Why Are My Celeriac Leaves Curling?

Celery leaf miner

Celery leaf miner (Euleia heraclei) attacks celeriac just as it does celery, parsnip, and parsley. The larvae mine between the upper and lower leaf surfaces, creating pale, blistered, translucent patches that turn brown and papery as the consumed tissue dies. The leaves curl around the mined areas. There are two generations per year (May to June and July to August). On celeriac, leaf miner attacks during the early growing season can seriously set back the plant's growth if the young foliage is severely damaged before the root has had time to begin swelling.

What to do

• Cover celeriac transplants with fine insect mesh from planting through to late September. This single measure is the most effective way to prevent leaf miner damage on celeriac, where the long growing season makes repeated exposure to two full generations of leaf miner a significant problem.
• Remove and destroy mined leaves promptly, killing any larvae by crushing the mined section. On well-established plants from July onward, leaf miner damage to the outer leaves has little effect on root development.
• Inspect plants weekly from May for the first signs of mining: small, pale, blister-like patches that precede full mining damage.

Septoria leaf spot

Septoria leaf spot (Septoria apiicola) on celeriac produces small, brown, circular spots on the leaves and stalks that enlarge and merge in cool, wet conditions. Severely infected leaves yellow and curl as the disease kills the leaf tissue. The fungus is seed-borne and can also spread by rain splash from infected plant debris in the soil. It is worst in wet summers and on crowded plants with poor airflow.

What to do

• Space celeriac plants 30 to 40 centimetres apart to ensure good airflow between plants and reduce the humid conditions in which leaf spot thrives.
• Avoid overhead watering and do not work among plants when they are wet. Water at the base of plants.
• Remove and destroy affected leaves promptly. Rotate carrot-family crops each year.
• Use commercially produced seed from reputable suppliers, which is treated to reduce seed-borne septoria infection.

Aphids

Carrot-willow aphid (Cavariella aegopodii) attacks celeriac and causes the young, emerging leaves at the growing tip to curl and distort. In heavy infestations, the growing tip may become so distorted that the plant fails to develop further. Aphids on celeriac are most common from May to July.

What to do

• Inspect the growing tips of celeriac plants weekly from May for aphid colonies. Apply insecticidal soap spray to infested growing tips, covering all surfaces. Repeat every 5 to 7 days.
• Insect mesh protection that prevents leaf miner also protects against aphids simultaneously.

Drought

Celeriac requires consistently moist soil throughout its long growing season. Drought causes the leaves to wilt and curl and, critically, causes the swelling root to develop cracks and rough, hard patches. Irregular watering that alternates between drought and heavy rain causes the most severe root cracking. The smooth, well-formed roots that make celeriac easiest to prepare are produced only by an even, consistent water supply.

What to do

• Water celeriac every 5 to 7 days in dry conditions from planting through to root harvest. Never allow the soil to dry out completely. Apply a generous mulch of compost to retain moisture.
• Incorporate large quantities of compost into the bed before planting to improve moisture retention, particularly on free-draining soils.

Slugs

Slugs are a significant problem for celeriac transplants in the weeks after planting in May and June. They attack the stems and young leaves of newly planted celeriac, rasping irregular holes in the foliage and causing the plant to wilt and curl. Severe slug damage to young transplants can kill the plants outright before they establish.

What to do

• Apply nematode slug control (Phasmarhabditis hermaphrodita) to the planting area immediately before transplanting celeriac. This provides protection through the most vulnerable first 4 to 6 weeks after transplanting.
• Transplant celeriac as well-established, sturdy young plants with a good root system, rather than small, weak seedlings: larger transplants withstand slug damage more effectively.

Frequently asked questions

Why are my celeriac leaves curling?

Celeriac leaves curl most often from celery leaf miner damage or septoria leaf spot. Celery leaf miner (Euleia heraclei) is a fly whose larvae mine between the upper and lower surfaces of celeriac leaves, creating pale, blistered patches that cause the leaf to curl around the damaged area. Septoria leaf spot (Septoria apiicola) produces small, brown spots on the leaves that enlarge in wet conditions, causing yellowing and curling. Celeriac shares its pest and disease problems with celery, and many of the same cultural controls apply to both crops.

How do I get a large celeriac root?

Large celeriac roots require a long growing season, consistently moist soil, and adequate nutrients. Start celeriac under cover in February or March to maximise the growing window: celeriac needs 6 to 7 months of growth from transplanting to produce a root of good size before autumn frosts. Transplant by late May into a well-prepared bed with generous quantities of incorporated compost. Water consistently throughout summer, as drought significantly restricts root development. In August, remove the lower leaves of the plant to expose the top of the swelling root to light, which encourages the crown to develop properly. A consistent, even water supply without periods of drought produces the smoothest, least knobbly roots.

When do you harvest celeriac in the UK?

Celeriac is harvested in the UK from October onward, and the roots can be left in the ground through autumn and winter until needed, as they are frost-hardy to around -10 degrees Celsius when the ground does not freeze solid. Lift roots from October as they reach a good size (typically 10 to 15 centimetres across). In areas prone to hard frosts or waterlogged winter soils, it is safer to lift the roots in November and store them in boxes of barely moist compost in a cool, frost-free shed, where they keep for several months. Twist off the leaves and roots before storing; do not wash the roots until use.

How do I prepare celeriac for cooking?

Celeriac has a tough, knobbly outer skin that requires peeling with a sharp knife rather than a vegetable peeler: cut the top and bottom flat, then slice the skin away in sections from top to bottom, cutting deep enough to remove all the rough, knobbly surface. The peeled flesh discolours quickly when exposed to air; drop the pieces immediately into water acidulated with a squeeze of lemon juice or a splash of white wine vinegar to prevent browning. Celeriac can be eaten raw (grated as remoulade) or cooked: roast in chunks, boil and mash, add to soups and stews, or slice thinly and layer in a gratin.