Why Are My Sweet Alyssum Leaves Curling?

Aphid infestation

Aphids are the most common pest on lobularia. The dense, low-growing habit of the plant makes it ideal territory for aphid colonies, which shelter among the many fine stems where they are difficult to see and treat. Cabbage aphid and green peach aphid are both found on sweet alyssum. The small leaves curl and the shoot tips distort under aphid pressure. Because lobularia is often used as an edging plant in close contact with other plants, aphid colonies can spread quickly to neighbouring plants if not managed early.

What to do

• Part the dense foliage and look among the stems and shoot tips for aphid colonies hidden in the centre of the plant.
• Spray with insecticidal soap every four to five days for two to three weeks, directing the spray into the interior of the plant where colonies shelter.
• The flowers of sweet alyssum are an important source of nectar for hoverflies and parasitic wasps: spray in the evening to avoid harming beneficial insects.
• Shearing a plant back after an aphid infestation promotes a flush of fresh, clean growth that is less attractive to aphids than the old, stressed foliage.

Drought stress

Despite its Mediterranean origins and tolerance of dry conditions, lobularia in containers or in very free-draining soil can suffer from drought, particularly during the warm, dry weeks of midsummer. The small leaves curl inward, the plant begins to look dull and grey-green rather than fresh and bright, and flowering stops. Sweet alyssum is notably forgiving of drought and usually rebounds quickly once watered, often producing a flush of new flowers within a few days of irrigation.

What to do

• Water lobularia in containers when the top 2 cm of compost feel dry. Check daily in warm weather as the small, dense plants can dry out quickly in a container.
• In the ground, water during prolonged dry spells. Established border plants need less frequent watering than container-grown specimens.
• If the plant has stopped flowering due to drought and heat, cut it back by one third and water well: it will resprout and rebloom within two to three weeks.

Clubroot and downy mildew

Lobularia is a member of the brassica family and shares its susceptibility to clubroot and downy mildew with cabbage, kale, and other relatives. Clubroot causes yellowing, curling leaves and poor growth as the roots swell and can no longer function. Downy mildew causes yellowing of the upper leaf surface and a grey, downy growth on the underside, with the leaves curling slightly. Both are most likely in wet, cool conditions and in soil that has recently grown other brassica family plants.

What to do

• If clubroot is suspected, pull a plant and check the roots for swollen, irregular distortions. If present, remove all affected plants and bin them. Do not compost.
• Raise the soil pH to 7 or above by applying garden lime: clubroot is much less severe in alkaline conditions.
• For downy mildew, improve air circulation by thinning plants and spray with a copper-based fungicide at first signs of infection.
• Avoid growing lobularia in ground that has recently had clubroot-infected brassicas, or grow in fresh compost in containers.

Overwatering

Lobularia tolerates dry conditions better than wet ones and is susceptible to root rot in persistently waterlogged compost or heavy, poorly draining soil. Overwatered plants develop yellow, curling leaves, lose their characteristic honey-scent, and may collapse at the stem base. This is most common in containers without drainage holes or in hanging baskets that retain moisture at the roots for extended periods.

What to do

• Ensure all containers have adequate drainage holes. Lobularia should never be grown in saucers that hold water.
• Use a free-draining compost in containers. Reduce watering frequency during cool or overcast periods.
• In the ground, plant lobularia in well-drained soil. On clay, improve drainage with grit and organic matter before planting.

Flea beetle damage

Flea beetles attack lobularia in spring and early summer, chewing small round holes in the small leaves and causing the damaged tissue to curl and shrivel at the edges. Young seedlings are most vulnerable: heavy flea beetle pressure on small seedlings can significantly check growth and delay flowering. The damage is more severe during dry, warm spring weather and when plants are grown in soil that has recently grown other brassica-family crops.

What to do

• Cover young lobularia seedlings with fine insect-proof mesh immediately after germination or planting out to exclude flea beetles during the vulnerable early stages.
• Water regularly: flea beetle damage is significantly worse on drought-stressed plants than on well-watered ones.
• Once plants are established and in flower, they outgrow flea beetle damage rapidly and further intervention is rarely needed.

Frequently asked questions

Why are my sweet alyssum leaves curling?

Sweet alyssum leaves curl most often from aphid infestation or drought stress. Aphids cluster on the tiny shoot tips and among the dense stems, causing the small leaves to curl and the plant to look stunted. Drought causes the fine, grey-green leaves to curl and the plant to stop flowering, though lobularia recovers well once watered and will rebloom.

Why has my lobularia stopped flowering?

Lobularia naturally slows or stops flowering during the hottest part of summer, particularly in very warm, dry conditions. Shearing the plant back by about one third when it goes to seed in midsummer stimulates a fresh flush of growth and flowers in late summer and autumn. Adequate watering and feeding also help maintain continuous flowering.

Is sweet alyssum a perennial?

Lobularia maritima is a short-lived perennial or sub-shrub in its native Mediterranean habitat but is almost always grown as a hardy annual in the UK. It can sometimes survive mild winters in sheltered coastal positions, but it is most reliably and economically grown from seed each year, either sown directly or purchased as plug plants in spring.

When should I sow lobularia?

Sow sweet alyssum from March to May either under glass or direct in the garden. It germinates readily at cool temperatures and can be direct-sown once the soil has warmed in April. For the earliest flowering, sow under glass in March and plant out from late April or May. Lobularia also self-seeds freely and will often appear without any additional sowing in subsequent years.