When to sow Onion (Overwintering Japanese) in the UK
Onion (Overwintering Japanese) is sown relative to the average last frost, so the right week depends on where in the UK you garden.
Sowing and harvest dates by region
Onion (Overwintering Japanese) is timed from the average frost dates, so its timings shift across the UK. Plot works them out from the Midlands and Lowland Wales baseline using Met Office average frost dates.
| Where you are | Sow outdoors | Plant out | Harvest |
|---|---|---|---|
| South West Coast and Cornwall | mid September to early October | early October to early December | mid June to late July |
| South Coast and Channel | late August to late September | late September to late November | mid June to late July |
| Greater London (Urban Heat Island) | early September to early October | early October to late November | mid June to late July |
| South East and Home Counties | mid August to mid September | mid September to early November | mid June to late July |
| East Anglia | early August to early September | early September to late October | mid June to late July |
| Midlands and Lowland Wales | early August to early September | early September to late October | mid June to late July |
| Wales and West Coast | late August to mid September | mid September to mid November | mid June to late July |
| Northern England | early August to late August | late August to late October | mid June to late July |
| Scotland Lowlands and Central Belt | late July to late August | late August to mid October | mid June to late July |
| Scotland Highlands and Islands | early July to early August | early August to late September | mid June to late July |
| Northern Ireland | mid August to mid September | mid September to early November | mid June to late July |
| Channel Islands and Isle of Man | mid September to early October | early October to early December | mid June to late July |
Growing notes
Sold as overwintering or Japanese onions: sow seed in late summer or early autumn, or plant autumn sets from September to November, for bulbs in June and July - weeks ahead of spring-planted onions. They do not store as long as maincrop onions, and need free-draining ground to survive a wet winter.
At a glance
- Spacing
- 10 cm between plants
- Row spacing
- 28 cm
- Plants per m²
- 35
- Days to maturity
- 290 days
- Yield per plant
- 0.15 kg
- Sun
- Full
- Frost tender
- No
- Grows in a container
- Yes, 20 litres or more
- Minimum soil depth
- 20 cm
- Balcony rating
- 2 out of 5
What usually goes wrong
- rotting in waterlogged winter soil
- bolting
- onion downy mildew
- birds pulling up sets
Companion planting
Grows well near
Keep away from
Related guides
Timings cross-checked against RHS guidance for onion (overwintering japanese).