Stop Onions Bolting: UK Prevention Guide

Onions bolt when they think winter has come and gone. A cold spell after planting flips a switch, and the onion sends up a flower stalk instead of swelling. The good news is that the fix is mostly about choosing the right sets and planting at the right time.

What Actually Makes Onions Bolt

Bolting is when an onion flowers early instead of growing a nice fat bulb. The usual trigger is a cold spell that the plant mistakes for a whole winter.

An onion set is already a little bulb. Give it a run of cold nights after planting and it decides it’s time to flower. Then it puts its energy into a seed head, not the bulb.

Four things make bolting more likely:

• Cold after planting. A frosty fortnight in March is the classic trigger.
• Big sets. Anything thicker than about 2cm bolts more often.
• Dry soil. A dry spell checks growth and nudges the plant to flower.
• Planting too early. Sets planted in February sit through far more cold than April ones.

Day length plays a part too, but cold is the thing you can actually do something about.

Sorting sets in March. This one is about pencil-thick, which is just right. Plant any fat ones in a separate row and pull them as spring onions.

Causes of Bolting at a Glance

Nearly all bolting comes down to cold and set size. The good news is you can sort both before a single set goes in the ground.

Choosing Bolt-Resistant Varieties

Picking the right variety is the easiest win. Some onions are bred to cope with a chilly spring, and heat-treated sets are the safest of all.

Heat-treated sets have been stored somewhere warm by the supplier. That warmth stops them getting ready to flower, so they just get on with making a bulb. Look for “heat-treated” on the packet.

Planting Timing That Avoids the Cold

When you plant matters just as much. Plant too early and the sets sit through weeks of cold nights, which is exactly what sets off bolting.

Aim for mid-March to mid-April. If you plant in February and a hard frost follows, expect a few bolters. If you do plant early, cover the bed with horticultural fleece on frosty nights to take the edge off the cold.

A roll of fleece is cheap insurance for any onion grower.

What to Do the Moment an Onion Bolts

Catch bolters early. The flower stalk starts as a thicker, rounder shoot in the middle of the plant, usually in late May or June.

Snap or cut the stalk off as low as you can the day you see it. That stops the plant wasting energy on seed. It won’t undo the bolting, but it limits the damage.

Then mark that onion. A bolted onion has a hard, woody core and a thick neck that never closes up properly. Rot gets in through that open neck, so it won’t keep.

Lift bolted onions first and eat them within a couple of weeks. Cut out the woody middle and cook with the rest as normal. Save your good, tight-necked onions for storing.

Snapping a flower stalk off low in June. The bulb below will stay smaller, so eat it soon rather than storing it.

At harvest, it’s easy to tell them apart. Bolted onions have a stiff stem and a fat, open neck. Good ones flop over and seal up tight.

Lifting onions in August. The floppy, tight-necked bulbs are the keepers. Any with a stiff central stem should have been pulled and eaten earlier.

Watering and Soil to Keep Growth Steady

Keep growth steady and onions are less tempted to flower. A dry May is a common culprit.

Onions have shallow roots, so they feel dry soil quickly. In dry spells, give them a good soak once a week rather than a little splash every day. Let the surface dry out between waterings.

A thin mulch of compost or grass clippings holds in moisture and keeps weeds down. Weeds compete hard with onions and add to the stress.

Once the bulbs have swollen nicely in July, stop watering. You want the leaves to start drying off, ready for harvest.

Spring Sets, Autumn Sets and Shallots

The rules shift a little depending on what you grow and when.

Autumn onions like Senshyu and Radar go in around late September. They’re bred to cope with a UK winter. Plant them too early, though, and they grow too big before winter, then bolt in spring.

Shallots bolt for the same reason as onions. Our guide on growing shallots successfully in the UK covers their slightly different planting time.

Good storing starts with good bulbs. Our advice on drying and curing onions properly explains why a well-dried, tight-necked onion keeps for months while a bolted one rots in weeks.

Tight-necked Sturon drying on a slatted tray in August, ready to store for months. Bolted onions don’t belong on this tray.

In short: pick small heat-treated sets, plant once the worst cold has passed, water steadily, and snap off any stalk the day it shows. The RHS growing onions advice is a handy extra read on spacing and feeding.

Frequently asked questions

Why do my onions keep bolting?

Usually because of cold weather after planting. The onion thinks winter has come and gone, so it starts to flower. Big sets and dry soil make it more likely. Choose small sets, plant from mid-March and pick bolt-resistant varieties.

Can you stop an onion from bolting once it starts?

No, once the stalk appears there’s no going back. Snap or cut the stalk off low straight away so the plant stops putting energy into seed. The bulb will stay smaller and won’t store, so lift it and eat it within a couple of weeks.

Are bolted onions still safe to eat?

Yes, bolted onions are perfectly safe to eat. Cut out the tough, woody core running up the middle and use the rest as normal. The flavour is fine. Just use them first, as they rot quickly in store.

Which onion varieties are least likely to bolt?

Sturon, Hercules, Senshyu and Setton are reliable choices. Heat-treated sets are even better, as warm storage stops them getting ready to flower. For autumn planting, Senshyu and Radar are bred to sit through a UK winter.

Do onion sets or onion seed bolt more?

Onion sets are more likely to bolt than seed-grown onions. A set is already a small bulb, so a cold spell can push it into flowering. Onions grown from seed are less prone. Sets are much easier, though, so most gardeners happily accept the small risk.

Now plan your onion year

Start with the right sets and a sensible date in our guide to growing onions in the UK, then pick a variety from the best UK onion varieties compared. At harvest time, get the keeping right with our advice on storing onions, garlic and shallots. And for an autumn job that sets up next summer, read when to plant garlic in the UK.