Why have my onions bolted and gone to seed?

What triggers bolting in onions, whether a bolted bulb is still usable, and the planting choices that avoid it next year.

Difficulty: beginner
Time: 20 min
Read: 3 min

Short answer

A cold spell after planting tricks the plant into thinking it has been through a winter, so it flowers instead of bulbing. Snap the flower stem off, harvest those bulbs first and use them fresh — they will not store. Use heat-treated sets and plant a little later next year.

Onions are biennial: they bulb in year one and flower in year two. Anything that convinces a young plant it has already been through winter triggers the flowering response, and a cold snap after planting is the usual culprit.

Step by step

Recognise a bolting onion.A thick, round, hollow flower stem rising from the centre of the plant, quite different from the flat leaves. It goes on to form a round seed head.
Snap the flower stem off as low as you can.It will not reverse the process but it stops the plant putting further energy into seed, and the bulb makes a little more size.
Harvest bolted bulbs first.A bolted onion has a woody core and will not store. Use them in cooking within a few weeks and store the unbolted ones properly.
Understand the trigger.Cold weather after planting is the main one. Onions planted early in spring into cold soil, or autumn sets hit by a hard winter, are the usual cases.
Buy heat-treated sets.Heat treatment kills the embryonic flower inside the set. They are usually sold slightly later in the season and are much more reliable.
Plant at the right time, not the earliest.Wait for the soil to warm rather than planting in the first mild week of March. Onions planted into cold wet soil are the ones that bolt.
Choose smaller sets.Very large sets bolt more readily than small ones. Sets around 15 to 20mm across are the sweet spot.
Keep growth steady.Drought followed by heavy rain, or a check from transplanting, can also trigger bolting. Even moisture and a mulch help.
Consider growing from seed.Seed-raised onions bolt far less than sets, though they need starting under cover in late winter and take more attention.
Do the same for garlic and leeks.Garlic bolting is normal in hardneck types and the scape should be removed and eaten. Leeks bolt for the same reasons as onions.

Tips

Snap the flower stem off as soon as you see it — it saves some bulb size.
Bolted onions will not store. Eat them first and store the rest.
Heat-treated sets are the single most effective preventative.

Common mistakes

Planting onion sets as early as possible — Cold soil after planting is the main bolting trigger. A fortnight's patience in spring produces a far better crop than an early start.
Storing bolted onions with the rest of the crop — They rot from the woody core and take neighbouring bulbs with them. Keep them separate and use them fresh.

If it doesn't work

Thick round flower stem from the centre

Cause: Bolting — Fix: Snap it off low, harvest that bulb early and use it fresh.

Most of the crop bolted

Cause: Cold spell after planting, or untreated large sets — Fix: Use heat-treated sets, plant later into warmer soil, choose smaller sets.

Bulbs small as well as bolted

Cause: Energy diverted into flowering — Fix: Expected. Remove flower stems as early as possible next time.

Garlic producing a curly stem

Cause: Normal scape on a hardneck variety — Fix: Cut it off and cook it. Removing it increases bulb size.

Questions people ask

Can I still eat a bolted onion?

Yes. Cut out the woody core in the centre and use the rest. They just will not keep, so use them within a few weeks.

How do I stop onions bolting?

Use heat-treated sets, plant into warm rather than cold soil, choose smaller sets, and keep growth steady with even moisture.

Should I remove the flower head?

Yes, as low down as you can and as soon as you spot it. The bulb will not become a keeper, but it will put on a little more size.