Growing onions from seed, and why day length matters

Onions from seed need a long start. Sow indoors about 8 to 12 weeks before your last spring frost, so the seedlings are a decent size when they go out. They’re slower than sets, but a packet of seed costs less than a bag of sets and gives you dozens of varieties to choose from.

The one decision that matters more than the sowing date is the type of onion, because onions make bulbs in response to day length.

Choose the right day length

An onion starts to bulb when the days reach a certain length. Which length depends on the variety.

Type	Bulbs when days reach	Suits
Long-day	About 14 to 16 hours	Higher latitudes: the UK, northern Europe, Canada, the northern US, southern New Zealand
Intermediate	About 12 to 14 hours	Middle latitudes: much of the US, southern Europe, southern Australia
Short-day	About 10 to 12 hours	Lower latitudes: the southern US, subtropical Australia

Seed catalogues for your region usually stock the right types. If you’re buying elsewhere, check the packet. Most onion varieties sold in the UK, such as Ailsa Craig and Bedfordshire Champion, are long-day types.

Sowing indoors

1. Fill a tray or modules with seed compost.
2. Sow thinly about 1 cm (½ in) deep in a tray, or four to six seeds per module to grow on as a clump.
3. Keep them at room temperature. The grass-like seedlings appear in one to two weeks.
4. Give them good light, and trim them back to about 10 cm (4 in) whenever they flop over.

Sowing outdoors

From about 6 to 2 weeks before your last frost, you can sow straight into a well-prepared bed, in rows 30 cm (12 in) apart. Thin seedlings to 10 cm (4 in) apart. Outdoor sowings are later to crop and need more weeding while the seedlings are small.

Planting out

Onions are hardy, so they go out from about 6 weeks before the last frost until about a week before it, after a week of hardening off.

• Single seedlings: 10 cm (4 in) apart, in rows 30 cm (12 in) apart, or 16 to a square in square foot gardening.
• Module clumps: plant the whole clump, 30 cm (12 in) apart each way. The onions push each other apart as they grow and make medium bulbs.

Plant them at the same depth they were growing and water in. Carrots, beetroot, lettuce and parsnips grow well nearby. Keep onions away from peas and beans.

Care

Onions have shallow roots and few leaves, so they hate competing with weeds. Hoe carefully, or weed by hand close to the bulbs. Water in dry spells until the bulbs are swelling well, then ease off.

The more leaves a plant has before bulbing starts, the bigger the onion. That’s why an early start pays off.

Harvesting and curing

Onions are ready about 120 to 160 days from sowing, when the leaves yellow and flop over. Leave them for another week or two, then lift them on a dry day.

Spread them out somewhere sunny and airy, or in a greenhouse, for two to three weeks until the skins are papery and the necks are dry. Then store them in nets or trays somewhere cool and dry.

If you’d rather start with sets, see planting onion sets. Set your own frost dates on the onion page to turn these weeks into calendar dates.