Bolting in Alliums

Bolting can be a problem with onions, leeks, and related species, and generally occurs in response to cold weather stress. Sustained temperatures of less than 45° F may result in bolting with as few as five leaves present. If the temperature falls below 50° F for two weeks or more, mature plants with 7-10 leaves will bolt. A cold, wet spring followed by a hot summer can also result in bolting.

Critical Periods

The transplants you receive from Dixondale Farms usually have four leaves; the fifth will appear out of the center of the onion about four weeks after transplanting. Larger transplants tend to be more susceptible to bolting, since they reach this critical period sooner.

Besides planting time, onions and leeks also face a critical period about two months later, as the plant sends out more leaves. Cool weather during either of these periods may trick your plants into thinking they’ve gone through the two growing seasons necessary for them to completely mature. The result is the premature development of a seed stalk.

How to Avoid Bolting

There are several things you can try to avoid bolting. First of all, match the proper onion variety with your growing region, particularly in terms of day-length. Next, do your best to plant your onion plants at the proper time. You can’t control the weather, but your plants are least likely to bolt if you get them in at the right time. With spring of 2019 coming much later for many of our customers, this has not helped the bolting issue. Be careful not to over-fertilize, too, because overly vigorous growth may result in bolting. So can soil that is too loose; if the plant thinks the ground has been disturbed, it may respond by trying to spread its seed.

Dealing With Plants That Have Bolted

The development of flower stalks and seeds supersedes bulb development in onions and leeks, so the bulb simply isn’t going to develop any further. You might as well pull it up and enjoy it while you can. You can’t store bolted bulbs, either, because the seed stalk exits the top of the bulb, weakening it and leaving a place where bacteria can set in.

We hope this helps you with the bolting issue. Be careful with your planting date, onion type, soil and fertilizers, and pull and eat any onions or leeks as soon as they bolt. If you have any questions on the subject that we haven’t covered here, don’t hesitate to ask!

If you have growing questions, please don’t hesitate to give us a call. Contact Customer Service at (830) 876-2430 or email