How to grow onions

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.

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:

When to harvest onions

As a general rule, autumn-planted onions grown from sets are ready to harvest from early summer, and spring-planted onions grown from sets, as well as those raised from seed, are ready to harvest from late summer. Never harvest onions before their leaves have turned yellow and stopped growing, although it's best to harvest them before the foliage turns brown and completely dies down.

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.

Take care not to damage or bruise your onions as you harvest them, as damaged onions don't store well. Use any damaged onions immediately.

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.

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.