Growing onions is so satisfying. However, you may not know what to do when onions bolt. Seeing flower stalks forming on your growing onions can be disheartening.
A bulb on the top center stalk of your onion means it has begun the process of flowering and making seeds. This is called “bolting,” which is terrible news for onion growers.
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What is bolting?
Bolting is the term that describes when a vegetable plant starts to seed or flower before it should. When onions bolt, the bulb part of the onion stops growing and will not mature further. The energy that would make the onion bulb is now directed towards making the flower stalk.
Onions and shallots are a biennial crops, which means that the first year they grow and the second year they flower. When an onion or shallot forms a flower in the first year, it is called bolting.
Why do onions bolt?
An onion bolts in response to stress. Stress can happen in several ways; too hot, too cold, not enough water, or too much water. Temperature fluctuations can cause onions to bolt as well.
Swings in temperature may cause the onion to think it is in the second year of growth when it is supposed to flower.
For example, warm temperatures followed by cool weather can encourage onions to go dormant. Once warm temperatures return, the onion may believe it is beginning its second growing season and produce seeds.
The onion reacts to the stress by putting its energy into reproducing and making seeds.
What should I do when my onions bolt?
- Harvest and use (or preserve, see preserving tips below).
- Cut off the flower on top, or cut the entire bolting stem, so the onion stops producing seeds. (This won’t restart bulb growth.) You can leave the onion in the ground for a few weeks. The onion won’t continue bulbing or get any larger, but it will “keep” in the ground and can be used later.
- Do not wait to harvest until the onion leaves turn brown and fall over (as you do with onions that don’t bolt). Bolted onions may rot or become fibrous if left in the ground too long.
- Leave a couple of bolting onions to flower until they produce seeds. Save the seeds to plant next season.
How do I preserve bolted onions?
Use bolted onions right away, just as you would other onions. Bolted onions will not store well, and you need to preserve them differently than onions that did not bolt.
Here are a few ideas for ways to preserve bolted onions:
- Chop and freeze.
- Cut up the onions and dehydrate them. Use dehydrated onions whole, or process them into onion powder. Store the dehydrated onions whole for the most robust flavor, and then blend small amounts for powder.
- Slice onions and freeze dry them. (This is my FAVORITE way to preserve onions.) Freeze-dried onions can be stored and used “as is” or processed into a powder. Looking for more information about freeze-drying? Read this post, Freeze Drying Tips for Beginners.
If you would like to learn more about freeze drying, read Freeze Drying Tips for Beginners.
What can I do to prevent onions from bolting next time?
You don’t control the weather or other outside conditions that may cause onions to bolt. However, there are a few ways to help prevent bolting in the future.
- Choose onions suited to your area. Onions are typically grouped into short, long, and intermediate day onions. Use this map to help determine which type to plant.
- Plant at the correct time. Use your local planting guide to determine when to plant.
- Onion sets (small bulbs) tend to set more frequently, especially in hot climates. Grow onions from seed or transplant instead.
If you would like more information about how to grow onions, read this blogpost.