Onion Bolting

Onion bolting occurs when onions have decided to send up a flower stalk and go to seed, which happens for the following reasons:

  • They’re mature, have already made their bulb and want to make seeds before they go dormant.
  • They got a couple of weeks of below 50° weather
  • They had a cold wet spring and then it got suddenly hot
  • They got too dry, or they stayed too wet

In all these cases, the bolting onion thinks it's going to die before it can bulb, so it puts all its energy into trying to make seeds right away instead.

Some folks will tell you to just bend over the flower stalk when it forms, or to just prune them off. I've even read that you should bend over all the green tops.

But once onion bolting starts, onions cannot be convinced otherwise and it’s best to dig them up for immediate eating as “green onions”. Bolting onions will only very rarely recover and form a large bulb, and even if they do, the bulb storage quality will be low. This is because the plant took the energy from the bulb and used it to send up the flower stalk.

To keep onions from bolting in the first place, protect them over an extended cold, wet spell by covering them. The best way is with a little hoop tunnel, which keeps them out of the weather entirely and can easily add 10 degrees of temperature.

Alternatively you could cover them with Reemay (or other brand) floating row cover. This is let involved, but doesn't work as well.

And if the weather turns suddenly hot, make sure to keep your onions evenly watered. The soils needs to stay loose and friable. If it starts to get that baked-hard quality, onions will bolt because they can't expand.

To learn more about growing onions check out these related articles:

  • How to Grow Onions
    Onions need soil that is loose and evenly moist so that they can easily push it aside when they start to form bulbs. This is the main how-to article that describes the best soil conditions, how day length affects bulb growth, fertilizing correctly, and how proper watering can prevent disease.
  • Types of Onions
    To grow large bulb onions it’s very important to choose types of onions that match how far north or south you live, because onions need to experience a certain day length before they switch from growing green tops to forming bulbs. Find out which types (and varieties) will work for you!
  • Planting Onions
    All about planting onions: how deep and how far apart to plant, the difference between growing from seed, onion sets or onion transplants, and when to plant onions for maximum bulb growth.
  • Growing Onions from Seed
    There are some advantages to starting your own onions from seed, but you’ll need to start them indoors in order to get a jump on the season and give your onions enough time to develop bulbs. Find out how and when to plant, and when to transplant outside.
  • Onion Diseases
    Most onion diseases are a lot easier to prevent than to cure. Learn 5 ways to prevent onion diseases, and find out what to do if your plants do become sick.
  • Harvesting Onions
    Make sure to wait for harvesting onions until the tops start turning brown. Learn the proper way to harvest and cure onions for the most successful winter storage.
  • Storing Onions
    There are several cool and clever ways to store onions, but the most important things are to provide excellent air circulation and to keep them dry. Read about tips and tricks for storing onions.

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