UK sowing calendar
When to sow onions in the UK
Onions sowing calendar (UK)
| Sow under cover | Jan-Feb (from seed, in modules at 10-16C) |
|---|---|
| Sow / plant outdoors | Sets Mar-Apr (overwintering sets Sep-Oct); seed direct Feb-Apr |
| Harvest | Jul-Oct (autumn-planted sets Jun-Jul) |
Sets (small immature bulbs) are the easiest option, planted early to mid-spring. From seed, start indoors in modules in mid to late winter as UK springs are cool, or sow direct once the soil warms. Autumn-planted (Japanese/overwintering) sets crop earliest, in Jun-Jul.
UK timing runs on the last-frost date, not a USDA zone. Most of Britain sees its last frost around mid-May, with the north and exposed inland gardens a week or two later and the mild south-west and coast earlier. Tender crops start under cover and only go out once frost has passed; hardy crops can be direct-sown as soon as the soil is workable.
Timing aligned with the RHS grow-your-own guide for onions.
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When to sow onions in the UK — FAQs
When should I sow onions in the UK?
Sow onions under cover Jan-Feb (from seed, in modules at 10-16C), then outdoors sets mar-apr (overwintering sets sep-oct); seed direct feb-apr. Sets (small immature bulbs) are the easiest option, planted early to mid-spring. From seed, start indoors in modules in mid to late winter as UK springs are cool, or sow direct once the soil warms. Autumn-planted (Japanese/overwintering) sets crop earliest, in Jun-Jul.
Can you sow onions outdoors in the UK?
Yes — sow or plant onions outdoors sets mar-apr (overwintering sets sep-oct); seed direct feb-apr. Wait until after the last frost (around mid-May, later in the north) for anything tender.
When is onions ready to harvest in the UK?
Onions is typically harvested Jul-Oct (autumn-planted sets Jun-Jul) in the UK, depending on your sowing date, cultivar and region.
Is it too late to sow onions?
It depends on the month and your region. Onions has a UK outdoor window of sets mar-apr (overwintering sets sep-oct); seed direct feb-apr; northern and exposed gardens run a week or two behind the south. Open Growli and tell it your postcode for timing tuned to your own last-frost date.