How to Grow Onions in the UK

Onions are one of the most useful crops you can grow. They store for months, go into almost every meal and need very little looking after once they’re in. A few rows of sets planted in spring can keep your kitchen supplied well into winter.

You can grow them from sets (small, part-grown bulbs) or seed. They cope with most soils and our unpredictable springs, and they don’t take up much room. If you’re new to growing food, our guide to starting a vegetable garden covers the basics.

Sets or seed: which should you choose?

Choose sets for the easiest, quickest crop. Choose seed if you want more varieties for less money and don’t mind starting early indoors.

Onion sets are little bulbs you push into the soil in spring. By late summer they’ve grown into full-sized onions. They get going fast, keep ahead of weeds and are less fussy early on. Garden centres sell them from February.

Onion seed opens up lots of varieties you can’t buy as sets, and one packet holds hundreds of seeds. The catch is time. You sow indoors in January or February and plant out in April, and they take a few weeks longer to be ready.

Top tip: Growing onions for the first time? Start with sets, and buy heat-treated ones if you can. They’re less likely to bolt, which is the most common problem for new growers.

Best onion varieties for UK gardens

Good brown onions include Sturon, Stuttgarter Giant and Centurion. For red, try Red Baron. All are easy to find as sets or seed.

White and brown onions

• Stuttgarter Giant: a reliable main crop with flat, golden bulbs and a mild flavour. Keeps well.
• Centurion: a heavy cropper that’s slow to bolt. Round, firm bulbs. A great all-rounder.
• Setton: bred for long storage, with golden skin and a strong flavour. Slow to bolt.
• Sturon: round with a mild, sweet taste. A good keeper, sold as sets and seed.

Red onions

• Red Baron: deep red skin and pink-tinged flesh. Sweeter than most brown onions. Lovely fresh in salads.
• Red Karmen: flat, dark bulbs with stronger colour and flavour than Red Baron.

Shallots

• Golden Gourmet: big, long bulbs with a mild flavour. Each set grows into a cluster of shallots.
• Red Sun: copper-red skin and a slightly stronger flavour. Keeps well.

Top tip: For onions over a longer season, grow Centurion to use fresh through summer and Setton to store for autumn and winter.

How to plant onion sets

Plant sets in firm, sunny ground in spring, 10cm apart with just the tip showing. Get that right and they mostly look after themselves.

When to plant

Spring sets go in from mid-March to mid-April, once the soil has dried enough not to stick to your boots. Mid-March is fine in most of the south. In the Midlands, late March is safer. In northern England and Scotland, wait until early April. Our UK vegetable planting calendar has timings for every crop.

Overwintering sets go in during October or November. Varieties such as Radar, Shakespeare and Senshyu Yellow root before winter, then race away in spring. They’re ready in June or early July, well before spring-planted onions.

Preparing the ground

Pick a sunny, open spot with well-drained soil. Dig in well-rotted compost or manure a few weeks before planting and let the soil settle. If you’ve been making compost, autumn is a great time to spread it on the onion bed. Tread the soil gently before planting. In loose, fluffy soil the sets tend to pop back out.

Avoid fresh manure. It gives soft, lush growth that gets diseases and stores badly.

Spacing and depth

Push each set in so just the tip shows. Firm the soil around it.

• Between sets: 10cm for full-sized onions, 5cm for pickling onions
• Between rows: 25-30cm
• Shallots: 15cm apart, rows 30cm apart

Wider spacing gives bigger bulbs. The RHS onion growing guide suggests 10-15cm between sets.

Only the tip of each set should show above the soil. Leave about 10cm between sets if you want full-sized onions.

Keeping the birds off

Blackbirds love pulling up new sets, mistaking the tips for grubs. Cover new rows with fleece or netting for the first two or three weeks. Or snip off the dry, wispy tip of each set before planting, which takes away the bit that tempts them.

Is it too late to plant onion sets in May?

Early May is about the latest for spring sets, and you’ll get smaller bulbs. Onions start to swell when the days get long around midsummer, whatever size they are. A late set has less time to grow leaves first, and fewer leaves mean a smaller bulb.

If you’ve missed April, plant the sets a bit closer and use them as smaller cooking onions. Or wait and plant an overwintering variety such as Radar or Senshyu Yellow in October. Spring onions are another option, as you can sow them until August.

Growing onions from seed

Sow seed indoors in January or February, or outdoors from late March in mild areas. It needs an earlier start, but you get more choice for less money.

Sowing indoors

Start seeds indoors in January or February. Fill module trays with seed compost and sow four to six seeds in each cell. Cover thinly, water gently and keep them on a windowsill or in a propagator. They come up in a couple of weeks.

