The short answer
Onions are one of the easiest crops to grow. The simplest route is from sets (small immature bulbs): push them into a sunny, free-draining bed in early spring, 10 to 15cm apart with just the tips showing. Keep them weed-free, water only in dry spells and stop once the bulbs swell in mid-summer. When the leaves yellow and flop over, lift the bulbs, dry them for a couple of weeks, and they will store for months. For an earlier crop, plant overwintering sets in autumn.
Sets or seed?
Onions are grown either from sets, which are small part-grown bulbs, or from seed. Sets are the easiest and fastest way, and the one to choose if you are starting out. They are a little more prone to bolting (running to flower, which stops the bulb swelling), so look for heat-treated sets, which greatly reduce that risk.
Seed is cheaper and gives a far wider choice of varieties, but it needs more time and care, and onion seed does not stay viable for more than a year or two. Whichever you choose, pick a variety with an RHS Award of Garden Merit for reliability.
| Variety | Notes |
|---|---|
| 'Centurion' | Early cropping, bronze skin, stores well |
| 'Red Baron' | Red skin, strong flavour, stores well |
| 'Hercules' | Large onions, good bolting resistance |
| 'Sturon' | Reliable, excellent flavour, stores well |
| 'Rumba' | Medium yellow onions, grows and stores well |
Where and when to plant
Onions want a warm, sunny spot with free-draining soil. They have small root systems, so weed the ground thoroughly and work in a bucketful of garden compost or well-rotted manure per square metre first (avoid fresh manure). If you garden no-dig, mulch with organic matter and plant straight into it.
| What | When / how |
|---|---|
| Plant sets | Early to mid-spring; overwintering sets in autumn |
| Sow seed | Indoors mid-winter to early spring; outdoors early to mid-spring |
| Position | Full sun, free-draining soil improved with organic matter |
| Set spacing | 10 to 15cm apart (wider for larger bulbs), rows 20 to 30cm apart |
| Planting depth | Sets 2cm deep, tip just showing; seed 1.3cm deep |
| Harvest | Spring-planted July to October; autumn-planted June to July |
| In a container | At least 45cm wide and deep, peat-free soil-based compost |
Plant sets in early to mid-spring for a July to September harvest. For an earlier crop, plant overwintering (Japanese) sets in autumn, which cope with cold and are ready in June and July. On heavy, wet soil, skip autumn planting (the sets rot) and start them in modules indoors instead.
How to plant
Sets: push each one into loose soil 2cm deep so just the tip shows, 10 to 15cm apart in rows 20 to 30cm apart. Space them wider for larger bulbs. Firm the soil and water them in. Birds love to tug freshly planted sets out of the ground, so cover with fleece or netting until they have rooted.
From seed: sow indoors from mid-winter to early spring, 1.3cm deep in modules on a warm windowsill or in a heated propagator; seedlings show in about two weeks. Multi-sowing three or four seeds per module gives a handy clump of smaller bulbs, planted out without splitting. Harden off, then plant 10cm apart in rows 25 to 30cm apart. You can also sow direct outdoors from early spring, thinning the seedlings to 10cm.
No bed to spare? Onions crop well in a large container, at least 45cm wide and deep, filled with peat-free soil-based compost in a sunny spot. Our guide to planning a vegetable garden covers fitting beds in, and crop rotation explains why onions move to a fresh bed each year.
Looking after them
Onions need little feeding if the soil was improved before planting. Their main needs are water at the right time, a weed-free bed, and having any flowers nipped off:
- Water in dry spells while the plants are growing, but stop once the bulbs have swollen in mid-summer: watering late in the season makes the bulbs store less well. Try not to wet the leaves.
- Weed regularly. Onion leaves cast little shade, so weeds race away and compete; pull them by hand close to the bulbs.
- Remove any flower stems the moment they appear, or the plant puts its energy into the flower instead of the bulb, and it will not keep.
Harvesting and storing
You can pull onions young to use fresh at any point, but for storing, wait until the foliage yellows and topples over, which signals the bulbs are mature. Autumn-planted sets are ready in June and July; spring-planted sets and seed-raised onions from July to October. Lift them gently with a fork before the leaves die down completely, and take care not to bruise them. (Bending the tops over by hand, once common advice, is no longer recommended.)
Dry the lifted bulbs for about two weeks so the skins turn papery: spread them in a single layer in full sun on a rack, or in a greenhouse or airy shed if the weather is wet. Once dry, keep them in net bags or trays in a single layer, or plait them and hang them up, somewhere light, cool, dry and airy (not dark, which makes them sprout). Use any damaged onions straight away. Well-dried, spring-planted onions keep until the following spring.
Common problems
- Bolting: a cold spell in spring can trigger flowering, which spoils storage. Heat-treated sets and resistant varieties help.
- Birds pulling up newly planted sets: net or fleece them until rooted.
- Diseases and pests: onions are mostly trouble-free, but watch for onion white rot and downy mildew (worse in damp, crowded conditions), neck rot in storage, and allium leaf miner and onion fly. Good spacing, crop rotation and airy storage prevent most trouble.
Frequently asked questions
Is it better to grow onions from sets or seed?
For most gardeners, sets. They are quicker, easier and more reliable than seed, especially heat-treated sets, which rarely bolt. Seed is cheaper and offers more varieties, but needs starting earlier indoors and a bit more care.
When should I plant onions?
Plant sets in early to mid-spring for a July to September harvest. For an earlier crop, plant overwintering (Japanese) sets in autumn, which tolerate cold and are ready in June and July. Sow seed indoors from mid-winter, or direct outdoors from early spring.
When do I stop watering onions?
Water during dry spells while they are growing, but stop once the bulbs have swollen in mid-summer. Watering late in the season reduces how well they store. Try to keep water off the foliage to avoid fungal diseases.
How do I know when onions are ready to harvest?
When the leaves turn yellow and flop over, the bulbs are mature. Lift them gently with a fork before the foliage dies down completely, ideally in dry weather.
How do I store onions so they last?
Dry the lifted bulbs for about two weeks until the skins are papery, in full sun or an airy shed. Then keep them in net bags or trays in a single layer, or plaited and hung, somewhere light, cool, dry and well-ventilated. Well-dried onions keep for months.
How we checked this2 sourcesView sourcesHide
Our guides draw on expert bodies like the RHS and Garden Organic, plus experienced growers, read at source. We check every guide against those sources on the date shown, and where advice differs between them, we say so. Garden Plans is published by Limely.
- AuthorityHow to grow onions, Royal Horticultural Society. Checked 22 Oct 2026.
- AuthorityHow to grow onions, Gardeners' World. Checked 22 Oct 2026.
