaves, remove the mesh: mature plants tolerate flea beetle damage without significant impact.

Frequently asked questions

Why are my iberis leaves curling?

Iberis leaves curl most often from aphid infestation or clubroot disease. Aphids colonise the shoot tips and the undersides of the narrow, dark green leaves, causing them to curl and distort. Clubroot, caused by Plasmodiophora brassicae, infects iberis roots (as a member of the Brassicaceae family), causing the leaves to wilt and curl as the swollen, distorted roots are unable to take up water and nutrients properly.

What is the difference between annual and perennial iberis?

Iberis umbellata and Iberis amara are hardy annuals grown for their flat-topped flower heads in white, pink, purple, and red, which are excellent for cutting. Iberis sempervirens (perennial candytuft) is an evergreen sub-shrub that produces masses of white flowers in spring, covering the plant completely. The perennial species is long-lived and ideal for edging, rockeries, and walls, while the annuals are easy direct-sow plants for the cutting garden.

When should I sow iberis?

Sow annual iberis direct in its final position from March to May, or in September for an early spring display the following year. It germinates readily in cool conditions and dislikes root disturbance, so direct sowing is preferred over raising under glass and transplanting. Thin to 15 cm apart once established. Successive sowings every three to four weeks extend the cutting season through summer.

How do I prune perennial iberis after flowering?

Cut perennial Iberis sempervirens back by about one third immediately after flowering finishes in late spring. This prevents the plant from becoming woody and straggly, encourages fresh new growth from the base, and keeps the compact, mound-forming habit that makes it so useful as an edging plant. Do not cut back into old, leafless wood as it may not regenerate. Feed with a balanced fertiliser after cutting back.