The seedlings look like thin blades of grass. Don’t separate them. In April, plant each clump outside 25cm apart. The onions push each other apart as they grow and make a neat little cluster. This is called multi-sowing, and it saves lots of fiddly work.

Leave each clump of seedlings together and plant it out as one in April. The onions nudge each other apart as they swell.

Sowing outdoors

In mild areas, sow straight into the bed from late March to mid-April. Sow thinly in drills 1cm deep and 25cm apart. Thin the seedlings to 5-10cm apart once they’re big enough to handle.

Feeding, watering and weeding

Onions need little feeding, steady water while the bulbs swell and lots of weeding. They aren’t hungry plants. If you added compost before planting, they rarely need more. On poor soil, give one sprinkle of general fertiliser in May. Stop feeding by late June, as late feeding makes onions that store badly.

Water in dry spells in May and June, when the bulbs are swelling. A good soak once a week beats a little every day. Too much water can cause rot, especially on clay. Stop watering once the leaves start to yellow and flop. Wet bulbs won’t dry or store well.

Weeding really matters. Onion leaves are thin and upright, so they can’t shade out weeds. Hoe between the rows often, keeping the blade shallow so you don’t nick the bulbs, and hand-weed between plants. A raised bed helps keep creeping grass out.

Top tip: In May, spread a thin layer of well-rotted compost between the rows. It keeps weeds down and moisture in. Keep it thin, as a thick mulch holds damp against the bulbs.

Onion leaves are too thin to shade out weeds, so little and often with the hoe is the way. Keep the blade shallow so you don’t nick the bulbs.

What should you do if an onion starts to flower?

Snap or cut off the flower stem as soon as you see it, then use that onion first. Once it flowers, the onion puts its energy into the flower instead of the bulb, and it won’t store well.

The flower stem leaves a hard core through the middle. Lift bolted onions with the rest and use them in the kitchen within a few weeks. To cut bolting next year, buy heat-treated sets and don’t plant into cold, wet soil. Our guide to why onions bolt explains more.

That tall stem with a papery bud means the onion has bolted. Snap it off and use that bulb first, as it won’t store.

Why are my onions small?

Small onions usually come from late planting, crowding, weeds or dry weather in May and June. Bulb size depends on how much leaf the plant grows before midsummer, so anything that slows early growth shows at harvest.

Check these in order:

• Late planting: sets planted after April have less time to grow leaves.
• Too close: sets 5cm apart give pickling onions. Use 10-15cm for full-sized bulbs.
• Weeds: they quickly steal light, water and food.
• Dry spells: not enough water while the bulbs swell keeps them small.
• Poor soil: hard or tired ground limits the roots. Add compost before your next crop.

Pests and diseases

Onion fly

Onion fly is the most common pest. The flies lay eggs at the base of the plants from May to July, and the maggots tunnel into the bulbs and make them rot. Affected plants yellow and wilt. Pull up and bin any affected onions. Don’t compost them.

Prevention works best. Cover rows with fine mesh or fleece from May. Move onions to a new bed each year, too. Sets usually suffer less than seed-grown onions because they’re tougher by the time the flies arrive.

White rot

White rot is the most serious onion disease. It’s a fungus in the soil that causes fluffy white mould at the base of the bulb and yellow, wilting leaves. There’s no cure, and the RHS explains that it can stay in the soil for many years.

The answer is keeping it out of clean ground. It spreads in soil on tools and boots, which is a real risk on allotments. Never move soil from an affected bed to a clean one. Remove affected plants with the soil around them and put them in the household bin, not the compost heap. If your plot has had white rot, grow onions in raised beds filled with fresh compost.

Fluffy white mould at the base and yellowing leaves point to white rot. Bin the plant with the soil around it, never the compost heap.

Downy mildew

Grey-purple furry patches on the leaves, often worst in wet summers. Give plants space so air can move, and water the soil rather than the leaves. Remove badly affected leaves. Our guide to onion downy mildew has more.

Neck rot

A storage problem where the neck of the onion goes soft and brown after harvest. It gets in through damage or necks that haven’t dried properly. Dry your onions thoroughly before storing and handle them gently.

Month-by-month onion calendar

Harvesting onions

Lift onions once the leaves have yellowed and flopped, before they die right down. Too early and they won’t store. Too late and they may rot in wet autumn soil.

They’re ready when the leaves yellow, soften and fall over by themselves. For spring-planted onions, that’s usually late July to September. Don’t bend the tops over yourself, as this old habit damages the neck and lets in disease.

How to lift them

Slide a fork under the bulbs to loosen the soil, then lift them gently. Don’t pull them up by the leaves, which can tear the neck. Pick a dry day.

