How Do I Grow Celery and Celeriac Indoors from Seed?

Celery and celeriac are the vegetables most often described as difficult, and with some justification — their seeds are tiny, slow to germinate, and the plants need a very long growing season. But with the right method, both are achievable for any patient gardener. The rewards are worth it: home-grown celery has a depth of flavour commercial varieties cannot match, and a celeriac root the size of a grapefruit is one of the most satisfying harvests of autumn.

When to Sow

Celery and celeriac need the longest indoor start of any common vegetable — 10–12 weeks before the last frost date, which typically means late February to early March in the UK and similar timing in the US. Their slow early growth means anything later risks short-changing the growing season. Celeriac in particular benefits from an early March sowing to give it maximum time to develop its swollen root.

Sowing Method: Surface Only

Celery and celeriac seeds are so small that they are not covered with compost — they need light to germinate and burying even a few millimetres prevents germination. Mix the tiny seeds with dry silver sand to make them easier to distribute evenly, then scatter thinly across the surface of moist, fine seed compost. Press gently with a flat board so seeds make good contact with the compost, then cover the tray with glass or cling film and place in a propagator at 18–21 °C. Germination typically takes 14–21 days.

The Bolting Risk

Both celery and celeriac can bolt (run to seed prematurely) if exposed to cold temperatures as young seedlings. Temperatures below 10 °C (50 °F) for extended periods — more than a week — after germination can trigger premature bolting. Keep seedlings in a warm environment (minimum 12 °C at night) throughout the indoor phase and for the first few weeks after planting out.

Pricking Out and Growing On

Seedlings are tiny when they first appear — prick out when they have their first pair of true leaves (which look like miniature celery leaves), handling them by the cotyledon only. Move into module trays or 5 cm pots. Water consistently — celery and celeriac need constantly moist (never dry) conditions throughout their growing life. Begin dilute feeding at the third true leaf stage.

Planting Out

Harden off over 10–14 days and plant out after the last frost into moisture-retentive soil enriched with plenty of compost. Celery benefits from planting in a slight depression to allow for blanching; celeriac is planted with the crown at soil level. Both benefit from regular watering through summer — drought stress causes stringy stalks and split celeriac roots.