How to grow onions
All you need to know about sowing, growing, harvesting and storing onions, in our Grow Guide.
Onions are a versatile crop, coming in a range of colours for different uses in the kitchen. Onions are a staple in a huge variety of dishes, from hot curries to soups, salads and tarts. All onion varieties are easy to grow and they store well, too, so you can enjoy home-grown onions all year round.
How to grow onions
You can grow onions from seed, but it’s much easier and quicker to grow them from sets (small onions). Most are heat-treated, meaning they're less likely to bolt (produce flowers), which stops the onions bulking up. One small onion set grows into one larger onion. Plant these in autumn or spring, 10-15cm apart in well-prepared, moisture-retentive, fertile soil in full sun. Keep the area weed free and water in dry periods. Harvest the onions when they're big enough to eat or the foliage has turned brown and started to wither.
How long do onions take to grow?
Onions require 90-100 days to mature from seed, which is around four months. From sets, onions are ready to harvest after around 80 days, or just under three months.
How to sow onion seed
Sow onion seed indoors as early as January, so they are large enough to plant out in spring. Sow seed in a pot or tray of moist seed compost, about 1cm apart. When the seedlings are a few inches tall, prick them out and transplant into fresh, peat-free multi-purpose compost. Once established, transplant seedlings into the garden, 10-15cm apart.
How to plant onion sets
There's no need to soak onion sets before planting. Plant them 10-15cm apart, allowing 30cm between rows. Plant them just below the soil surface, with just the tips showing, in moisture-retentive, fertile soil, ideally with plenty of well-rotted organic matter such as garden compost.
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Watch Monty Don's video guide to planting onion bulbs (sets) in modules:
When to plant onion sets
Onion sets are usually available to buy for spring planting but a small number can also be planted in autumn in the UK. In spring, plant onion sets from mid-March to mid-April. There's a wide variety to choose from, including popular 'Red Baron', 'Sturon' and 'Hercules'. Spring-planted onion sets are ready to harvest from late summer.
Those suitable for autumn planting are more tolerant of cold conditions and can therefore be planted from October through to March. Common autumn-planting varieties include 'Autumn Champion' and 'Electric'. An advantage of planting onion bulbs (sets) in autumn is that they take up space that would otherwise be left unplanted, and getting them in the ground before Christmas saves you a job in spring. Autumn-planted onion sets tend to mature a few weeks sooner than spring-planted sets, typically from early summer.
Onions typically need about 100 days of growing to produce decent sized bulbs. You can get away with planting them as late as mid-May to get 100 days of growing before light levels fall in autumn, but bear in mind they will be smaller than earlier planted bulbs when you come to harvest them.
How to care for onion crops
In spring, apply a nitrogen-rich fertiliser to autumn-planted bulbs to give them a boost. Water well during dry spells and remove any flower heads that appear, as these divert the plant's energy from bulb development, to seed production. Onions are shallow rooting, so hand weed instead of hoeing between the rows.
Growing onions: problem solving
You may need to cover the sets with horticultural fleece, to stop birds from pulling them up.
Drooping yellow foliage is the first sign of onion fly larvae, but by then they'll already be eating their way through the bulb by the time you notice the damage. You can protect crops the following year, by growing them under fleece. Companion planting parsley among your onions can also ward off onion fly.
Onion-neck rot can be a problem in wet summers. Telltale signs are brown marks and fluffy grey mould. Don't overcrowd when planting, and dry bulbs thoroughly before storing.
Watch Monty Don's video guide to identifying white onion rot:
How to harvest onions
Harvest onions as soon as they’re big enough to use. The leaves will droop over and turn brown when they’ve stopped growing. Gently loosen the soil with a fork and lift the onions out of the soil, and leave them to dry on a drying rack or similar, before storing.
How to prepare and use onions
Peel and chop onions for soups, stews, pickles and sauces. Sweeter varieties, such as red onions, are best for using raw in salads.
Watch this 20-second video demonstration from our friends at olive magazine on how to chop an onion.
How to store onions
Spread onions and shallots out on newspaper or racks to dry. They’re ready when their outer skins rustle when you touch them. Hang or string them in nets in a cool, dark, dry place. They should last for months.
Advice on buying onions
- Decide if you want seeds or sets. Sets are easier to grow but are more expensive to buy
- If buying sets, ensure you buy autumn- or spring-planting sets for planting at the right time
- Discard any sets with signs of mould or softness before planting
Where to buy onions
Great onion varieties to grow
- ‘Red Baron’ – a beautiful red onion with a nice flavour that's sweet enough for salads and s superb grilled or roasted. A reliable cropper and it stores well. Plant sets in spring.
- ‘Setton’ – a good keeper and easy to peel, with a strong but sweet flavour. Onions are well rounded and often used for the show bench. Plant sets in spring.
- ‘Snowball’ – a white-fleshed onion with mild, succulent flesh that's perfect for sandwiches and salads. The bulbs store well. Plant sets in autumn.
- 'Autumn Champion' – a great all-rounder, this onion has a mildish flavour but is very robust. Plant sets in autumn.
- 'Electric' – this gorgeous red onion is perfect for using in salads and also pickles well. Plant sets in autumn.
- 'Senshyu Yellow' – a very reliable Japanese over-wintering onion, bearing pale brown bulbs that store well. Plant sets in autumn.
This special edition Year Planner 2023 contains advice for every month of the year, with tips for delicious harvests, plus, a sowing calendar is included for the best results all year round. Only £7.99.