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How to stop onions from bolting

Onion sets 'Pink Panther' from Suttons

Once an onion decides to set seed there’s really nothing you can do to reverse the process. But by taking a few simple precautions, you can reduce the chances of it happening in the first place. Here’s how to stop your onion sets or your onion seeds from bolting and what you should do if they start to flower.

Why do onions bolt?

Onion seeds ‘Walla Walla’ from Suttons
Sow ‘Walla Walla’ seeds for large, juicy and mild onions that can be eaten raw or cooked
Image: Onion seeds ‘Walla Walla’ from Suttons

An onion is what’s known as a biennial – its lifecycle spans two growing seasons. Under perfect conditions, it would grow during the first season and flower and set seed during the second. Bolting happens when the plant feels stressed. It reacts by rushing to reproduce by flowering and setting seed before it’s fully grown. 

It’s easy to recognise a bolting onion. The plant shoots up a solid stem and a flower pod appears at the top. This soon produces a globe-like flower and, shortly after pollination, seeds form. The problem for growers is that once it seeds, the plant stops growing and you’re left with undersized bulbs that don’t store very well.

How to prevent your onions bolting

Onion sets ‘Red Arrow’ from Suttons
‘Red Arrow’ is less likely to bolt than other red varieties
Image: Onion sets ‘Red Arrow’ from Suttons

When we say onions are sensitive to stress, we mean that if it’s too hot and dry, or if it’s too cold, your onions will run to seed. The plant feels threatened by drought or the onset of an early winter and so shortcuts its lifecycle to ensure it reproduces before it dies.

The simplest way to prevent bolting is to protect your onions from undue stress. When it’s very cold, use a fleece to insulate your crop from the worst of the frost, and when it’s hot, make sure you water regularly. Not only does watering keep the soil nice and moist so that your onion plants have plenty to drink, but evaporation of water from the soil cools the plants, stopping them from getting too hot.

Onions grown from seed have been shown to bolt less often than those grown from sets although, if you are growing from sets, choosing ones that have been heat-treated much reduces the likelihood that your onions will bolt. 

What to do if your onions flower

Onion sets ‘Sturon’ from Suttons
‘Sturon’ has an RHS Award of Garden Merit 
Image: Onion sets ‘Sturon’ from Suttons

Unless you wish to save the seeds for use next season, cut the flowers off any onions that have bolted. This won’t reverse the bolting process but it does stop the bulb from dividing. Bolted onions are perfectly edible and although they’ll last for a month or so in the ground, they won’t store.

The best thing to do with bolted onions is to dig them up and eat them right away, or make them into chutney.

Best bolt-resistant onion varieties to try

Onion seeds ‘F1 Santero’ from Suttons
‘Santero’ onion seeds are resistant to bolting and disease
Image: Onion seeds ‘F1 Santero’ from Suttons

No onion is completely resistant to bolting but some varieties are less likely to run to seed than others. If you’ve been having problems with bolting onions, try the following varieties:

  • Sturon – a plump, medium sized round onion that stores well, Sturon is renowned as a flavoursome all-rounder.
  • Stuttgarter – a true favourite with UK growers, this onion has a lovely flavour that lacks the harshness which comes with some onions. 
  • Red Arrow – red onions have a reputation for bolting, but it really depends on conditions in your own garden, so don’t write them off. In fact if you’re looking for a bolt-resistant red onion, this one is a good selection, and with its mild taste is a delicious addition to salads. 
  • F1 Santero – growing from seed? Try this one. F1 Santero is bolt and downy mildew resistant, crops heavily, and stores well.

Bolting onions is a frustration all veg growers face at one time or another. With a little care it’s avoidable, but even if it does happen, the good news is, you can still eat bolted onions – they may be a little on the small side but they’re just as delicious as fully grown veg. 

Lead image: Onion sets – Pink Panther from Suttons

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