Why Onions Bolt and How to Stop It
Updated June 2026
A flower stalk on your onion means it has run to seed. Here is why, and how to grow onions that bulb instead.
When an onion sends up a hard, round flower stalk, it has bolted, switching from growing a bulb to making seed. Bolted onions stop sizing up and will not store. The causes are specific to onions and mostly come down to timing and variety.
Why onions bolt
- Cold stress on established plants. Onions that are already a decent size when they hit a cold spell read it as a winter, then bolt when it warms. Planting too early, so plants are large going into cold, is the classic trigger.
- Temperature swings. A warm patch, then a cold snap, then warmth again unsettles the plant into flowering.
- Wrong day-length variety. Onions are day-length sensitive. Growing the wrong type for your latitude, or sowing at the wrong time, leads to poor bulbing and bolting.
- Old or stressed seedlings. Transplants that sat too long, dried out or were checked are more likely to bolt.
How to stop onions bolting
Sow at the right time
Timing is everything with onions. Sow so plants go through the cold while still small, then size up as it warms. Follow the sowing window for your region rather than planting whenever seedlings appear in the shops. The app calendar gives the right window for where you garden.
Grow the right day-length type
Match the variety to your latitude and season, short-day, intermediate or long-day. Using the right type for your region is the single biggest factor in good bulbs and no bolting.
Keep growth steady
Even water and feeding keep onions growing calmly without the checks that trigger bolting. Avoid letting seedlings get old, dry or pot-bound before planting.
Can you eat a bolted onion?
Yes. A bolted onion is safe and tasty, it just will not get bigger or keep. Use bolted onions first in the kitchen, removing the tough central stalk, and let the unbolted ones size up for storage.
Catch problems before they cost you a crop
Track every bed in the Planting Season app, log what is going wrong, and get region-specific reminders so the same problem does not bite twice.
Open the App →Frequently Asked Questions
Why is my onion growing a flower?
It has bolted, switching from making a bulb to making seed. This is usually triggered by cold stress on plants that were already large, by temperature swings, or by growing the wrong day-length variety for your region.
Can you still eat onions that have bolted?
Yes. Bolted onions are safe and tasty, they just will not grow bigger or store well. Use them soon and cut out the tough woody core in the centre where the flower stalk grew through.
How do I stop my onions from bolting?
Sow at the right time so plants are still small through the cold, grow the correct day-length variety for your latitude, and keep growth steady with even water and feeding. Avoid planting old or stressed seedlings.
Does planting onions too early cause bolting?
Yes, it is the classic cause. Onions that are already large when a cold spell hits read it as winter and bolt when it warms again. Sowing to the right window for your region keeps plants small through the cold.
What does day-length have to do with onions?
Onions bulb in response to day length, so short-day, intermediate and long-day types each suit different latitudes and sowing times. Growing the wrong type for your region causes poor bulbing and more bolting.
See also: How to Grow Onions and Why Lettuce Bolts
Get next month's planting calendar, free
One email a month with exactly what to plant in your region, plus seasonal tips and harvest reminders. No spam, and you can unsubscribe any time.