Onions are a kitchen staple that can be grown by the basketful and stored for months. In Britain, choosing reliable varieties and giving them a long, steady growing season helps produce good bulbs.

When and how to sow

Plant onion sets from March to April in well-prepared ground, or sow seed under cover from January for an earlier start. Sets may also be planted in autumn in suitable areas for an early summer crop. Onions tolerate cool weather, but protect very young plants from hard frost with fleece if required.

Care

Give onions full sun, fertile but well-drained soil and regular watering while bulbs are swelling. Keep weeds down carefully, as onions do not compete well. Stop watering once bulbs are mature and the foliage begins to yellow.

Common problems

Onion fly, thrips, white rot and downy mildew can be troublesome. Rotate the crop, avoid planting onions repeatedly in the same bed and cover young plants with fine mesh where onion fly is a concern. Remove badly affected plants and consult the RHS or a local garden centre for difficult disease problems.

Harvest

Lift onions once the tops yellow and bend over naturally, usually from late summer. Dry them in a warm, airy, sheltered place until skins and necks are papery. Store only firm, unblemished bulbs in a cool, dry frost-free place.

Growing Onions: frequently asked questions

When should I sow onions in the UK?

Plant onion sets from March to April in well-prepared ground, or sow seed under cover from January for an earlier start.

What are common problems when growing onions?

Onion fly, thrips, white rot and downy mildew can be troublesome.

When is onions ready to harvest?

Lift onions once the tops yellow and bend over naturally, usually from late summer.

General guidance for UK home gardens and allotments. Reference figures are typical values; your exact timing depends on your region, last-frost date and variety. Page updated 19 Sep 2026.