Onions Bolting: What Are the Causes, and Can You Still Save Them?

Onions have a hard-earned reputation for being an easy-to-grow vegetable. But even if you provide them with nutrient-rich, well-draining soil, full sun, and keep them safe from pests and diseases, there’s still one growing problem that can easily spoil your harvest: bolting. While there’s nothing you can do to reverse this process, understanding why do onions bolt is the first step in preventing it.

The main factor that causes onions to bolt is temperature-related stress. Hot weather and sudden frosts in the spring will make onions start flowering before they’re ready to harvest. Similarly, not watering enough will also stress the plants, and result in bolting. If you see a flowering stalk on your onion, that’s a sign that the bulb has stopped growing, and that you should pick it as soon as you can.

In this growing guide, we’ll take a look at the factors that trigger onion bolting, and what to do if you want to salvage your crop. And, to spare you the headache of having to deal with it in the next growing season, we’ll share a few tips on how to prevent your onions from bolting.

How To Tell if Your Onions Are Bolting

While the majority of an onion’s leaves are similar the flower stalk will be slightly different. You will notice it grows from the center of the plant and has a water-drop-shaped flower bud at the end – this is the flower bud before it opens. A bolted onion may have the flower already open, in this case, it will be a white, blue, or purple globe-shaped flower.

Understanding an Onions Growing Cycle

While most vegetables are short-term crops, onions are considered biennial, meaning they will grow over two seasons. During the first season, an onion grown from seed will mature into a plant and form a storage bulb. Then the plant goes into dormancy during the fall and winter and begins growing again in the spring. During the second season, the onion bulb will enlarge in size and once the season comes to a close the onion will flower and die back.

While the full life cycle of an onion takes two years, most onions grown at home are germinated and harvested in the same year. The average harvest time for an onion grown in the spring is around 100 days while an overwintered onion will take three times as long, or around 300 days. The amount of time it takes for your onions to grow is dependent on the latitude and season you are growing in. Be sure to track how long you’re growing your onions and what is listed on the seed packet, flowering before its maturity date is a sign of bolting because of stress.

3 Common Reasons Your Onion Has Flower Buds Too Early

Onions are biennial plants. They spend the first year of their life growing leaves and developing a large, round bulb. They then go dormant over winter, and in the second year, they will use up the energy stored in the bulb to produce flowers and go to seed. If this happens before the onion is harvested, the process of going to flower is called bolting.

Bolting is a natural phase in the onion’s lifecycle. But if the plant is experiencing any stress during its growing stage, it will produce flowers prematurely, even in its first year.

Here are three things to keep an eye out for.

1: A Surprise Spring Frost

The most common cause for bolting is temperature fluctuations. For example, if you have warm, sunny weather in spring, but your region experiences a sudden cold snap, it will trick onions into thinking that it’s winter. This will make them enter a brief dormancy. And when the weather warms up again, the onions will think they’ve entered their second growing year, and start producing flowers.

2: Whether Being Too Hot or Too Cold

High temperatures can trigger bolting in onions. They are a cold-weather crop that grows best in temperatures ranging from 13°C to 24°C (55°F to 75°F). But if onions experience temperatures higher than 24°C (75°F) before the bulb has fully developed, they will start bolting. 

At the other end of the spectrum, low temperatures can also cause the onions to bolt, especially if young plants experience temperatures below 4°C (40°F) while they have 6 or more leaves. This can be a problem if you’ve planted onions in the fall and left them in the ground over winter. As long as temperatures gradually rise come spring, your onions should be fine. But as we mentioned earlier, a cold snap is guaranteed to trigger bolting.

3: Not Enough Water and Nutrients

Water and fertilizer-related stress can also be responsible for bolting. If your onions are underwatered during their growing stage, or if they’re not receiving enough nutrients, the plants will decide to spend all their remaining energy on flowering and going to seed.

Is It Bad if Onions Start Flowering?

A flowering onion is a bad sign for any gardener. When the plant decides to bloom, it will stop spending energy on bulbs and leaves. This means that the onion bulb will stop growing, and you’ll have a considerably smaller harvest. Not only that, but the bulb won’t last long in storage either, and will probably keep for about a month before it starts rotting.

