Celeriac needs at least six months from seed to a finished bulb. That means sowing indoors in late February or March, and sowing too late is the most common reason the bulb ends up half the size of the one in the shop. Keep to the calendar and you get a bulb of 10-14 cm that keeps in store until March.
Six months from seed to bulb
Celeriac belongs to the Rapaceum Group within the species Apium graveolens. Sown in February, harvested September to November: seven months, and that time cannot be shortened. Other root vegetables can be sown in May and still finish. Celeriac cannot.
| Timing | What happens | Why then |
|---|---|---|
| Late February to mid-March | Sow indoors, in good light | The bulb needs at least six months |
| March to April | Prick out into individual pots, around 16 °C | Cool and bright gives compact plants |
| Late May to early June | Plant out after hardening off | Nights must be reliably above 10 °C |
| June to August | Steady moisture and feeding | A single dry spell shows up in the bulb at harvest |
| Mid-August | Clear the soil off the top of the bulb | The bulb should sit largely above ground |
| September to November | Harvest at 10-14 cm across | The flavour deepens after the first cold nights |
At Florea we sow celeriac in mid-February, under grow lights. In most of the UK and northern Europe a windowsill gives too few hours of light that month for the plant to be ready to go out by late May. There is more on early sowings in our article on starting seeds indoors.
Check the seed packet before you sow
Three different vegetables are sold under the name celery and they often sit side by side on the seed rack. Buy the wrong packet and you will never get a bulb. The difference is in the Latin name.
Rapaceum Group: Celeriac. Gives a bulb of 10-14 cm and is the one you want for puree and soup.
Variety dulce: Stalk celery, usually sold simply as celery. Gives thick stalks but no bulb.
Variety secalinum: Cutting celery, also called leaf celery. Gives leaves to use as a herb and still manages a sowing as late as April.
All three are the same species, but they differ in care and harvest time, which we go through in Growing celery. You will find seed among our vegetable seeds.
Germination cannot be hurried
Celery seed needs light to germinate and should stay at the surface, with at most a thin scattering of seed compost over it. Allow 2-3 weeks for germination. It comes unevenly, so do not write the tray off because only half of it is up after two weeks.
More heat does not solve it, hold 15-20 °C. Stand the tray on a radiator and it gets so warm that the seed goes dormant, and germination stops altogether instead of speeding up. That is a common reason a February sowing has to be done again in March, which costs a month of a tight season.
Celeriac cannot be sown direct outdoors. There is not enough time. Vegetables that take a sowing straight into the bed are among our seeds for direct sowing.
A cold spring makes the plant flower instead of forming a bulb
Celeriac is biennial. In the first year the plant builds the bulb, in the second it sends up a flower stem and sets seed. A young plant kept too cold reads the chill as a winter having passed and jumps to year two. The result is a flower stem in July and a bulb the size of a walnut.
The line is 10 °C. Plants should be held above it both indoors after germination and outdoors after planting. In practice that means waiting until late May or early June to plant out, and having horticultural fleece to hand if the forecast shows cold nights the week after.
Planting out, depth and spacing
Despite the name, the bulb is not a root. It grows on the lowest part of the stem and should sit largely above the soil surface, so set the plant with the base of the stem right at ground level. Plant it deeper and roots form all over the surface instead of only at the base, which leaves you more to peel away.
Plant spacing: 30 cm between plants.
Row spacing: 40-50 cm.
Soil: Deep, rich in organic matter and moisture retentive. The soil should never be allowed to dry out completely over summer. Work compost into the bed in spring.
Position: Sun or light partial shade.
Spacing governs size more than most people expect. Closer than 30 cm and the bulbs come out smaller, because the plants compete for both water and room. If you are growing for storage it is worth giving them a little more.
Why the bulb ends up small or knobbly
Five mistakes account for most failed celeriac crops, and they show up in different ways in October.
| What you see | Likely cause | What to do instead |
|---|---|---|
| The bulb stopped at 4-5 cm | Sown too late, the season ran out | Sow in late February, by mid-March at the latest |
| Flower stem in July and no bulb | The plant sat below 10 °C when young | Plant out no earlier than late May, keep fleece to hand |
| Roots all over the surface of the bulb | The bulb ended up under the soil | Set the stem base at ground level, clear the top from mid-August |
| Hard, woody rings inside the flesh | A dry spell in mid-season | Water before the soil dries out, and mulch the bed so the moisture stays |
| Even but small bulbs down the whole row | Too little room between plants | 30 cm between plants, 40-50 cm between rows |
A sixth fault only shows when you cut the bulb open: brown, hollow patches in the middle while the surface looks perfect. That is boron deficiency, and we cover it in the article on celery.
Four cultivars and what separates them
Choose a cultivar for whatever usually goes wrong for you, not for the picture on the packet. Four cultivars hold the Award of Garden Merit from the Royal Horticultural Society in the UK, and they differ on points that matter in a cool climate.
Asterix: Matures earlier than the others. The one to choose if you know the sowing rarely gets done in February.
Prinz: Resistant to bolting, so less inclined to run to flower after a spell below 10 °C. Large bulbs and white, crisp flesh.
Ibis: Fast growing, smooth skinned and resistant to disease.
Monarch: Smooth skin, juicy flesh and a nutty flavour. A smooth skin means less waste when you peel.
If you are only picking one: take Prinz if cold is your problem and Asterix if the calendar is.
Harvest and storage over winter
Harvest from September, once the bulb measures 10-14 cm across. The bulbs take light frost and the flavour deepens after the first cold nights, so there is nothing to gain from rushing the harvest in mild weather. Twist the tops off rather than cutting them.
From there you have two routes, and the choice depends on the soil.
Leave the bulbs in the bed: Works on free-draining soil in mild spots. Cover with 10-15 cm of straw or leaves in October and lift as needed until March. On heavy clay that gets waterlogged the bulbs rot instead.
Lift and store indoors: The safer choice, and what you do where the ground tends to freeze deep. Pack the bulbs in damp sand or damp, peat-free compost in a box and keep it dark.
Stored at 0-5 °C and high humidity the bulb keeps for 6-8 months, as long as it is not allowed to dry out. That reaches March, when the seed packets for the next season go in. The same box of damp sand works for swede.
Author: Emma Vogiatzi
Fact-checked by Erik Hoekstra
Last updated 2026-09-25