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.

Oignon

Allium cepa

Plantez les oignons à repiquer de mi-mars à mi-avril dans le sud de l'Angleterre, et fin avril dans le nord — ou semez les graines en intérieur dès janvier pour les repiquer en avril. Les oignons sont rustiques (RHS H5) et supportent le gel lorsqu'ils sont encore jeunes. Ils ont besoin d'une plate-bande ensoleillée et bien drainée, pauvre en azote ; un sol trop riche favorise le développement de feuilles luxuriantes au détriment de la taille des bulbes. Plantez les plants à 10 cm les uns des autres, en laissant les pointes dépasser légèrement du sol. Récoltez de fin juillet à début septembre, lorsque le feuillage jaunit et se penche. Faites sécher les bulbes au soleil pendant une semaine, puis stockez-les dans un endroit aéré et à l'abri du gel ; les oignons cultivés au Royaume-Uni et correctement séchés se conservent jusqu'en avril, voire plus longtemps.

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