Learn · Onions
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How to Grow Onions in the UK

Onions are one of the easiest and most useful crops to grow. Whether you start from seed or sets, the principles are the same: good drainage, firm soil, and patience.

Sow indoors
January – February
Sow direct
March – April
Plant out
March – April
Harvest
July – September
Days to maturity
120–120 days
Position
Full sun
Frost sensitive
No
Min soil temp
5°C

Sowing and planting calendar

MethodFromToNotes
Sow indoors (from seed)JanuaryFebruarySow in modules under cover, prick out and harden off before planting.
Plant sets (spring)MarchAprilPush sets into firm soil with tip just showing. Easiest method for beginners.
Plant sets (autumn)SeptemberOctoberJapanese overwintering varieties for an earlier harvest the following summer.
Regional variation: Spring-planted sets are the most reliable method across all UK regions. Autumn-planted sets work best in milder areas — in Scotland, stick to spring planting.

Growing tips

Sets vs seed
Sets (small bulbs) are quicker and easier than seed — ready to harvest 2–3 weeks earlier and no thinning required. Seed gives more variety choice and is cheaper for large quantities.
Firm soil is essential
Onions need firm, well-drained soil. Loose, rich soil produces lots of top growth but small bulbs. Avoid fresh manure.
Stop watering before harvest
Once the foliage starts to yellow and fall over, stop watering. This helps the bulbs ripen and improves storage life.
Cure before storing
After lifting, lay onions on wire mesh in a warm, dry place for 3–4 weeks until outer skins are papery. Only store firm, dry bulbs.

Common problems

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Onion fly

Maggots tunnel into the base of plants, causing yellowing and collapse.

Fix: Grow under insect-proof mesh. Avoid growing in the same spot as last year. Sets are less susceptible than seedlings.
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White rot
ACT NOW

Fluffy white fungal growth at the base of plants. Persists in soil for 20+ years.

Fix: No cure. Remove and destroy affected plants. Do not grow alliums in that spot for many years.
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Downy mildew

Grey-purple mould on leaves in cool, wet weather.

Fix: Improve airflow. Avoid overhead watering. Choose resistant varieties where available.

Frequently asked questions

Should I grow onions from seed or sets?
Sets are easier and more reliable for most growers — less work, earlier harvest, and better suited to short growing seasons. Seed gives more variety choice.
When should I harvest onions?
When the foliage yellows and falls over naturally — usually July to September. Don't bend the tops deliberately; let the plant do it in its own time.
How do I store onions?
Cure in a warm, dry place for 3–4 weeks until outer skins are completely papery. Store in nets or plaits in a cool, dry shed. Well-cured onions keep for 6–9 months.

Related guides

→ How to Plan Crop Rotation on an Allotment→ How to Grow Garlic in the UK

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