Welcome to Top of the Crops - today, you will learn how to grow celeriac in a polytunnel. For more gardening insights, be sure to check out our blog Polytunnel Gardening too!
Celeriac, (Apium graveolens var. rapaceum) is not the easiest of vegetables to grow but a polytunnel where the soil has been amended and cared for well could be the ideal place to grow this root vegetable.
Celeriac is also known by several other common names including celery root and turnip-rooted celery. This vegetable's bulbous 'roots' (technically engorged stems called hypocotyl) are good in soups, stews and a range of other recipes.
While this is not a vegetable that is going to be winning awards for its looks, what it lacks in visual appeal it can make up for its pleasing, mild, celery-like flavour.
Celeriac originally comes from the Mediterranean region. It is now grown in many regions around the world. As well as harvesting the 'root' or bulbous stem, the leaves are also edible and can, like the leaves of celery, be used as a garnish or pot herb.
Celeriac is often considered to be a fairly challenging crop to grow mostly because it requires an extremely rich, fertile and moisture-retentive soil in full sun. Provide these conditions however and suddenly, it will not seem so challenging at all.
Choose your planting location carefully and make sure that the soil is rich and fertile, with a good proportion of organic matter. Make sure it is not a location that gets waterlogged in winter, nor one that dries out too much during the summer months.
The first thing you need to think about if you wish to grow celeriac is where to grow them to provide the above preferred conditions. But you also need to think about which specific variety of celeriac you will grow.
Celeriac seeds come in a range of different varieties. Some are more bolt and disease resistant than others. Modern varieties have also been bred to reduce the knobbles on the bulbs and make these ugly vegetables easier to prepare in the kitchen.
Suggestions on good varieties to grow are included a little later in this guide.
If you wish, celeriac can also be grown from plug plants purchased from a garden centre or plant nursery. But if you take this route then you will usually only have a very limited choice when it comes to variety, and often no choice at all. And of course this is a more expensive and less sustainable option than growing from seed.
Once you have chosen a suitable growing location and picked the right variety of celeriac for you then you can sow your seeds.
In locations with a longer growing season, celeriac can be sown outdoors where it is to grow, but here in the UK, we need to sow indoors in order to make sure that the plants have time to mature before the end of the growing season.
Celeriac seeds should be sown indoors where temperatures can be maintained at around 15 degrees C. for germination to take place. A heated propagator will make it easier and can increase the rates of germination, which can be slow and erratic.
Seeds should ideally be sown in March, into a suitable seed starting potting mix, and covered only lightly.
Once the seedlings emerge, they should be moved into a bright location where temperatures remain above 10 degrees C.. If the temperatures drop lower than this, it can lead the celeriac plants to bolt a little later in their lives. Make sure that you water your seedlings regularly and do not let them dry out.
Once they are large enough to handle, prick out and pot on seedlings into their own individual pots or soil blocks of multi-purpose potting mix.
Seedlings should be transplanted into their final growing positions only when all risk of frost has passed in the planting area.
This will usually be in late May or early June, and can be a little earlier in a polytunnel than it is if you are growing entirely outdoors.
When placing seedling in their final locations, make sure that the crown is not buried and firm the soil well around each plant. There should be a spacing of around 30cm between plants, 45cm between rows.
It can be a good idea to cover this crop with cloches or row covers during early growth in order to keep them warm and reduce the chances of bolting due to a cold snap in the spring. Your celeriac crop may also need protection from slugs and snails, especially while the plants are still small.
Celeriac certainly has a reputation for being rather challenging to grow successfully. But as long as you provide the right growing conditions and maintain consistent soil moisture around your crop then you should get a worthwhile harvest by the end of the year.
It is important to keep celeriac well watered during the summer, especially during particularly dry periods. In fact, this is often the problem area where things can go awry when you are growing celeriac at home. Make sure that you maintain consistently moist but not waterlogged conditions for your celeriac in order to achieve the best results.
Celeriac needs a soil or growing medium that is rich in organic matter, and adding an organic mulch can help ensure that the soil remains healthy and fertile. A good quality mulch will not only add slow-release fertility over time and protect the soil, it will also help to conserve soil moisture throughout the summer months. Just make sure when mulching that you do not bury the crown.
