Recently, I began growing a few onions in my backyard (to learn more about how I started growing several onions from one sprouted onion, check out this article I wrote about what to do with a sprouted onion). After about six weeks, I found that my onions were beginning to form what looked like flower buds. I did not expect that to happen, so I did a little research and learned that my onions were bolting.
Onion bolting is a term used to describe an onion that has prematurely sent out a flower stalk. This is undesirable because the onion bulb will not continue to grow once the plant has produced a flower. When you see a flower bud, snip the flower bud off to prevent the onion bulb from splitting. Then harvest the onions as soon as possible. Bolting is also common when growing garlic.
Unfortunately, bolting is a problem for even professional onion growers. However, there are some things you can do to minimize the chance of bolting. The rest of this article will provide you with a detailed list of the potential causes of bolting and some tips for preventing bolting.
Onion Growing Cycle
To understand why bolting occurs, it is helpful to know a little bit about an onion’s growing cycle. Onions are biennial growers which means that their life cycle spans two growing seasons. During the first year of an onion’s life, it will grow from a seed into a plant and finally into a dormant bulb during the late fall/winter. In the spring, the onion plant will come out of dormancy and continue to develop the bulb. Finally, the onion will flower. Fertilized flowers will produce seeds, and the plant will die.
As gardeners, we grow onions and harvest them during the same season because we are interested in consuming the bulb rather than waiting for the flower. For this reason, flowering is undesirable.
Onion sets are immature bulbs that were grown from seed during the previous year. These bulbs are pulled from the soil and stored in a dormant state during the winter months.
Bolting occurs when certain triggers make the plant “think” it is in its second growing season. Bolting is more common when growing onions from sets (or sprouted onions) because they have been stored for several months prior to planting.
What Causes Bolting?
Onion bolting is a response to some sort of environmental stress. The onion thinks it is dying and quickly sends up a flower so it can reproduce. In an effort to quickly flower and produce seeds, the onion begins putting all of its energy towards the flower rather than developing the bulb. Some of the environmental stresses that cause bolting are listed below.
Sudden Changes in Weather
Because onions are typically planted during the early parts of the growing season, it is common for there to be a cold snap in some areas. A cold snap can trigger a genetic response that makes the plant flower like it would during the fall. The best way to prevent bolting during a cold snap is to cover the plant.
Onions also bolt when the weather gets too hot. The plant becomes dry and begins to flower in order reproduce before dying. Although you cannot change the weather, watering your onions during hot weather will minimize the chance of bolting. Not only will it prevent the plant from going dry, the evaporation of water from the soil surface will keep the air cool around the plant.
Loose Soil
Loose soil can cause an onion’s roots to be disturbed more easily. This can trigger the onion to bolt because the plant believes that it is starving.
Improper Fertilization
Over-fertilizing can cause your onions to grow too quickly during the early part of their development. You should also stop fertilizing once the onion has formed a bulb.
Set Storage and Temperature
As previously discussed, bolting is more common when onions are grown from sets rather than seed. According to a study conducted by researchers at the National Agricultural Research Center in Pakistan, set size and storage temperature also influences bolting. The results of the study indicate that bolting increased with set-size and colder storage temperatures (Khokhar, 2008).
How to Prevent Bolting
Bolting is a natural occurrence and is inevitable if you plant enough onions. However, there are some ways to prevent or at least minimize the amount of bolting in your garden. Some of these preventative measures are described below.
Plant at the right time
The best way to prevent bolting is to plant your onions at the correct time. Planting time depends on the variety of onion you are growing. In general, you should plant your onions 4-6 weeks before the last estimate spring frost. You can determine this date by entering your zip code into this page of The Old Farmer’s Almanac.
Select the right variety of onion for your location
Onions are photothermoperiodic which means that they are sensitive to temperature and length of daylight. Onions will form a bulb after it has received a certain amount of daylight each day for a certain number of days. The amount of daylight require depends on the onion variety.
Onions varieties are characterized as long-day, intermediate, and short-day. Short-day varieties are best for the southern region of the United States, intermediate varieties are best for the middle of the country, and the long-day varieties are best for northern regions.
| Onion Variety | Latitude | Length of Daylight |
|---|---|---|
| Short-Day | 25° – 35° | 10-12 hours |
| Intermediate | 32° – 42° | 12-14 hours |
| Long-Day | 37° – 47° | 14-16 hours |
Short-Day Varieties of Onions
- Texas 1015Y
- Contessa
- Giant Red Hamburger
- Red Creole
- Southern Belle
- Texas Early White
- Texas Legend
Intermediate Varieties of Onions
- Candy
- Red Candy Apple
- Super Star
- Sierra Blanca
- Cabernet
- Zoey
Long-Day Varieties of Onions
- Ailsa Craig
- Blush Onion
- Copra
- Highlander
- Red River
- Rossa di Milano
- Talon
- Red Carpet
- Walla Walla Sweet
- Yankee
Fertilize properly
Fertilize your onions every 2-3 weeks after planting with 5-1-1 fertilizer or with Miracle Gro’s All-Purpose Plant Fertilizer.
Onions need a lot of nitrogen after their roots have formed. Compost is rich in nitrogen and can also be added as a supplement to regular fertilizing. Researchers at the University of Georgia conducted a study that examined the effect of nitrogen content on bolting. The results of the three-year study showed that onions with a higher foliage nitrogen content were less likely to bolt (Diaz-Perez, 2004).
In addition to nitrogen, onions also need plenty of phosphorus and potassium to promote root growth.
According to the professional growers at Dixondale Farms, you should stop fertilizing your onions after the bulb has developed.
Grow onions from seeds
Some gardeners say that onions grown from seed will acclimate better to temperature fluctuations than onions grown from transplants or sets. However, I have not been able to find any research to support this assertion. Thus, you will, in theory, not get as much bolting during the first year of these onions growing.
Related Questions
Can you eat an onion after it flowers?
Yes, you can eat an onion after it flowers. However, the onion should be eaten as soon as possible. When an onion begins to flower, the onion puts its energy towards flowering and producing seeds rather than developing the bulb. As a result, the flavor of the onion bulb begins to diminish, and the onion becomes more difficult to store.
Can you treat a bolted onion?
Unfortunately, there are no treatments or chemicals that will reverse bolting. Once an onion has bolted, there is no going back. The best thing you can do is cut off the flower bud and harvest the onion as soon as possible.
What does a bolted onion look like?
The first sign of a bolted onion is a capsule on top of one of the leaves. A flower will emerge from these capsules. After the flower dies off, the plant will produce seeds. These onion seeds can be harvested and planted during the next growing season.
