Onion bolting is a common problem in the vegetable garden. Flower and seed stalks grow from the center of the onion. Once the onion has sent up the central stalk, the plant begins to focus its energy on producing and spreading seed. Once bolting begins, there is no stopping the process. You can cut off the flower stalk, but the onion bulb will not continue to grow.
Weather is the trigger that sets off the blooming process in onions. Temperatures between 40 and 45 degrees Fahrenheit are usually necessary to start the bolting process. Cold weather in the spring followed by a hot summer may cause onions to bolt prematurely. Fluctuating weather patterns that go from cold to hot to cold again may also cause onions to bloom prematurely. Crops seeded or transplanted in the fall may be more susceptible to bolting in the spring if the weather is warm during the fall and cold during the winter. They may produce a large bulb, but the onion may weigh as much as 50 percent less than one that has not bolted.
Harvest and Storage
Onions are still edible once they have bolted, but are prone to spoilage. Harvest any onions that have begun to form stalks. Cut off the flower stalks and leaves and use as soon as possible. The seed stalk that has penetrated the onion bulb will be hard and green immediately after harvest, but will quickly begin to soften and decay. The decay will spread throughout the onion bulb, making it unsuitable for storing.
The best way to prevent onions from blooming is to plant the right variety at the right time of year for your area. Onions are classified as short day, intermediate day and long day, referring to the length of daylight hours necessary for the onions to grow and produce bulbs. Planting the wrong variety at the wrong time may result in crop failure. According to Texas A&M University, long day onions are recommended for U.S. Department of Agriculture Plant Hardiness Zones 1 through 4, and are best planted in April and May. In USDA Plant Hardiness Zone 5, plant long or intermediate day onions from March 15 to April 15. In USDA Plant Hardiness Zone 6, plant intermediate or short day onions from March 1 to April 1 and from February 15 to March 15 in USDA Plant Hardiness Zone 7. In USDA Plant Hardiness Zone 8, plant short day onions from January 15 to February 15 and from January 1 to January 15 in USDA Plant Hardiness Zones 9 through 11.