Mulching well will also help suppress weed growth to a degree. But you should also make sure that you stay on top of weeds in your vegetable patch so that celeriac can grow without too much competition for water and nutrients over time.
In the summer, remove the lower, outer leaves from the plants and draw some soil around the bulbous base of each plant which is the part that you will be looking to harvest later on. Removing the outer leaves when they flop outwards helps to expose the crown so it can develop.
Celeriac can be lifted as soon as they mature fully, which will be from October onwards.
Do not lift and store celeriac but rather leave them in the soil over winter until they are needed. Cover the plants with straw or fleece to shelter them from harsh winter weather and protect the harvest until it is required.
If you must lift celeriac for storage to make space in your garden then you can also twist off their leaves and then bury them in boxed of damp potting mix or coir in a cool location until they are needed.
When property stored, at temperatures between 0 and 5 degrees C. and not allowed to dehydrate, celeriac can keep for around 6-8 months. But rotting can easily occur through the centre of the celeriac so it is not easy to store celeriac for longer periods of time.
Once you decide to use celeriac you will discover that it can be used in a wide range of recipes. It is often mashed like potatoes, boiled or steamed or roasted, added to stews and casseroles or blended into soups. But it can also, interestingly, be grated and eaten raw in winter salads or slaws, for a somewhat different texture and flavour.
Highly regarded varieties of celeriac for UK growers include:
All of these four varieties have an award of garden merit from the RHS.
The most common issues that arise when growing celeriac do so because there is some issue with the growing conditions or care. Choose the right location and care for your crop correctly as above and you should find that this crop is far more likely to provide a successful harvest towards the end of the year.
However, a number of pests can also be a problem for celeriac and so growing them in a polytunnel can also help as it is easier to control the environment and manage pest problems in a contained space.
A diverse and resilient ecosystem is the best defence against excessive numbers of slugs, which are one pest that can commonly pose a problem for celeriac plants. Defend your plants against slugs by making sure you attract plenty of birds, and other slug predators to your garden.
Celery leaf spot is one other common problem for celeriac as well as for its relatives. Do not grow celeriac where there has been celery leaf spot in previous years and be sure to maintain a good system of crop rotation to protect against disease. If you do discover that you encounter problems with disease, you may choose to opt for a treated seed and more resistant varieties in the future.
To grow celeriac successfully in a polytunnel, you must have the right irrigation or watering system in place to maintain the consistent moisture that this crop requires. A drip irrigation system can be a good solution to conserve water while consistently making sure that the celeriac gets what it needs.
Correct spacing and good airflow are also important when growing celeriac in a polytunnel since overcrowding can cause a number of issues.
Companion planting with celeriac can also help you to provide optimal conditions and increase the overall yields that you are able to achieve within a polytunnel garden.
Leeks like similar growing conditions to celeriac and can be a good companion plant along with certain fragrant herbs which could help stop problems with carrot fly and other pests. Other alliums may also help to confuse and/or deter pests.
It typically takes around 6 months from sowing to harvest, so this is a crop that requires a long growing season. This is why we usually get a head start by sowing seeds indoors in earlier spring here in the UK. If we direct sowed this crop it would not usually have time to reach maturity and harvestable size by the end of our growing season.
Celeriac is ready to harvest once the bulbous lower stem is around the size of a large apple – approximately 7-14cm in diameter – and the skin has a rough and knobbly appearance. Remove a little soil around the base of the plant and you should be able to see this enlarged stem section and determine whether you wish to harvest or will wait a little longer.
Yes, slugs will unfortunately often eat celeriac and the plants can be particularly vulnerable during wet conditions. Slugs will often eat the leaves of celeriac, especially when the plants are still small, and can also sometimes eat the main 'root' part we wish to harvest.
As mentioned above, celeriac grows well alongside several other crops, including leeks and other alliums. But it should not usually be planted alongside celery or carrots since it shares a number of pests and diseases with these other related crops.
BBC Good Foods. (2020) Celeriac Recipes. [online] Available at: https://www.bbcgoodfood.com/recipes/collection/celeriac-recipes