When to Plant Onion Sets in Autumn: A UK Guide
In the UK, plant overwintering onion sets from late September to November; October is the usual month. Plant spring sets from mid-March to mid-April. Put each set 2cm deep with the tip showing, spaced 10cm to 15cm apart. Autumn sets are ready to harvest from June to July. Choose autumn varieties for overwintering and give them sunny, free-draining soil.
Key takeaways
- Only plant sets sold for autumn planting between late September and November.
- One onion set grows into one onion; wider spacing gives bigger bulbs.
- Waterlogged winter soil can rot sets, so use raised beds or modules where drainage is poor.
- Cover newly planted sets with netting or fleece until rooted, because birds often pull them out.
- Autumn-planted onions keep until about early winter; spring-planted onions can keep into the following spring.
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Expert tip
Trim any long, loose papery tip off each set before planting. Birds use it as a handle to pull sets out. Removing it gives them less to grab.
When should you plant onion sets in the UK?
Plant overwintering sets in autumn and standard sets in spring.
The autumn planting window runs from late September to November, with October the usual month. Standard spring sets go in from mid-March to mid-April, once the soil is workable.
If you are asking "Can I plant onion sets now?" in October, the answer is yes, with two conditions. They must be autumn or overwintering sets, and the soil must drain well.
Autumn planting gives you onions in June and July. Spring-planted onions are ready from July to September. The earlier crop is useful, but there is a storage trade-off.
Autumn-planted onions store until about early winter. Spring-planted onions keep well into the following spring. Many growers plant both for a longer supply.
Choose your planting season around both the harvest date and how long you want to store the crop. Our October gardening jobs guide covers other work for the same month.
What are overwintering onion sets?
Overwintering onion sets are small, immature onion bulbs bred to cope with cold.
They are often sold as Japanese onions. Each set grows into one onion, rather than a cluster of bulbs.
Popular autumn-planting varieties include 'Electric', a red onion, and 'Autumn Champion'. Check that your chosen sets are sold for autumn planting before putting them in.
Ordinary spring sets planted in autumn are more likely to bolt. Bolting means the onion sends up a flower stem, and the resulting bulbs do not store well.
Low temperatures in spring can trigger bolting. Many sets are heat-treated to make this less likely, but remove any flower stems as soon as they appear.
Shallots and garlic are planted around the same time as autumn onions. Our guide to growing shallots covers that separate crop.
For the other planting job, see our guide to growing garlic.
Can you plant onion sets too early?
Yes, spring sets planted in cold, wet soil can rot, and cold spells increase the chance of bolting.
Wait until the soil is workable rather than planting simply because the calendar has reached mid-March. Sets sitting in cold, wet ground can rot before they get established.
For autumn planting, the variety matters as well as the date. Use overwintering types within the late September to November window.
Planting ordinary spring sets in autumn raises the risk of bolting. An earlier planting date does not make those sets suitable for overwintering.
How do you plant onion sets?
Plant onion sets about 2cm deep, with only the pointed tip showing above the soil.
- Choose a sunny spot with free-draining soil.
- Weed thoroughly and dig in plenty of garden compost. Onions have a limited root system, so prepare the soil before planting.
- Make shallow drills or loosen the soil where each set will go.
- Push sets gently into the loose soil, about 2cm deep, with the pointed tips showing. Space them 10cm to 15cm apart, in rows 20cm to 30cm apart. Wider spacing gives bigger bulbs.
- Firm the soil around each set and water well.
- Cover with netting or fleece until the sets have rooted in. Birds often pull newly planted sets out of the ground.
Keep the planting shallow rather than covering the pointed tips. Leave enough room between rows to weed regularly as the onions grow.
What if your soil is heavy or wet in winter?
Use a raised bed or start sets in modules if your soil waterlogs in winter.
Avoid autumn planting straight into heavy soil that stays waterlogged. The sets are likely to rot, even though overwintering varieties can cope with cold.
Raised beds improve drainage and reduce bending. They suit gardens with clay or poorly structured soil, and you can fill them with good compost or garden compost.
Our raised beds and planters collection includes options for growing above the existing soil. The metal vs wood raised beds guide explains the material choices.
The other answer is one set per module of peat-free multipurpose compost. Keep the trays cool and bright, then plant the young onions out in spring with a head start.
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Can you start onion sets in modules in a greenhouse?
Yes, autumn onion sets can overwinter in modules in a cool, bright, unheated greenhouse.
Plant one set per module of peat-free multipurpose compost. This is useful where winter drainage makes direct planting unsuitable.
When deciding when to plant onion sets indoors, use the autumn window for overwintering types. They do not need heat; an unheated greenhouse is ideal.
Our guide to what to grow in an unheated greenhouse covers other crops for this space. Plant your young onions outside in spring after hardening them off.
