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

Plant onions in early spring once the ground is workable. In-ground gardens and raised beds are both excellent options for growing onions.

Space onion plants 6 inches apart in rows that are 12 inches apart. Grow them in a sunny spot that has fertile, well-drained soil with a pH of 6.0 to 6.8.

Onions aren't great at taking up water, so it's important to keep soil moist so their shallow roots can drink up. Water whenever the top inch of soil becomes dry.

Onions can be eaten at nearly any size so harvest when they're the right size for your next culinary creation.

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.

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.

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.

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.

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.

Soil, Planting, and Care

Growing onions requires abundant sun and good drainage, and they grow best when the soil pH ranges between 6.0 and 6.8. Raised beds or raised rows made by mounding up soil are ideal, especially if your soil is heavy clay. For mounded rows, mix a 3-inch layer of compost into the top 6 inches of soil. Set plants 1 inch deep, so that their roots are well covered with soil but the top of the plant's neck is not buried too deeply. You don't want the part of the neck where the leaves grow away from the clear sheath to collect soil or water down between the young leaves, or they can rot. Space plants 6 inches apart in furrows 12 inches apart.

Onions roots are shallow and not very efficient at taking up moisture, so they need a steady supply of water to grow without interruption. Although they actually recover well from drought and start growing again when watered, it is best to keep the soil consistently moist until the bulbs enlarge.

You may mulch with a light layer of weed-free and herbicide-free grass clippings or another fine mulch. Onions naturally push toward the surface as they form bulbs, and it's best if the tops of the bulbs are allowed to bask in dry sun. Remove mulch that might keep the expanding bulbs excessively moist.

Seedlings that are about the diameter of a pencil produce the biggest, most beautiful bulbs, so some gardeners sort seedlings by size before planting. Plant the largest ones together only 2 inches apart to start enjoying as green onions in just two or three weeks. Very small seedlings set at close spacing can serve as a second crop of scallions. Use the pencil-sized plants to grow full-sized onions that will produce extra-juicy slices.

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.

Problem solving

As onions leaves expand, they may be found by tiny black onion thrips, which suck sap from onion leaves. These are hard to see because they hide down in the folds and neck of the leaves. Also be on the lookout for aphids. Finally, weak plants that slowly wilt may be infested with onion root maggots, the larvae of a common fly.

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.

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.

You can harvest young onions just a few weeks after planting if you want to use them as spring onions or scallions. There is no perfect size, just pull when they are big enough to suit you.

For full-sized bulbs, let onions grow and mature. They are ready to harvest when the bulbs are big and the tops begin to turn yellow and fall over. Pull them up, shake off the soil, and lay them out to cure with the tops still attached. Any warm, airy location is a good place to do this; you can even sling them over a fence as long as they aren't rained upon. Bulbs must stay dry and have good air circulation. As the onions cure, the roots will shrivel and the necks above the bulbs will slowly dry – a natural process that helps to seal the top of the bulb, making the onions less likely to rot. After 7 to 10 days, clip off the tops of the onions and the roots with pruning shears, remove as much dry dirt as possible without taking off the papery outer skins, and store your onions in a cool place. Very sweet, juicy onions may be stored, wrapped in newspaper or paper towels, in the fridge.

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.

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