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Onion

Allium cepa

In a typical UK garden, sow indoors late January to late February, sow outside early March to early April, plant out early March to mid-April, and harvest early June to early September.

When to sow, plant and harvest

Last frost around 20 April, first frost around 31 October.

Sow indoors
late January to late February
Sow outdoors
early March to early April
Plant out
early March to mid-Aprilmid-September to mid-October
Harvest
early June to early September
Spacing
10 cm apart, rows 30 cm apart
Square foot
16 per square (30 cm squares)
Sowing depth
1 cm
Ready in
90 to 120 days from planting out
Grows to
50 cm
Hardiness
Hardy to about -10 °C
Sun
Full sun
Crops for
One harvest

Growing tip

Plant sets with the tip just showing, and net them until they root as birds pull them up.

Neighbours

Alliums are one crop rotation group. Try not to grow them where any of the family grew in the last two or three years. How crop rotation works

Guides

How to grow

Growing onions from seed, and why day length matters

Sow onion seed indoors 8 to 12 weeks before your last spring frost, or outdoors 2 to 6 weeks before it. Plant seedlings out from about 6 weeks before the last frost. Choose a long-day, intermediate or short-day variety to suit how far you are from the equator, or the bulbs won't form properly.

When to plant

Onion sets go in from March, or in autumn for an earlier crop

Plant onion sets from early March to mid-April for onions that store well, or from mid-September to mid-October with a hardy overwintering variety for an early summer crop. Shallot sets go in from early October to late November, or February to late March.

More alliums

Where these figures come from

Every crop in VegLog was checked against published growing advice. Dates are worked out from average frost dates, so they shift with your garden. For onion, we used:

Grow onion with VegLog

Add onion to a bed and VegLog reminds you when to sow, plant out and pick, timed to your own town's frost dates.