Lay the bulbs on the soil in the sun, or on a wire rack, for one to two weeks. This curing dries the skins and seals the neck. If rain is on the way, move them into an airy shed, greenhouse or cold frame. Good airflow is key.

A week or two on a rack lets the skins go papery and the necks seal. If rain is coming, move them under cover somewhere airy.

How to tell they’re dry

The skins should be dry and papery and rustle when you touch them. The neck should be thin, tight and completely dry. If the neck still feels damp, give them longer. A damp neck is the main cause of rot in store.

Storing onions

Keep dried onions in nets, trays or plaits somewhere cool, dry, airy and frost-free. They’ll keep for months.

Trim the dry leaves to a few centimetres above the bulb, or plait them together to hang in strings. Keep them in the light rather than total darkness, which encourages sprouting. Hang them in nets, open crates or old tights somewhere cool, dry and frost-free, such as a garage, shed or cold utility room. Avoid the warm, steamy kitchen.

How long they keep: storage varieties like Setton and Sturon keep the longest. Stuttgarter Giant keeps well too. Centurion is better used sooner, and red onions don’t last as long.

Check your store every few weeks. Remove any onions that are soft, mouldy or sprouting, as one bad onion soon spreads rot to its neighbours.

A cool, airy, frost-free spot keeps onions for months. Check them every few weeks and take out any that turn soft.

Growing spring onions

Spring onions (also called salad onions or scallions) are grown for their mild, slim stems and eaten fresh. They’re quick, easy and fit in the smallest space.

Sow seed straight outside from March to August, in drills 1cm deep and 10cm apart. Thin to 2cm apart, or leave them in clumps. They’re ready in a couple of months. For a steady supply, sow a little every three or four weeks through summer.

Good varieties include White Lisbon (the classic, mild and reliable), North Holland Blood Red (red-skinned and pretty in salads) and Ishikura (long, straight stems with no bulb).

Spring onions are perfect for pots and window boxes. They also make a handy follow-on crop after early potatoes or broad beans.

Common onion mistakes

Planting too deep

Burying sets completely slows them down and can make them rot. Only the tip should show. If sets pop out of the soil, push them gently back in.

Forgetting to weed

Onion leaves are too thin to keep weeds down, and an unweeded bed soon gets swamped. A little weeding often is much easier than one big rescue job.

Watering late in the season

Watering once the leaves start to yellow makes the necks soft and prone to rot. Stop watering when the tops begin to fall.

Skipping the drying stage

Bringing onions straight indoors traps moisture, and they soon rot. Give them one to two weeks drying in a warm, airy spot first. Don’t skip it.

Growing them in the same spot every year

Pests and diseases build up if onions stay in one bed. Move them to a different bed each year and leave a gap of at least three years before using the same spot again.

Fancy another crop that stores through winter? Read our guide on how to grow parsnips in the UK.

Frequently asked questions

When should I plant onion sets in the UK?

Plant spring sets from mid-March to mid-April. Wait until the soil isn’t cold and sticky. In northern England and Scotland, plant a couple of weeks later. Overwintering sets go in during October or November for an earlier crop the following June.

How deep do you plant onion sets?

Push them in so just the tip shows. About two-thirds of the bulb sits below the soil. Too deep slows them down, and too shallow lets birds pull them out. Firm the soil gently around each one.

Should I grow onions from sets or seed?

Sets are easier and ready sooner. Seed gives you a much wider choice of varieties and costs less per onion, but needs an earlier start indoors. If you’re new to onions, start with sets.

Why are my onions bolting?

Bolting means the onion flowers instead of making a good bulb. Cold snaps after planting are the main cause, especially with spring-planted sets. Choose bolt-resistant varieties like Centurion or Setton, and buy heat-treated sets when you can.

How do I know when onions are ready to harvest?

They’re ready when the leaves yellow and flop over by themselves. That’s usually between July and September. Don’t bend the tops over yourself, as it can let in disease. Lift them gently with a fork on a dry day.

How long do home-grown onions last in storage?

Well-dried onions keep for several months in nets. Storage varieties like Sturon and Setton keep the longest. Red onions don’t last as long. Check them regularly and use or remove any that go soft.

Can I grow onions in containers?

Yes, in a container at least 20cm deep. Use multipurpose compost and plant sets 8-10cm apart. Water regularly, as pots dry out faster than the ground. Spring onions are especially good in pots and window boxes.

Related guides

For more on drying and keeping your crop, see storing onions, garlic and shallots, which covers curing, plaits versus nets and the best keepers. Our guide to storing garden produce covers every kind of store.