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From the team at Gardeners' World Magazine
How to grow onions

How to grow onions

Published: Monday, 28 March, 2022 at 11:04 am
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All you need to know about sowing, growing, harvesting and storing onions, in our Grow Guide.

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Plantyesyesyesyes
Harvestyesyesyesyes
Average Yield:30 large onions per 3m row
Spacing:10cm apart30cm between rowsleave tip showing

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.

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How to grow onions

You can grow onions from seed, but it’s much easier and quicker to grow them from sets (small onions). 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

Sowing onion seed
How to grow onions - sowing 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

How to grow onions - planting onion sets

Onion sets are available to buy for spring or autumn planting. Most are heat-treated, meaning they're less likely to bolt (produce flowers), which stops the onions bulking up. Plant onion sets 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.

More like this

Watch Monty Don's video guide to planting onion sets in modules:


How to care for onion crops

How to grow onions - watering onions

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

How to grow onions - harvesting onions
How to grow onions - harvesting 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

How to grow onions - storing onions
How to grow onions - storing 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

How to grow onions - onion 'Setton'
  • ‘Red Baron’ – nice flavour, sweet enough for salads and superb grilled or roasted. A reliable cropper and it stores well
  • ‘Setton’ – a good keeper and easy to peel, with a strong but sweet flavour
  • ‘Snowball’ – the mild, succulent flesh is perfect for sandwiches and salads. The bulbs store well

You can also see some of our tried-and-tested gardening tools below, with a selection of great kit for planting and support.

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