Celeriac is a cool-season biennial grown as an annual. It is similar in growing habit and requirements as celery. Celeriac is best when it comes to harvest in cool weather. It is best started indoors and later transplanted into the garden. Sow seed indoors as early as 10 weeks before the average last frost date in spring. Celeriac requires 90 to 120 days to reach harvest.
Most importantly when you first learn how to grow celeriac is that it needs to be sown under cover in March. The seedlings are then planted out after the the last frosts in late May / early June. The young plants are started off early and spend a relatively long time indoors so we've a little bit of work to do to get them going.
Broadcast sow (Sprinkle) the seeds over a seed tray or pot containing a fine seed compost. Don't cover the seeds as they need light to germinate. You need to place the pot or tray in a warm place, a south facing windowsill is ideal or in a propagator if you have one.
Celeriac may take 2 or 3 weeks to germinate so don't worry if they seem to be taking a bit long! Keep the compost moist and don't let it dry out.
You will need to prick the seedlings out into modular trays about 2 weeks after germination.
To do this you need to gently loosen the roots beneath the seedling with a suitable stick. (We used a pen) and lift it gently by the leaves.
Fill a 6 cell modular tray with a good multipurpose compost and make a hole in the cells you want to plant about the size of the seedling root.
Place the seedling root in the prepared hole and carefully fill the compost in around the root. Plant 1 seedling per individual cell.
We have used lettuce seedlings in the photos but the principle is the same. It is important the seedlings are pricked out at an early stage to avoid root disturbance which can lead to bolting later on. It is important to prick out the seedlings at an early stage otherwise there will be too much root disturbance which can lead to bolting at a later stage.
Celeriac plants should be moved to the garden when about 10cm tall and hardened off before planting in their final positions. Plants that have been raised indoors will need to get used to the outdoor temperature and conditions before they can be planted outside, this will take about a week to 10 days depending on the weather.
The best way is to use a cloche or mini greenhouse. You can leave the cloche off the plants on dry frost free days and replace at night. Gradually increase the time with the cloche removed until the end of the week when you leave it off day and night. If the weather is mild you may not need the cloche, just move the plants outside for longer periods each day.
If you have started your seeds on a windowsill you will need to leave them in an unheated room for a day or two before moving outside to the cloche.
Celeriac should be planted out at a spacing of 35cm between plants and 35cm between rows. This plant likes moist soil conditions and plenty of compost to thrive. Will benefit from regular watering if the Summer is dry. Roots will grow larger in wet Summers.
Water your plants well an hour before planting. To plant your seedling make a hole in the soil the approximate size of the seedling 'plug'. You need to push the soil in around the roots firmly with your fingers to get good contact with the soil. Don't firm down on the top of the soil as this can compact it and prevent moisture getting down to the plants roots.
Water the plants after planting but do not soak them. You are better to transplant on a dull day or in the evening to prevent the plants wilting on a hot, dry day.
Young plants will need to be protected from slugs but otherwise it's generally a healthy and resistant vegetable.
Celeriac is a shallow rooted plant so make sure you water in dry spells. Be careful when hoeing around the plant as the shallow roots are easily damaged. It is important to keep the plants weed free so don't use that as an excuse!
The plants will also benefit from a top dressing of seaweed / poultry manure pellets during the summer.
Remove the outer leaves to expose the crown and encourage the bulb to develop.
Celeriac can be harvested from October onwards. You can store them in boxes of sand in a frost free shed or garage. In milder areas they can be left in the ground and lifted as required unless you have a slug problem in which case you should lift them all.
A knife or spade is handy as Celeriac has tight masses of fleshy roots which will need to be severed as you gently pull the plant up. If you are harvesting to store don't clean off too much as similar to carrots a thin layer of soil will help prevent roots drying out.