How to grow celery from seed

Celery needs a long season, so it is always started indoors. Sow from mid-March to mid-April. With a heated propagator or a warm, bright windowsill you can start in February; earlier sowings mean an earlier harvest, but only if you can keep the seedlings warm.

Warmth is the whole trick. Celery is a biennial: a spell of cold tells it that winter has passed, and it then runs to seed in its first summer instead of making stalks. Temperatures around 41–46°F are the most risky. Keep seedlings above about 59°F while you raise them, and never let them sit below 50°F for more than about 12 hours until they are well established.

Plant out in late May or June, once nights stay above 50°F. In Germany, Austria and Switzerland that means after the Ice Saints at the earliest; in the UK, late May to mid-June.

Celery needs light to germinate. Scatter the tiny seeds thinly on the surface of moist seed compost and just press them in, or cover them with the barest dusting of compost. Keep the tray at 64–68°F and the surface moist; covering it with a lid or clear bag helps. Seedlings take two to three weeks to appear.

When they are big enough to handle, prick them out into individual small pots or modules and grow them on somewhere bright and warm. Harden them off gradually before planting out, and watch the night temperatures.

Plant into fertile, humus-rich soil that holds moisture well. Dig in plenty of well-rotted compost beforehand, ideally the autumn before. Self-blanching and green varieties go in a block rather than a single row, 10–12 in apart each way, so they shade each other's stems. In a 120 × 80 cm raised bed that is a block of four by three, 12 plants.

Set the plants at the same depth they were in the pot, water them in well and protect them from slugs straight away.

Celery wants a steady supply of water. Keep the soil moist all summer and water generously in dry spells. A mulch helps hold the moisture in.

On good soil the compost you dug in is enough. On poor soil, or if growth stalls, give a light dressing of an organic nitrogen-rich fertiliser; plants in containers need a balanced liquid feed every two weeks.

Most celery sold for gardens today is self-blanching or green and needs no earthing up; the block planting keeps the stems pale enough. For paler stems, tie a collar of cardboard or newspaper round the plants once they are about 12 in tall. Traditional trench celery is planted in a trench about 4 in deep and earthed up from late summer; it is more work and seldom grown now.

Self-blanching and green celery is ready from mid to late summer, about 11 to 12 weeks after planting out, and you can keep harvesting into autumn. Lift whole plants as you need them, before the outer leaves start to yellow.

These types are not frost-hardy. When nights turn cold, cover the plants with fleece or cloches, and finish harvesting before the first real frost. Trench celery is tougher and is traditionally lifted from October into winter.

Celery leaf spot (Septoria) is the main disease: brown to grey-brown spots spread over the leaves, and in warm, wet weather a whole bed can go down within weeks. Buy fresh seed from a good supplier, remove spotted leaves, water at the base and clear the bed at the end of the season.

The celery fly, or celery leaf miner, lays eggs in the leaves; its maggots eat broad, pale blotches that dry out. Pick off mined leaves as soon as you see them.

Slugs and snails love celery, especially just after planting out.

Celery belongs to the carrot family (Apiaceae) with carrots, parsnips, parsley, dill, fennel and celeriac. Leave at least three years before growing any of them in the same bed again; that also helps against leaf spot.

Mixed-planting tables often pair celery with leeks, cabbages, tomatoes or beans, and claim celery keeps cabbage whites away. We found no solid trials behind these pairings; treat them as tradition.

What does matter: don't plant celery right next to or straight after carrots, parsnips or parsley, which share its pests and leaf diseases, and don't put it where taller crops will shade it or compete for water.