Overwintering onion sets, often sold as Japanese onions, go into the ground through September and October, root while the soil still holds some warmth, and then sit through winter as small green shoots doing very little. Come spring they take off weeks ahead of anything sown in the new year, bulbing up by early summer while maincrop onions are still leafing. The only real trick is timing: plant too early and soft autumn growth gets caught by the first hard frost, plant too late and the sets haven't rooted enough to sit out a cold snap.

Plant Overwintering Onion Sets in an Autumn Bed

What you need

60 overwintering onion sets look for 'Senshyu' or 'Radar', bred not to bolt over winter
100 g blood, fish and bone fertiliser optional, raked into the surface on tired ground
1 garden line or taut string between two canes

Step by step

Fork over the bed to loosen the top 20cm, pulling out weeds, stones and the remains of the last crop.
20 min
Rake the surface level, firm it by walking over it flat-footed, then rake again to leave a fine, crumbly tilth.
15 min
Stretch the garden line across the bed to mark the first row, moving it to mark each row 25-30cm apart.
5 min
Using a dibber, push each set into the soil along the line, tip up, so just the papery point shows above the surface. Space sets about 10cm apart.
20 min
Firm the soil gently around each set with your fingers. Planted too loose, a set is easy for a bird to pull out by morning.
15 min
Water the row once if the soil is dry, then leave it alone; overwatering now rots sets faster than dry soil ever will.
5 min
Lay fleece, netting, or a few twiggy sticks across the bed for the first fortnight if pigeons are a problem in your garden.
10 min

Afterwards

Check the row weekly through autumn: frost heave and birds both lift loose sets back to the surface, and a set left sitting on top of the soil will dry out and die. Press any you find straight back in and firm around them. Hand-weed through winter, since the thin young leaves don't compete with faster-growing weeds and a hoe is too easy to catch a shallow-rooted set with.

Questions

Can I use ordinary (spring-planted) onion sets instead?

Better not to. Ordinary sets are bred to bulb up through long summer days, and if they sit through a cold snap followed by a mild spell as autumn-planted sets, a large share will bolt - run to flower - instead of forming a usable bulb. Overwintering varieties like 'Senshyu' and 'Radar' are selected specifically to avoid this.

A set has come out of the ground overnight. Is it ruined?

No. Birds and frost heave both do this in the first couple of weeks. Just push the set firmly back into the soil and firm around it; a few hours out of the ground doesn't harm it.

How much space does a 3m double row actually need?

A double row 25-30cm apart, sets spaced 10cm within each row, fits comfortably across a standard 1.2m-wide bed with room either side for access, and gives around 60 onions from 3m of row.