What To Do When Onions Start Bolting

Onion bolting is irreversible. Once the plant decides to flower and go to seed, there are no chemicals or treatments that will turn back the onion’s internal clock. However, you can do a few things so that your onion crop isn’t completely wasted.

1: Remove the Scapes

When you start seeing flowering stalks — or scapes — on your onion, use a pair of scissors to cut them off. This won’t make the onion bulb grow any bigger, but it will prevent it from splitting and rotting while still in the ground.

Don’t throw away the flowering stalks, though. Onion scapes are edible and have a crunchy texture and a mildly sweet, chive-like taste. You can use them the same way you would use scallions, or turn them into delicious onion scape pesto.

2: Harvest Your Onions

Ideally, you’ll want to harvest your onion bulbs as soon as the plants have started bolting. If you leave them in the ground until the leaves turn brown and start to fall over, the bulbs will turn woody, and start rotting.

If you don’t have the time to harvest all your bolted onions straight away, there’s one trick you can use to make them last in the ground for up to a month. Trim the top of the scape with a pair of scissors, then bend the remaining stem so that it’s pointing down. This way, rainwater won’t run into the hollow stem and reach the bulb, which will prevent premature rotting.

Bolted onions are safe to eat, but they don’t store well, and can’t be cured. Try to use them straight away, or keep them in the fridge for up to 2 weeks. For long-term storage, your best choices are chopping and freezing, dehydrating, freeze drying, or pickling.

Got more questions about how to harvest onions? Check out our guide to harvesting and curing.

3: Let Some Onions Go To Seed

If your onions have bolted, it’s worth letting a few of them do their thing and go to seed. Two or three onion plants will produce enough seeds to help you start a new crop.

6 Ways to Prevent Onion Bolting in the Future

The only way to truly prevent an onion from bolting is to grow it in the correct latitude during the right time with all the water and nutrients it needs. However, managing all these factors can be difficult. Here are simple remedies to help prevent your onion from bolting.

1. Pick Onion Varieties Suited to Your Region

There are 3 types of onions you can grow in your garden: long-day, short-day, and intermediate varieties. This refers to how many hours of light each variety needs each day to produce a bulb. Long-day onions need 14–16 hours, short-day onions need 12–14 hours, and intermediate varieties need between 12 and 13 hours of sunlight per day.

Long-day onions are best for growing in northern regions or in latitudes of 37°–47°. Top varieties include Blush, Frontier, Raven, Red Carpet, Talon, Walla Walla Sweet, and Yankee.

Intermediate-day onions are better suited for latitudes of 32°–42°. Some great varieties you can grow are Candy, Cabernet, Monastrell, Siera Blanca, and Zoey.

Short-day onions grow best at latitudes of 25°–35°. Try growing varieties such as Desert Sunrise, Gabriela, Southern Belle, Texas Legend, White Castle, and Yellow Granex.

2. Grow Onions from Seed

This is a great way to reduce the risk of bolting, especially in areas that may experience surprise frosts. Onion starts and sets mature faster and are more vulnerable to bolting. Onions grown from seed will still be in their juvenile stage when a late frost hits and are less likely to bolt.

3. Use Heat-treated Onion Sets

Heat-treated sets are less prone to bolting, grow bigger bulbs, and resist pests and diseases better. Though slightly more expensive, they are ideal for growers in fluctuating climates.

4. Protect Onions from Frost

Use a frame covered with white or transparent polyester tarp to shield seedlings from sudden cold. For overwintered onions, a January mulch layer will keep the soil warm and boost spring growth.

5. Provide Shade in Hot Weather

Shade cloth helps prevent onions from overheating once temperatures rise above 24°C (75°F). Keeping them cool ensures better bulb development and reduces the risk of bolting.

6. Keep Soil Moist During Heatwaves

In hot or dry weather, check onions daily and water deeply in the morning if the top inch of soil is dry. Mulching helps maintain soil moisture and protect plants from stress.

Conclusion

While it’s disappointing anytime your harvest doesn’t grow how it should, gardening is a learning experience! Take this information and use it next season, begin planning now by finding varieties that grow best in your area and plan when you will sow your seeds. This season’s crop may not have been what you were hoping for but next season you’ll be growing onions like a pro!