How to grow onions

Sets are very easy to grow, ideal for both novice and experienced gardeners

More time and care is needed to grow onions from seeds, but seeds are generally cheaper to buy and you'll have a greater choice of varieties. Although, onion seeds don't stay viable for long, with germination rates declining after a year or two.

Onion sets are widely available in spring in garden centres and from online retailers. A limited range is also available in autumn, usually sold as overwintering or Japanese onions, for an earlier crop. If you want a specific variety, be sure to buy or order early, as popular ones often sell out quickly.

Seeds are readily available from similar sources all year round. Young plants are also sold in spring and early summer – ideal if you only want a few.

You can sow onions outdoors from early to mid-spring, when your soil starts to warm up. Sow seeds 1.3cm (½in) deep, in rows 20-30cm (8-12in) apart. Thin out the seedlings first to 5cm (2in) apart, and later to 10cm (4in) apart. Closer spacing will result in smaller individual bulbs, but more of them.

In prepared ground (see above) plant sets 2cm (¾in) deep in drills or gently push them into loose soil, so the tip is just showing at the surface. Space them 5-15cm (2–6in) apart (wider spacing will give you larger bulbs), in rows 20–30cm (8–12in) apart. Firm the soil around them and water well. Birds will often peck newly planted sets out of the ground, so protect with biodegradable fleece or netting until they’ve rooted in.

If your soil is very wet and cold in early to mid-spring, you can plant onion sets indoors, then transplant them outside when soil conditions improve.

Avoid autumn planting if your soil is heavy and prone to waterlogging during the winter months, as the sets are likely to rot. But they can be planted in modules indoors instead, then transplanted outdoors in spring, which still gives them a head-start. 

Use modular trays filled with peat-free multipurpose compost and plant one set per module. Keep somewhere cool and bright, an unheated greenhouse is ideal.

Water during prolonged dry spells, particularly during the early stages of growth when roots are forming, but stop watering once the bulbs have swollen in mid-summer. Watering spring-planted crops after mid-summer can mean they store less successfully. Try not to wet the foliage when watering, as this can encourage fungal diseases.

Adding organic matter, such as garden compost, before planting should provide sufficient nutrients for a good crop of onions. If your soil is poor, you could add an organic-based, nitrogen-rich fertiliser to autumn-planted onions in late winter. This not only boosts growth but can also suppress premature flowering.

Remove any flower stems as soon as they start to form, otherwise the plant’s energy will go into producing the flower rather than swelling the bulb. It also means the bulb won’t store well.