Why Are My Radicchio Leaves Curling?

Downy mildew

Downy mildew (Bremia lactucae) is the most common disease on radicchio in UK gardens, particularly in the cool, damp conditions of autumn when radicchio is approaching harvest. It causes angular, pale yellow patches on the upper surface of the leaves, bounded by the leaf veins, with a soft greyish-white mould visible on the undersides of the patches. The affected leaves curl, yellow, and eventually collapse. The disease spreads rapidly in dense plantings with poor air circulation and under cloches or cold frames with inadequate ventilation.

What to do

• Space plants at least 30 centimetres apart to allow air to move between them. Water at the base of the plant, not overhead. Remove and bin infected outer leaves promptly. Ventilate protected crops well, particularly in mild, damp autumn weather. Choose downy mildew-resistant varieties for autumn-winter crops under protection.

Aphids

Peach-potato aphid colonises radicchio from spring through to autumn, preferring the growing tips and the undersides of the innermost young leaves. In radicchio that is heading up, aphids can become trapped within the developing head, causing curling of the innermost leaves and creating problems at harvest. The developing head's tight structure makes it difficult to treat aphids once they are inside; prevention and early treatment are more effective than responding to a well-established infestation.

What to do

• Check plants regularly in spring and summer. Grow under fine insect mesh to exclude winged colonisers. Apply insecticidal soap to the leaves and growing tips before the head begins to form. At harvest, remove and separate any aphid-infested outer leaves and wash the head thoroughly.

Cold damage

Radicchio is cold-hardy and the tight inner head typically survives temperatures of -8 to -10°C without permanent damage. However, the outer leaves may freeze, turn translucent, and curl as they thaw. This is not a disease or pest problem: the outer leaves have been killed by frost while the inner head, protected by the outer layer, is usually undamaged. After a hard frost, leave the plant to thaw naturally without handling, then peel off the damaged outer leaves to reveal the sound inner head.

What to do

• Do not pick or disturb radicchio immediately after a hard frost; allow it to thaw first. Remove the frost-damaged outer leaves once thawed. A cold frame or cloche extends the season and protects the outer leaves from repeated hard frosts, while still allowing the cold exposure that triggers red colouration.

Bolting

Radicchio bolts if sown too early in spring or if summer temperatures are prolonged. A bolting plant produces a tall, branching flower stalk and the tight head form is lost: the leaves become loose, small, increasingly bitter, and may curl at the margins. Radicchio is sensitive to day length and temperature for heading up: plants sown too early may produce a head in summer, then bolt before autumn harvest; plants sown too late may not head up before winter. The optimal UK sowing window for reliable autumn heads is mid-June to mid-July.

What to do

• Sow radicchio in June to July for the most reliable UK results. Avoid spring sowings unless using bolt-resistant varieties specifically bred for spring production. Once a plant has bolted, remove it; the leaves are edible but very bitter and the plant will not form a head.

Frequently asked questions

Why are my radicchio leaves curling?

Radicchio (Cichorium intybus var. foliosum) leaves curl and distort most commonly because of downy mildew, aphids, or cold damage. Downy mildew (Bremia lactucae) is the most frequent cause of leaf curling on radicchio in UK gardens: it produces angular yellow patches on the leaf upper surface with a grey-white downy mould on the undersides, and the affected leaves curl and wilt as the infection spreads. Aphids (particularly peach-potato aphid) colonise the growing tips and undersides of young leaves, causing curling and puckering of newer growth. Cold damage can cause the outer leaves of radicchio to curl, soften, and turn translucent after a hard frost; the inner head typically survives and hardens off, and frost-damaged outer leaves can be removed to reveal undamaged tissue underneath. Bolting in hot, dry conditions causes the leaves to become small, curl, and lose the tight head form.

How do I grow radicchio in the UK?

Radicchio (Cichorium intybus var. foliosum) is the red-leafed chicory from northern Italy, prized for its bitter, peppery leaves and striking crimson and white patterning. It performs best in UK conditions when sown in mid to late summer (June to July) for harvest in autumn and winter: radicchio actually needs cold to develop its characteristic red pigmentation, and plants that experience warm temperatures throughout their life tend to remain green and loose. Sow in modules or direct, thin or transplant to 30 centimetres apart, and grow in full sun in well-drained, moderately fertile soil. Unlike lettuce, radicchio is very cold-hardy and can withstand several degrees of frost without damage; the outer leaves may freeze and look ruined, but the tight inner head survives and recovers. Some varieties (particularly the Treviso types) develop their best colour and flavour only after frost. Harvest by cutting the whole head at the base. Leaving the root in the ground often produces a flush of smaller, tender leaves in spring.

Why is my radicchio green instead of red?

Radicchio that remains green or predominantly green rather than developing its characteristic red and white coloration is most commonly the result of insufficient cold exposure, sowing too early, or growing a variety not well suited to UK conditions. Radicchio produces its red anthocyanin pigments in response to cold temperatures: plants that experience warm conditions throughout their growing period may fail to colour up and remain green or red-streaked. Sowing radicchio in June to July (rather than in spring) means the plant matures into autumn when cooling temperatures trigger the colour change. Varieties differ considerably in how reliably they colour in UK conditions: varieties described as 'Chioggia' types are among the most reliable for UK gardens. Lighter levels at the end of the season (shorter days, lower sun angle) also contribute to the colour change. A plant that remains stubbornly green is still edible, though the flavour will be milder.

Is radicchio a perennial?

Yes, radicchio (Cichorium intybus var. foliosum) is botanically a perennial plant: the roots survive the UK winter and will regrow in spring if left in the ground after the head has been harvested. The regrowth produces smaller, looser leaves than the original head, typically in spring, which can be used as a cut-and-come-again salad leaf crop. However, most UK growers treat radicchio as an annual, sowing fresh each year for reliable, full-sized heads. After a year or two in the ground, the overwintered roots may produce increasingly loose, bitter regrowth and are usually better dug up and replaced. The parent species, Cichorium intybus (common chicory), is a familiar blue-flowered perennial weed of roadsides and verges across the UK, and the same perennial vigour carries into the cultivated radicchio varieties.