When should I plant onion sets in the UK?

Mid-March to mid-April in southern England, late March to early May in the Midlands, mid-April to early May in northern England and Scotland. Earlier sowings risk bolting; later ones don't have time to make a decent bulb. Heat-treated sets resist bolting and can be planted from late March even in the south.

Why are my onions bolting (going to flower)?

Bolting (premature flowering) usually means cold weather hit the sets when they were young, signalling that they've been through a winter. Heat-treated sets resist bolting. If you find one bolting, pull it; the bulb won't store but is fine to eat fresh. Other causes: planting too early, water stress, planting larger sets (smaller sets bolt less).

Sets or seed — which is better?

Sets are easier, faster, more reliable, and what nine out of ten UK gardeners grow. Seed gives access to more cultivars (especially storage varieties like Bedfordshire Champion), often produces firmer bulbs that store longer, and avoids the bolting tendency of sets. Sow seed indoors in January to February, transplant in April. For first-year gardeners, sets are the answer.

How big do UK onions get?

Most main-crop varieties form bulbs 5–8 cm across — fist-sized. Sturon and Stuttgarter Riesen are reliable medium-sized croppers. Exhibition varieties (Kelsae, Ailsa Craig) can reach 15–20 cm with sustained feeding and watering. Smaller, dense bulbs store longer than huge fluffy ones.

When are my onions ready to harvest?

Late July to early September. Watch the foliage — when about three-quarters has yellowed and toppled, the bulbs are mature. Don't bend the necks deliberately to speed ripening; you'll wound the bulb and ruin storage life. Lift on a dry day with a fork; lay on top of the soil to dry in the sun.

How long do home-grown onions store?

Properly cured storage varieties (Bedfordshire Champion, Sturon, Red Baron) keep 6–9 months in cool dry conditions. The trick is full curing — necks must be papery and fully dry before you stack them; any green left in the neck rots downwards into the bulb. Check stored onions monthly; soft ones go to the kitchen immediately.

Onion — Grow

Onion

Allium cepa

vegetable☀️ full_sun🪴 well_drained📏 smallUSDA 3–9

📋Quick Facts

Water

💧💧 Average watering

Hardiness

Zone 6-9

About

Plant onion sets from mid-March to mid-April in southern England, late April in the north — or sow seed indoors from January for transplanting in April. Onions are hardy (RHS H5) and tolerate frost as young plants. They want a sunny well-drained bed with low nitrogen; rich soil makes lush leaves and undersized bulbs. Plant sets 10 cm apart with the tips just showing above the soil. Harvest from late July to early September when the foliage yellows and topples. Dry the bulbs in the sun for a week, then store somewhere airy and frost-free; properly cured UK-grown onions keep until April or longer.

Top tip

Plant sets or sow seed in fertile soil; keep weed-free and let tops die back before lifting and drying bulbs.

Also known as: Cebolla, Oignon, Zwiebel, Cipolla, Cebula, Cebola, Allium cepa, Ui

How to grow onion

1
Choose sets or seed
Sets (small immature onions) are faster, easier, and what most UK gardeners grow. Seed gives more variety and a longer storage life but needs an early indoor start. Heat-treated sets resist bolting.
2
Prepare the bed
Late winter. Fork over a sunny, well-drained spot. Don't add fresh manure — high nitrogen produces lush leaves, not bulbs. Rake to fine tilth.
3
Plant the sets
Mid-March to mid-April in southern England, late April in the north. Push individual sets into the soil so just the tip shows, 10 cm apart, in rows 30 cm apart. Firm in. Birds sometimes pull them out for the wisps of dry root — net for a fortnight after planting.
4
Weed
Hand-weed regularly throughout the season. Onions don't compete well with weeds. Hoe carefully between rows.
5
Water if very dry
Onions are reasonably drought-tolerant but a dry June reduces the eventual bulb size. Water deeply once a week if there's no rain.
6
Stop watering at maturity
When you see the foliage starting to yellow and topple in late July, stop watering. Bulbs ripen and store better in dry conditions.
7
Lift
Once foliage has yellowed and toppled (don't crush it to speed the process — that introduces disease), lift the bulbs with a fork and lay them on top of the soil to dry. A week of sunny weather is ideal; if it's wet, move them to an airy shed.
8
Cure and store
When the necks are fully dry (papery, no green), trim leaves to 5 cm and roots flush with the base. Don't wash. Store in mesh bags or woven trays in a cool, dry, well-ventilated place. Inspect monthly and remove any with soft necks.

Common questions

Pest Resilience

4/5 — Good resilience

Allium chemistry deters most pests; onion fly and allium leaf miner are the main risks.

Companion Planting

Grows well with

Beetroot Carrot Lettuce Wild strawberry

Visual Characteristics

🍳

Culinary

Culinary Use

Soups, stir-fries, curries, caramelised, salads, sauces, rings, pickled

The onion year in your garden

Dispatching imaginary bots to check your garden out...

What to do now

How to Propagate

🌰Seed

Easy

🌱Bulb

Moderate

Hardiness Zones

H1a (tender)H7 (very hardy)

H1a

H1b

H1c

RHS H5–H6

USDA 6 equivalent

Onion

Onion

📋Quick Facts

About

How to grow onion

Common questions

Pest Resilience

Companion Planting

Visual Characteristics

The onion year in your garden

How to Propagate

Hardiness Zones

Names in Other Languages(7)