Why Do Onions Bolt? How to Prevent Onion Flowering
What onion bolting is, why onions send up flower stalks (cold spells, large sets, early planting), how to prevent bolting and what to do with bolted onions.
Onions "bolt" when they send up a flower stalk instead of putting energy into the bulb. The main trigger is a cold spell after the plant has reached a certain size (about pencil thickness), which makes it think a winter has passed and it is time to flower. Large sets and early planting make bolting much more likely.
Causes
- Temperature swings: several days below about 10 °C (50 °F) after plants are established.
- Large sets: sets bigger than about 2 cm bolt more.
- Planting too early or overwintering the wrong variety.
- Stress: drought, heat, poor nutrition.
Prevention
- Choose small sets or heat-treated sets.
- Plant at the recommended time for your zone.
- Use bolt-resistant varieties.
- Keep plants evenly watered and fed.
What to Do With Bolted Onions
Snap off the flower stalk as soon as you see it. The bulb will still be edible but has a hard core and will not store; use bolted onions first. Or let a few flower for pollinators and seed.
More Onion Guides
- How to Grow Onions: The Complete Guide
- When to harvest and cure onions
- How to store onions
- Why onions stay small
Frequently Asked Questions
Can you eat onions that have bolted?
Yes, but use them soon; they do not store well.
Should I cut the flower off my onion?
Yes, to slow bolting, though the bulb will not grow much larger.
TarlaTapan guides focus on practical, field-tested growing advice. Corrections and suggestions are welcome at info@tarlatapan.tr.