Why Are My Erysimum Leaves Curling?

Drought stress

Erysimum is reasonably drought tolerant once established in open ground, but young transplants and container-grown plants are much more susceptible to water stress. In a dry spring or after planting into dry soil, the leaves lose their firmness, curl inward from the edges, and may develop a slightly greyish tinge. Established biennial wallflowers planted in autumn and growing through winter are occasionally stressed by dry spells followed by freezing winds, which cause the leaf edges to curl and brown.

What to do

• Water transplants thoroughly at planting and again if dry weather follows for more than a week or two.
• Mulch around established plants with 5 cm of garden compost to retain soil moisture through dry spring spells.
• Container-grown erysimum needs watering when the top 2 to 3 cm of compost feel dry. Water before the leaves show stress symptoms.
• The plant recovers well from drought if watered promptly. Leaves that have already crisped and browned will not recover but healthy new growth will continue from the crown.

Flea beetle

Flea beetles are a very common pest on erysimum and all other members of the brassica family. The small, shiny metallic beetles — typically 2 to 3 mm long and either black or striped yellow and black depending on species — feed on the leaves, creating large numbers of tiny round holes that give the foliage a shot-gun appearance. Heavily damaged leaves become weak and papery, curl, and may eventually wither. Young plants and seedlings are particularly vulnerable, and severe attacks in spring can kill newly transplanted wallflower plants.

The beetles jump vigorously when disturbed, making them difficult to catch but easy to identify. They overwinter in plant debris and soil and emerge in early spring, coinciding with the period when biennial wallflowers are putting on their main growth before flowering.

What to do

• Cover young transplants with fine insect-proof mesh or fleece for the first few weeks after planting to prevent beetle access during the most vulnerable stage.
• Keep the soil around plants moist: flea beetles are more active and damaging in dry conditions.
• Remove plant debris and fallen leaves from around plants in autumn to reduce overwintering habitat for the beetles.
• On established plants, the damage is rarely severe enough to warrant chemical treatment. On seedlings or very young transplants, a pyrethrin-based spray can be used as a last resort.

Club root

Club root is a soil-borne disease caused by a slime mould that affects all members of the brassica family, including erysimum. Infected plants develop swollen, distorted, club-like growths on the roots, and the above-ground symptoms include wilting, yellowing, and curling leaves even when the soil is adequately moist. The disease persists in the soil for many years and is spread by infected soil, tools, and plant material. It is far more prevalent in acid soils with poor drainage.

What to do

• To diagnose club root, gently pull up an affected plant and examine the roots. Swollen, distorted, club-like growths on the roots are the definitive sign.
• There is no chemical cure for club root once established. Remove and bin all infected plants and roots without composting them.
• Raise the soil pH to 7.0 or above by applying garden lime: club root is greatly suppressed in alkaline soil conditions.
• Avoid growing brassica-family plants in the affected area for at least six years.
• Grow wallflowers in containers with fresh, sterilised compost to avoid soil-borne infection.

Downy mildew

Downy mildew affects erysimum in cool, damp conditions, particularly during wet springs and on plants in sheltered positions with poor air circulation. Infected leaves develop yellowish patches on the upper surface and a white or grey downy coating on the underside. The affected areas curl, turn brown, and die back. Young seedlings can be killed outright by severe infection. It is most common in nursery or greenhouse conditions and on plants that have been overwatered.

What to do

• Improve air circulation around plants by spacing them adequately and removing any overcrowded growth.
• Water at the base of plants rather than from above, and in the morning so that leaves dry off during the day.
• Remove and bin infected leaves and stems promptly. In severe cases, remove the whole plant to prevent spread.
• A copper-based fungicide provides some protection for remaining healthy growth when applied at the first signs of infection.

Aphid infestation

Aphids, particularly the cabbage aphid (Brevicoryne brassicae), colonise erysimum and other brassica-family plants in spring and summer. Cabbage aphids are a distinctive mealy grey colour and form dense colonies on the undersides of leaves and on shoot tips. Their feeding causes the leaves to curl and pucker around the colony, and heavily infested plants become weakened and flower poorly. The aphids spread quickly from plant to plant during warm, dry spells.

What to do

• Check the undersides of curling leaves for the characteristic mealy grey colonies of cabbage aphid or smaller green aphid colonies.
• Knock aphids off with a strong jet of water from a hose and pinch out the most heavily infested shoot tips.
• Spray with insecticidal soap every four to five days for two to three weeks for persistent infestations.
• Encourage natural predators: lacewings, hoverflies, and parasitic wasps all predate aphids and are attracted to the flowers of erysimum.

Frequently asked questions

Why are my erysimum leaves curling?

Erysimum leaves curl most often from drought stress or flea beetle damage. In dry spells, the leaves curl and the plant wilts. Flea beetles create tiny round holes in the leaves and the damage causes the foliage to curl and look ragged. Club root and downy mildew are also common causes in wet seasons.

What are the tiny holes in my erysimum leaves?

Tiny round holes shot through the leaves of erysimum are caused by flea beetles. These small, shiny beetles jump when disturbed and feed on the leaves of brassica-family plants, including erysimum. In severe infestations the foliage looks peppered with holes and the leaves curl and wither.

Why is my erysimum wilting and the leaves are curling?

Wilting with curling leaves in dry conditions almost always indicates drought stress. Water the plant thoroughly and it should recover within a few hours. If the soil is moist and the plant is still wilting, check for club root by gently pulling up a plant and examining the roots for swollen, distorted club-like growths.

Is erysimum a perennial?

Erysimum cheiri, the traditional biennial wallflower, is grown from seed one year and flowers the next before dying. Many modern erysimum cultivars such as the Bowles's Mauve types are short-lived perennials that flower for several years. These perennial forms often become woody and tired after two to three years and are best replaced with new plants.