Why Are My Swiss Chard Leaves Curling?

Beet leaf miner

Beet leaf miner (Pegomya hyoscyami) is the most common pest of Swiss chard in UK gardens, and the same species that attacks beetroot and spinach. The larvae of a small fly mine between the upper and lower leaf surfaces, consuming the green tissue inside and creating characteristic pale, blistered, translucent patches that turn brown and papery as the mined tissue dies. The surrounding leaf curls around the mined areas. There are two to three generations per year from May through to September. The damage is visually striking but rarely causes serious harm to established chard: the plant continues to produce new leaves from its centre even as outer leaves are mined.

What to do

• Remove and destroy badly mined outer leaves promptly, killing any larvae inside them by crushing the mined sections. The plant will produce new, healthy leaves from the growing centre.
• Cover chard sowings with fine insect mesh to prevent the adult fly from accessing the leaves for egg-laying. This is most valuable for young plants in April and May during the first-generation attack.
• Check leaves weekly from May onward and remove mined leaves before the larvae pupate and give rise to the next generation.

Downy mildew

Downy mildew on Swiss chard (Peronospora farinosa f. sp. betae) produces pale, angular, yellow patches on the upper surface of the leaves, with a characteristic grey-purple downy fungal growth on the underside. Affected leaves curl and yellow, and in severe infections, entire leaves collapse and die. Downy mildew is most common on crowded, poorly ventilated plants in cool, wet conditions. It is worse on overwintered chard in spring and on closely planted autumn sowings.

What to do

• Thin chard seedlings to the recommended spacing (15 to 20 centimetres for full-sized plants, closer for baby-leaf harvesting) to improve airflow and reduce the humid conditions in which downy mildew thrives.
• Remove and destroy affected leaves promptly. Avoid overhead watering: water at the base of plants to keep foliage dry.
• Rotate beet-family crops (chard, beetroot, spinach) each year to new beds to reduce soil-borne spore populations.

Aphids

Black bean aphid (Aphis fabae) and peach-potato aphid (Myzus persicae) both colonise Swiss chard, typically on the underside of the youngest leaves and at the growing tips. Dense aphid colonies cause the young leaves to curl around the feeding sites and may stunt the emerging growth. Aphid honeydew coats the leaf surface and promotes sooty mould growth.

What to do

• Inspect the underside of young chard leaves weekly from May for aphid colonies. Small colonies can be squashed by hand or removed with a strong jet of water from a hose.
• Apply insecticidal soap spray to larger colonies, covering the underside of affected leaves thoroughly. Repeat every 5 to 7 days until the infestation subsides.
• Natural predators including ladybirds, lacewings, and parasitic wasps provide excellent aphid control on chard by mid-summer without any intervention.

Drought

Swiss chard tolerates drought better than many other vegetables, but extended dry conditions cause the large leaves to droop and curl at the margins and reduce the quality of the harvest. The midrib stalks become more fibrous and chewy in drought conditions. In very hot, dry weather the plant may bolt prematurely.

What to do

• Water chard during dry spells to maintain consistent soil moisture, giving a thorough soaking every 7 to 10 days in warm, dry conditions.
• Apply a mulch of compost around the plants to conserve soil moisture and suppress weeds.

Bolting

Swiss chard is a biennial and is designed to flower and set seed in its second year. It bolts prematurely (in its first year) when it experiences cold temperatures followed by warming conditions, typically overwintered autumn sowings in spring. Once chard bolts, the leaves become progressively tougher and more bitter and the stems become stringy.

What to do

• Remove flower stems as soon as they appear to delay bolting and keep the plant producing edible leaves for a few more weeks.
• Sow chard in spring (March to May) or midsummer (July) rather than sowing in late summer for overwintering as a small plant: small plants exposed to a full cold winter are the most prone to spring bolting.
• Choose bolt-resistant varieties, particularly for spring and autumn sowings: 'Perpetual Spinach' (a chard type) has notably good bolt resistance.

Frequently asked questions

Why are my Swiss chard leaves curling?

Swiss chard leaves curl most often from beet leaf miner damage, downy mildew, or aphids. Beet leaf miner (Pegomya hyoscyami) larvae mine between the upper and lower leaf surfaces of chard, creating pale, blistered patches that cause the leaf to curl around the mined areas. Downy mildew (Peronospora farinosa) produces pale, angular patches on the upper leaf surface and a grey-purple downy growth beneath; affected leaves curl and yellow. Aphids colonise the underside of young leaves, causing the leaf edges to curl inward around the feeding sites.

What are the blistered patches on my Swiss chard leaves?

Blistered, pale, papery patches on Swiss chard leaves are almost certainly caused by beet leaf miner (Pegomya hyoscyami), the same pest that attacks beetroot. The larvae of a small fly, they mine between the upper and lower leaf surfaces and create translucent patches that turn brown and papery as the mined tissue dies. The surrounding leaf curls around the damage. Leaf miner is the most common pest of Swiss chard in UK gardens. Remove and destroy badly mined leaves promptly, killing any larvae by crushing the mined section. The plant will produce new leaves from the centre, and established chard continues to crop well even when outer leaves are heavily mined.

Can you eat Swiss chard raw?

Yes, Swiss chard can be eaten raw, though the flavour is stronger than salad leaves like lettuce. Young, tender chard leaves are pleasant in salads, wraps, and grain bowls: remove the central midrib from larger leaves and use the green leaf blade. Older, larger chard leaves are better cooked: they can be steamed, wilted, stir-fried, or added to soups, pasta, and gratins. The colourful stems (midribs) of rainbow chard and ruby chard are edible both raw and cooked, though they are more fibrous than the leaf blade and benefit from slightly longer cooking.

How do I stop Swiss chard bolting?

Swiss chard bolts (runs to seed) when it experiences cold temperatures in early growth, followed by warming conditions that trigger flowering. Autumn-sown chard that overwinters and then experiences spring warming is particularly likely to bolt in April and May. Prevent bolting by: sowing in spring (March to May) or midsummer (July to August) for autumn and winter harvest rather than sowing in late summer for overwintering as a young plant; choosing bolt-resistant varieties; and removing any emerging flower stems immediately when they appear to delay the plant's commitment to seeding. Once chard has bolted fully, the leaves become increasingly bitter and tough, and it is best replaced.