Celeriac Too Small? Try These Tips to Help It Swell

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Celeriac, or celery root, is the unsung hero of root vegetables thanks to its taste and versatility in the kitchen. It can also easily compete for the ugliest vegetable ever grown. Popular in Europe, this hardy plant dates back to the celery leaves growing wild in the Mediteranean.

The hardest part about growing celeriac is getting it to swell to a decent size. And to do that, you need to understand what helps it thrive – which is what you’ll learn in this article.

Celeriac is a long-season crop, so to grow large celery roots, start it early and prick once into larger modules. Spacing it at least 30cm apart will give it room to grow, and regular thorough watering will help it swell. Fertilize at least once with organic fertilizer and harvest it in late autumn.

It’s also crucial to pick a variety that forms big bulbs, and to adjust your expectations – but we’ll talk about that later on. No matter what variety you choose, your celeriac should at least reach a 4-inch diameter.

The first hurdle to overcome is germinating celeriac seeds. They are very difficult to sprout for novice gardeners, because they require precise temperatures, constant moisture, and, most importantly, exposure to light – so don’t bury them in the soil! To kickstart your celeriac season, check this article on germinating celery seeds – it applies just as well to celeriac seeds, which are a bit bigger.

If you’ve already managed to germinate celeriac seeds only to end up with a golf-ball size celery root – first of all, congratulations, you’ve at least managed to grow some celeriac. But let’s have a look at what might have gone wrong:

Why celeriac stays small

Here is a summary of possible causes why your celery roots are unimpressive:

  • You started the seeds too late.
    Remember, celeriac is a long season crop – a 6-month long project – and you need to get started as soon as the gardening season begins, and harvest them as late as possible.
  • You didn’t prick the seedlings or thin them.
    Even young seedlings take a long time to grow – think 10-12 weeks from sowing to final transplating. The seeds are small, so if you don’t sow them precisely, thin them in time or prick them into individual pots, they will compete for root space and light and stay leggy and small.
  • You spaced the seedlings too close together.
    Celeriac roots will compete for space and their foliage will need room to develop.
  • Your seedlings got damaged by frost, wind or slugs.
    While mature celeriac is frost hardy, the seedlings are more susceptible to frost, so plant them out once the danger of frost has past. Celeriac also does better in more sheltered locations, as strong winds can stunt their growth. Slugs can also feast on young celeriac leaves and it will take the seedlings time to recover.
  • You’re not watering nearly enough.
    Scarce and irregular watering is a recipe for disappointment when it comes to celeriac. Celeriac is a marsh-loving plant and grows wonderfully in boggy conditions. You can only mimic that environment by giving it plenty of water from transplant to harvest.
  • Your soil is depleted of nutrients.
    Celeriac is a hungry plant that needs lots of organic matter and extra fertilizers from time to time.
  • You’re not being patient.
    Celeriac only starts to swell in late summer and it takes it a couple more months to grow to mature into a full size.

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How to grow large organic celeriac

First of all, you need to choose a variety that can get to a decent size. I recommend growing Prinz, Monarch or Giant Prague – these are the most popular options for home gardeners. But even with these varieties, celeriac is considered to have reached maturity when it gets to 4 to 5 inches in diameter.

Now, you can certainly grow a celery root that’s bigger than that, but don’t expect to see the same results as commercially-grown celeriac that’s sold in supermarkets, laden with chemical fertilizers and pesticides.

Here are the main steps of growing large healthy celeriac plants, the organic way:

  1. Sow the seeds on time, under cover, or better yet indoors, under lights. Sow mid to late February, prick out in March and pot on in larger pots as necessary.
  2. Harden off the seedlings in May before planting to final location.
  3. Space seedlings 30cm apart in every direction.
  4. Cover seedlings with a layer of horticultural fleece to protect from the cold and winds, until weather improves.
  5. Protect seedlings from slugs, using a sprinkle of zeolite around the plants or wool pellets.
  6. Begin a regular watering regimen, or better yet, install a drip irrigation system, as raised beds tend to dry out.
  7. When the root swells to the size of a walnut, remove the dirt from the top half. Drying of the top root promotes even more growth.
  8. Prune wilted leaves regularly – cut them at the base to direct the plant’s energy to the roots. Keep the foliage nice and tidy.
  9. Aside from planting in a soil rich in organic matter, fertilize at least once with organic fertilizer such as chicken manure pellets or vermicompost before it’s ready for harvest.
  10. When they’re ready for harvest, celeriac roots should be mounding off the ground – especially varieties like Monarch or Prinz
  11. Roots begin to swell in late-summer, early autumn, so hold off on harvesting until at least October. Celeriac can store in the ground over the winter in mild temperate climates – zone 8 and up.

Companion planting for celeriac

If you’re looking to grow celeriac as part of a polyculture, you certainly can. Just make sure you don’t grow it alongside other root crops or hungry plants like corn because it will compete for nutrients.

The best companion plants that you can interplant in between your celeriac are:

  • Onions and spinach – they act as a low crop, and their shallow roots won’t disturb celeriac’s growth.
  • Tomatoes – as a high crop. Make sure you train them up a stake and prune them heavily. This way, their base will get good airflow and not get tangled up with celeriac’s foliage.


If you’ve reached the glorious moment of harvest, use a small garden fork to lift the roots from the ground. It will look tangled up and weird, but all you have to do it cut the root part off and peel until you reach white flesh. Don’t forget to use the green part as well – celeriac produces thinner celery stalks, but they’re just as delicious as regular celery.

You can store this wonderful root crop for as long as 8 months and it will make a delicious base for soups, stews and purees, or consumed raw in salads and slaws. All in all, growing celeriac is definitely worth the trouble – and the patience!

Check out these must-have gardening products

You don’t need much to start gardening, but some tools and products will make a difference in how comfortable and effective gardening can be for you. Here are my favorites:

  • Garden Trowel. A good garden trowel will last you many years. I love how sturdy this hand trowel from WOLF-Garten is, the metal doesn’t bend and it has a nice grip.
  • Trimming Scissors. I use them for delicate pruning and harvesting all summer long, and they’re super handy. These Teflon Trimming Scissors are extra nice because they don’t rust as easily.
  • Dutch Hoe. Dutch hoes may seem old-fashioned, but there’s nothing like a quick sweep through the topsoil to get rid of small weeds – no bending required. I love WOLF-Garten’s selection: this dutch hoe coupled with their universal handle.
  • Grow Lights. These grow lights from Mars Hydro are super strong, yet dimmable, so they fit every stage of growth. They don’t put out too much heat and are very economical.
  • Seedling Trays. There’s an art to choosing the best size for seedling trays so that it holds the perfect amount of water and gives the roots enough room to grow. These germination plugs are perfect when coupled with 1020 bottom trays
  • Liquid Fertilizer. You’ll need to feed your plants from the seedling stage, all the way to fruiting. This organic fish & seaweed blend is a very versatile option. Use it half-strength for young plants and full-strength for established plants.

Browse our list of tools, fertilizers & pesticides, indoor growing products and seed shop recommendations – we hope you find our selection useful and it saves you some time!

Adriana Sim

Hi, I'm Adriana Sim, owner of Tiny Garden Habit. I practice my green thumb in beautiful Transylvania, Romania, zone 6b. While my garden is not quite tiny, it's definitely compact and super-productive. You can grow a lot of food in a small space, and it's my mission to teach you how!

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