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.

Cebula

Allium cepa

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.

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