Growing Onions in the UK: Sets, Sowing and Storing

Onions are the sort of crop that makes you feel like a proper gardener with almost no effort. Stick a few sets in the ground in spring, keep the weeds down, and by late summer you'll be lifting fat bulbs to hang in the shed. They store for months, they go in nearly everything you cook, and they rarely throw a tantrum. If you're just starting out on a plot or a raised bed, onions are a brilliant place to begin.

Should I grow onions from sets or seed?

There are two ways to grow onions, and the choice matters more than you'd think.

Sets are small, immature onion bulbs that were part-grown last year and stopped early. You buy them in little net bags, push them into the soil, and they carry on where they left off. They're the easy, dependable option: they shrug off cold weather, they don't need heat to get going, and they crop reliably even in a rubbish summer. For a first-timer, sets are the clear winner.

Seed is cheaper by a mile and opens up a far wider choice of varieties, including some cracking heritage and exhibition types you'll never find as sets. The trade-off is effort. Onion seed needs an early start indoors, usually from January or February, plus warmth, light and patience while the seedlings grow on. It's a lovely thing to try once you've got a season or two behind you.

When should I plant onion sets in the UK?

This is where a lot of beginners come unstuck, so it's worth getting clear. There are two separate planting seasons, and they give you two different harvests.

Autumn or overwintering sets go in during September and October. These are often sold as Japanese onions, and they're bred to sit through a British winter without bolting. They root before the cold sets in, tick over through the dark months, then bulk up fast in spring to give you an early crop in June and July. If you love the idea of pulling your own onions before most people have even planted theirs, these are the ones.

Maincrop sets go in during March and April, once the worst of the winter is behind you and the soil is starting to warm. These are your workhorse onions, ready to harvest from late summer, and they're the type most people grow. They also tend to store better than the autumn crop, so if you want onions in the kitchen through winter, plant plenty of these.

Two planting windows, two harvests. Autumn sets for an early treat, maincrop sets for the ones you'll be eating at Christmas.

You can grow both if you've the room, staggering your harvests across the year. Just don't plant maincrop sets too early into cold, wet ground, as that's a fast route to bolting.

How do I plant onion sets?

Onions aren't fussy, but they do want a sunny, open spot with soil that drains well. Heavy, waterlogged ground makes them sulk and rot. Dig over the bed, pull out every weed you can find, and rake it to a reasonably fine, crumbly surface. If your soil is on the poor side, a bit of well-rotted compost worked in beforehand helps.

1. 1

   Prepare the bed

   Choose a sunny, well-drained spot and clear it of weeds. Rake the soil to a fine, soft tilth so the sets go in easily.

2. 2

   Space them out

   Position the sets about 10cm apart, in rows roughly 30cm apart. Wider spacing gives you bigger bulbs; closer spacing gives you more, smaller ones.

3. 3

   Push them in

   Gently press each set into the soft soil, pointed tip upwards, so just the very tip is still showing at the surface. Don't bury them completely.

4. 4

   Firm and protect

   Firm the soil around each one. Birds love to tug freshly planted sets out of the ground, so cover with netting or fleece until they've rooted in.

5. 5

   Water in if dry

   If the soil is dry, give the bed a gentle watering to settle the sets and encourage them to root away.

Don't be alarmed if a bird pulls one or two out anyway in the first week. Just push them back in. Once the roots take hold, they'll stay put.

How do I look after growing onions?

Here's the single most important thing to know: onions hate competition. Their roots are shallow and they don't like sharing water and nutrients with weeds, so keep the bed clean. Hand weed or hoe carefully and often, taking care not to nick the bulbs, and your onions will reward you.

Watering is simple. In a normal damp British summer you may barely need to water at all once they're established. During a proper dry spell, give them a good soak every couple of weeks to keep the bulbs swelling. As the onions approach maturity in mid to late summer, ease right off. Stop watering and stop feeding, and let the bulbs firm up and ripen. Too much water late on makes for soft onions that won't keep.

You generally won't need to feed if the soil was in decent condition to start with. Overdoing the nitrogen produces lush leaves and soft bulbs that store badly, so go easy.

When and how do I harvest onions?

Your onions will tell you when they're ready. As summer wears on, the leaves stop growing, then flop over, yellow and begin to die back naturally. That collapse is the signal that the bulbs have finished swelling. Resist the old advice to bend the tops over yourself. Just let them go over in their own time.

Pick a dry day and ease the bulbs up gently with a fork, loosening the soil so you don't bruise them. Damaged onions rot in store, so handle them like eggs.

Now for the step that makes all the difference: curing. The bulbs need to dry out properly so their skins go papery and their necks seal. Lay them out in a single layer somewhere warm, dry and airy: a sunny spot outdoors if the weather's kind, or a greenhouse, porch or airy shed if it isn't. Leave them for around two weeks, until the skins rustle and the tops are fully dry. Cured onions keep for months. Skip this and they'll be soft by autumn.

How do I store onions so they last?

Once cured, storage is easy. Onions want somewhere cool, dry and airy, never warm and damp. Pop them in net bags, spread them in shallow trays, or plait the dry tops together and hang them up the old-fashioned way, which looks lovely in a pantry. Good keepers, especially maincrop varieties, will happily last right through winter and into spring.

Check them over now and then and pull out any that feel soft or start to sprout. Use those first. Eat any thick-necked or bolted onions early too, as they won't store well.

What problems should I watch out for?

Onions are largely trouble-free, but a few things are worth knowing about.

Onion white rot is the one to respect. It's a soil-borne fungus that causes fluffy white mould and rotting at the base of the bulb, and once it's in your ground it can survive for a great many years. There's no cure, so it's all about prevention. Rotate your onions and other alliums around the plot, keep your tools and boots clean, and never, ever plant supermarket onions that might bring it in.

Downy mildew shows up in wet summers as pale, furry patches on the leaves. Good spacing and airflow help, so don't cram your sets in too tightly, and clear away any badly affected foliage.

Bolting is when a plant sends up a flower stalk instead of bulking up. It's usually caused by cold after planting or by going in too early. Choose heat-treated sets where you can, hold off planting maincrop sets until the ground warms, and keep plants growing steadily. A bolted onion is still edible, just use it quickly.

Onions really are a gentle introduction to growing your own food, and there's a proper thrill the first time you lift a row of them. Once you've got the hang of it, weave them into a sensible rotation with your other crops so the soil stays healthy year after year. Have a look at our crop rotation guide for a simple plan, and browse more beginner-friendly crops over on the Grow hub.