When to sow celeriac

Celeriac needs a long season, so it is always started indoors. Sow in February or March; commercial growers with heated glasshouses start as early as January. In the UK, March is the usual month.

The single most important thing is warmth. If young plants spend a few weeks below about 50–59°F, especially in the 41–46°F range, they take it as winter and bolt into flower the following summer instead of forming a root. Keep seedlings at a steady 61–68°F or more until they go outside, and don't harden them off too early.

Plant out once the frosts are over: from mid-May in Germany, Austria and Switzerland, usually after the Ice Saints, and in late May or early June in the UK. Young plants can go in until the end of June.

Celeriac seed is tiny and needs light to germinate. Scatter it thinly on moist seed compost and press it in, covering it at most with a light dusting of compost. At 64–68°F it comes up in about two to three weeks; keep the surface moist, ideally by watering from below.

When the seedlings are big enough to handle, prick them out singly into modules or small pots. Grow them on warm and bright until they have around six true leaves.

Choose a sunny spot with rich, moisture-retentive soil and dig in plenty of compost. Plant at the same depth as in the pot, with the base of the stem at soil level. Planted too deep, the root forms poorly.

Space the plants 12–16 in apart in rows 16–20 in apart. A 120 × 80 cm raised bed takes two rows; at 12 in that is 8 plants, at 16 in six. Closer spacing gives smaller roots.

Celeriac is thirsty. Keep the soil evenly moist all season; a thick mulch helps, as long as it doesn't cover the crown. The plants need the most water from mid-July to early September, when the root swells, and a dry spell then means small, woody roots.

Water at the base rather than over the leaves, which helps keep leaf spot away. From late summer, remove the outer leaves once they lie flat, and any side shoots, so the top of the root stays clear.

Celeriac grows slowly and puts on most weight in autumn, so leave it as long as you can. Lift from October, when roots are around 4–6 in across.

It copes with light frost. In mild areas, including much of the UK, you can leave it in the ground into winter under a thick mulch of straw and dig as needed. Celeriac is not reliably winter-hardy, so where winters are colder, lift it before hard frost.

Twist off the leaves, trim the roots and store celeriac cool, at 32–41°F, in boxes of damp sand or compost so it doesn't dry out. Kept like that it lasts for several months.

Celery leaf spot (Septoria) is the main disease: brown spots spread across the leaves, starting on the older ones. It is carried on seed and splashed by water, so buy good seed, water at the base and choose resistant varieties where you can.

Celery leaf miner larvae make pale blotches in the leaves; pick off affected leaves. Slugs go for young plants, and carrot fly can tunnel into the root. Fine insect mesh keeps both flies off.

Bolting in the first summer almost always goes back to cold seedlings in spring (see above).

Celeriac belongs to the carrot family (Apiaceae), with carrots, parsnips, parsley, dill and fennel. Never follow it directly with itself or another of these; leave several years, ideally four.

Celeriac is traditionally paired with leeks, cabbages or beans, and some tables claim it deters cabbage white butterflies. We found no good evidence for that. Treat such pairings as tradition.

The rule with a real reason is rotation: keep celeriac away from other carrot-family crops, with which it shares pests and diseases.