A cold frame works too. Read our cold frame gardening guide or browse our cold frames.
For another autumn job under glass, see our guide to sowing sweet peas in autumn.
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Can you grow onion sets in pots or planters?
Yes, onions grow well in sunny containers at least 45cm wide and 45cm deep.
Use peat-free soil-based compost and plant the sets the same way as those in the ground. Keep the tips showing and allow 10cm to 15cm between sets.
When choosing when to plant onion sets in pots, follow the same seasonal windows. Autumn varieties go in during autumn, while standard sets go in during spring.
A planter brings the growing area within easier reach. Container onions still need regular weeding and water during long dry spells.
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How do you look after onions over winter and spring?
Weed regularly, protect newly planted sets from birds and water during long dry spells.
Onion leaves cast little shade, so weeds can compete with the crop and slow bulb growth. Weed by hand close to the bulbs.
Water during long dry spells, especially early in growth, and try not to wet the leaves. Stop watering once the bulbs have swollen in mid-summer.
On poor soil, give autumn-planted onions a nitrogen-rich organic feed in late winter. The feed advice applies to poor soil rather than every onion bed.
Autumn-planted onions are hardy and do not need heat. Frost is not the main worry; waterlogged soil and birds are.
Keep netting or fleece over newly planted sets until they have rooted in. Check for flower stems as the crop grows, and snap them off as soon as they appear.
Use bolted onions first, as they will not keep. Removing the flower stem does not make them good keeping onions.
What does the onion set year look like?
Autumn sets start in late September to November, while spring sets start in mid-March to mid-April.
The calendar below puts planting, care and harvest dates together. Soil conditions still matter, especially before spring planting.
| Month | Autumn onion sets | Spring onion sets |
|---|---|---|
| Late September to November | Plant | Not planted yet |
| December to February | Weed, protect from birds, feed poor soil in late winter | Not planted yet |
| Mid-March to mid-April | Weed and water in dry spells | Plant |
| May | Snap off any flower stems | Weed and water in dry spells |
| June to July | Harvest and dry | Keep weeding |
| July to September | Store, use by early winter | Harvest and dry |
When are onions ready to harvest and how do you store them?
Onions are nearly ready to harvest when their leaves turn yellow and flop over.
Expect autumn-planted onions in June and July, with spring-planted onions ready from July to September. Watch the leaves as well as the calendar.
Lift the onions with a fork before the leaves die right down. Take care not to bruise the bulbs, and use any damaged onions first.
Dry the crop for about two weeks in full sun on racks. Air needs to circulate underneath the bulbs.
In damp weather, move drying onions into a greenhouse or a well-ventilated shed. A greenhouse provides a dry, airy place to finish the crop during a wet summer.
Wait until the skins and leaves are papery before storage. Put the onions in net bags, arrange them in single layers in trays, or plait and hang them.
Keep stored onions somewhere light, cool, dry and airy. Darkness encourages sprouting.
Plan to use autumn-planted onions by about early winter. Keep spring-planted onions for the longer storage period into the following spring.
What should you not plant next to onions?
Most gardeners keep peas and beans in a different bed from onions.
Peas and beans belong to a separate rotation group. Onions share their rotation group with garlic, shallots and leeks.
Avoid growing onions in the same bed as last year's onions. Onion white rot can remain in the soil for years.
This applies to onion sets in raised beds as well as those planted directly in the ground. Include the previous crop when choosing this autumn's onion bed.
Our team at Greenhouse Stores can help you choose a raised bed, cold frame or greenhouse for winter growing.
Frequently asked questions
Can I plant onion sets now?
In October, yes, if they are autumn sets and your soil drains well. The autumn planting window is late September to November. Standard spring sets go in from mid-March to mid-April.
How many onions do you get from one set?
One onion set grows into one onion. A set is a small, immature onion bulb. Space sets 10cm to 15cm apart; wider spacing gives bigger bulbs.
What is the trick to growing onions?
Give onions sun, free drainage, shallow planting, bird protection and regular weeding. Plant with only the pointed tip showing. Weed by hand near the bulbs because weeds slow their growth.
Do autumn onion sets need protecting from frost?
Autumn onion sets are hardy and do not need heat or frost protection. Wet, waterlogged soil and birds are the main worries. Use netting or fleece until newly planted sets have rooted in.
How deep should you plant onion sets?
Plant onion sets about 2cm deep, with only the pointed tip showing. Push them gently into loose soil, then firm around them and water well. Keep rows 20cm to 30cm apart.
Can you plant spring onion sets in autumn?
Only plant varieties sold for autumn planting in autumn. Ordinary spring sets planted in autumn are more likely to bolt. Plant standard spring sets from mid-March to mid-April, once the soil is workable.
