The name "alyssum" covers two very different groups of plants in UK gardens. Sweet alyssum (Lobularia maritima) is a compact annual, 10 to 15 cm tall, that produces masses of tiny honey-scented white or purple flowers continuously from June to October. It is one of the most useful UK edging plants, softening path edges, filling gaps in borders, and flowering with almost no attention from summer to the first frosts. Gold dust alyssum (Aurinia saxatilis, formerly Alyssum saxatile) is a completely different plant: a hardy perennial growing to 20 to 30 cm that produces spectacular mounds of intense golden-yellow flowers in April and May, making it a classic for dry stone walls, rock gardens, and south-facing slopes. The genus Alyssum itself, to about 30 cm with yellow flowers, includes some species grown in UK rock gardens. All three groups belong to the family Brassicaceae, all need full sun, and all need excellent drainage.
Leaves curling on either type signal one of a handful of problems. The two most important are flea beetle damage, which is the most distinctive and common cause on sweet alyssum and other Brassicaceae, and drought stress with heat scorch. Both are recognisable once you know what to look for.
Flea beetle damage
Flea beetles (Phyllotreta species) are tiny metallic black or bronze-green jumping beetles, 1 to 3 mm long, that feed on the foliage of Brassicaceae family plants. Sweet alyssum is a member of this family and is susceptible. The beetles chew small, round, clean-edged holes scattered across the leaf surface. As feeding stress accumulates, the leaf edges curl and the plant looks tatty, holed, and exhausted. The beetles are so small and jump so quickly that gardeners often notice the damage before they see the insects. If you tap the foliage gently over a sheet of white paper, tiny beetles may fall onto it and jump away.
Seedlings and young plants are most vulnerable, especially during dry spring weather when plants are stressed and beetles are at peak activity. Established plants with a full root system can usually produce new growth faster than the beetles remove it, and they tend to outgrow the damage as the season progresses. Young transplants or plants just emerging from seed have no such buffer.
Protect young plants with fleece or fine insect mesh immediately after planting or germination. The cover prevents beetles reaching the leaves during the vulnerable early weeks. Remove it once the plant is well established and growing vigorously. Water regularly: plants under water stress are more severely affected and recover more slowly. Insecticidal soap sprays can reduce beetle populations but must contact the insects directly to work, which is difficult given how quickly the beetles jump and hide. Good autumn garden hygiene, clearing plant debris and leaf litter where flea beetles overwinter in the soil, reduces populations the following spring.
Drought stress and heat scorch
Sweet alyssum has thin, delicate leaves that lose water rapidly in hot conditions. In a heatwave or in a container that has dried out, the leaf margins curl upward and inward, the plant wilts, and the foliage looks scorched at the edges. This is not a disease and it is not permanent damage: sweet alyssum recovers well from drought stress once watered, provided the roots have not been allowed to dry out completely for a prolonged period.
Containers are the most common setting for this problem. Pots in full sun on a south-facing terrace can dry out completely within a day or two in hot July and August weather. Check sweet alyssum in containers daily during hot spells and water as soon as the surface of the compost feels dry. A slightly larger container helps buffer against rapid drying.
The shearing technique is particularly useful for sweet alyssum that has been damaged by heat. After the first main flush of flowers, typically in July, cut the whole plant back by roughly half with scissors or garden shears. This removes heat-damaged and flea-beetle-tatty growth in one go and stimulates a fresh flush of young leaves and new flower buds. With adequate water after shearing, sweet alyssum typically produces a second flush of flowers from late August through to October.
Gold dust alyssum (Aurinia saxatilis) is a Mediterranean rock plant adapted to baking summer conditions and is considerably more tolerant of drought than sweet alyssum. Extreme heat in a dry UK summer can still cause the leaves to curl and mottle slightly, and the plant may go almost entirely dormant in midsummer, dropping leaves and looking very sparse. This is normal behaviour. New growth typically appears in cooler autumn weather, and the plant will flower spectacularly again the following spring.
Other causes
Aphids form soft colonies on the shoot tips of sweet alyssum, particularly in spring and early summer. Feeding causes young leaves at the tips to curl inward and yellow. The colonies are visible on close inspection as clusters of small green or yellowish insects. Dislodge them with a strong jet of water, rub them off with your fingers, or leave them for a week and allow natural predators to arrive. Lacewing larvae and ladybird larvae are effective aphid controllers in most UK gardens. Persistent heavy infestations can be treated with insecticidal soap applied directly to the insects.
Caterpillars of cabbage white butterflies (Pieris species) will occasionally feed on sweet alyssum as a member of the Brassicaceae family. The damage is more irregular than flea beetle holes: ragged, larger areas of missing leaf, often with visible frass. Check under the leaves for caterpillars and remove by hand.
Downy mildew (Peronospora species) can affect sweet alyssum in cool, humid conditions. Infected plants show pale yellowish patches on the upper leaf surface and a grey-purple mould on the underside. Improve air circulation by not planting too densely, avoid wetting the foliage when watering, and remove affected material promptly.
Clubroot (Plasmodiophora brassicae) is a soil-borne disease affecting all Brassicaceae including sweet alyssum. Infected plants develop distorted, swollen, discoloured roots and show progressive yellowing and wilting despite regular watering. Affected plants cannot be saved. Remove and dispose of them without composting. Do not grow any Brassicaceae in the infected soil for at least seven years.
Over-watering in containers causes root rot and waterlogged compost. Roots suffocate and the plant wilts as if dry even though the compost is wet. Always use free-draining compost with added grit in containers, and ensure pots have generous drainage holes. Allow the compost to partially dry out between waterings.
Prevention and long-term care
Both types of alyssum share the same fundamental requirements. Plant in a position that receives full sun for most of the day. Sweet alyssum in partial shade becomes drawn, flowers less freely, and is more susceptible to disease. Gold dust alyssum in shade performs poorly and rarely flowers well.
Drainage is the most critical factor for gold dust alyssum. It is native to rocky, dry slopes and walls across southern Europe and dislikes having wet roots in winter far more than summer drought. On heavy UK clay soils, grow it in a raised bed, a dry stone wall, or a gritty, very free-draining raised border. Winter wet on poorly draining heavy soil is the primary cause of gold dust alyssum failure in the UK.
Sweet alyssum, by contrast, benefits from consistent moisture, especially in containers and in hot summers. Water regularly. Protect seedlings and young transplants from flea beetle with fleece or fine netting in spring. Shear the plant back after the first flush of flowers to encourage fresh growth and a second flush. In warm, sheltered positions, sweet alyssum self-seeds gently and will return each year without replanting.
Gold dust alyssum needs little attention beyond ensuring excellent drainage. Popular varieties worth seeking out include Citrinum, with pale lemon-yellow flowers that combine well with other spring plants; Dudley Nevill, with unusual biscuit-apricot flowers; and Tom Thumb, which forms a particularly compact mound. All three are reliably hardy in UK conditions when planted with adequate drainage.
Frequently asked questions
What is the difference between sweet alyssum and gold dust alyssum?
Sweet alyssum (Lobularia maritima) is a low-growing annual or short-lived perennial, typically 10 to 15 cm tall, producing masses of tiny honey-scented white or purple flowers from June to October. It is used for edging paths, softening hard surfaces, and filling gaps in borders. It self-seeds gently in warm sheltered spots. Gold dust alyssum (Aurinia saxatilis) is a different plant entirely: a hardy perennial growing to 20 to 30 cm, producing spectacular mounds of intense golden-yellow flowers in April and May, best suited to dry walls, rock gardens, and south-facing slopes. Both need full sun and excellent drainage, but gold dust alyssum is far more drought-tolerant and dislikes winter wet. Popular varieties of gold dust alyssum include Citrinum, Dudley Nevill, and Tom Thumb.
What are the tiny holes in my alyssum leaves and why are the edges curling?
Small round holes scattered across the leaves combined with curled, tatty-looking foliage is the classic sign of flea beetle damage. Flea beetles (Phyllotreta species) are tiny metallic black or bronze-green jumping beetles, 1 to 3 mm long, that attack Brassicaceae family plants including sweet alyssum. They puncture the leaf surface to feed, creating small round shot-holes, and the feeding stress causes leaf edges to curl. Seedlings and young plants are most vulnerable, especially in dry spring weather. Protect young plants with fleece or fine netting immediately after planting. Water regularly to reduce plant stress. Established plants usually grow through the attack without lasting damage.
Why are my sweet alyssum leaves curling and going limp in hot weather?
Sweet alyssum has thin, delicate leaves that lose water quickly in hot conditions. In a heatwave, or in containers that have been allowed to dry out, the leaves curl at the margins, wilt, and the plant looks exhausted. Water sweet alyssum regularly, especially in containers. If the plant looks exhausted after its first flush of flowers, shear it back by roughly half: this removes tatty heat-damaged growth and stimulates a fresh flush of young leaves and new flowers as temperatures cool in late summer. Sweet alyssum will often recover completely and flower again from late August through to the first frosts.
Can alyssum get aphids?
Yes. Soft colonies of greenfly cluster on the growing tips of sweet alyssum in spring and early summer. Feeding causes the young leaves at the tip to curl inward and yellow. For light infestations, dislodge aphids with a strong jet of water or by rubbing them off with your fingers. Leaving the plants alone for a week or two often allows natural predators such as lacewing larvae and ladybird larvae to bring the population down. Heavy or persistent infestations can be treated with insecticidal soap spray applied so it contacts the insects directly.
Why is my sweet alyssum wilting and yellowing even though I am watering it?
Wilting and yellowing despite regular watering points to one of two root problems. Over-watering in a container without adequate drainage causes waterlogged compost and root rot: the roots suffocate, cannot take up water, and the plant wilts as if dry even though the compost is wet. Check that the pot has large drainage holes and that compost is not permanently saturated. The other cause is clubroot (Plasmodiophora brassicae), a soil-borne disease affecting all Brassicaceae including sweet alyssum. Infected plants develop distorted, swollen roots and show progressive yellowing and wilting. Affected plants cannot be saved. Remove and dispose of them without composting, and do not grow any Brassicaceae in the same soil for at least seven years.