Onions are a versatile and popular kitchen staple, I think I use them in almost every dish I cook. When growing onions in your vegetable garden you can sometimes see something unexpected, something known as bolting.
Bolting refers to the premature development of a flowering stalk, diverting energy away from bulb growth. What to expect in this blog post? The life cycle of onions, causes of onion bolting, its implications on bulb quality, and practical solutions to prevent and manage this phenomenon effectively.
What is the growth cycle of an onion?
To understand why your onion(s) are bolting you first must understand the growth cycle of an onion.
In general, onions are classified as biennial plants, meaning they complete their life cycle over two years. However, many gardeners grow them as annuals, harvesting them in their first year.
First year (annual cultivation)
When onions are grown as annuals, the entire growth cycle occurs within one year. Here are the key stages:
Onion seeds or sets (small bulbs) are planted in early spring, typically a few weeks before the last expected frost date. Onions prefer well-drained soil and full sun.
After planting, the onion will focus itself on vegetative growth. This means the roots will develop, and the green shoots emerge from the ground.
During this stage, the onion plant forms leaves and develops the bulb. Keep in mind, that onions have shallow roots, so proper watering and weed control are essential.
As the onion plant continues to grow, it will put its energy towards bulb development. Onions store nutrients in the bulb, which swells and becomes the onion we use in the kitchen.
Maturation and harvest
Onions are usually ready for harvest in late summer to early fall, depending on the variety and growing conditions. Once the onion tops (foliage) start to turn yellow and fall over, it’s a sign that the bulbs are mature and ready for harvest.
Second year (biennial cultivation)
If you leave the onion in the ground after its first year in the garden, it will continue its growth cycle:
Bolting and flowering
In the second year, the onion plant transitions to its reproductive phase. It sends up a tall flowering stalk known as a scape. This scape will produce flowers, and if pollinated, it will produce seeds.
The seeds mature and can be collected for planting in subsequent years. Note, don’t save the seeds of an onion plant which bolts in its first year. These seeds will most likely grow into super fast bolting onions again. If you want to save seeds you should save one or two onions, depending on how many seeds you want to save. These onions will stay in the ground and will produce seeds in the second growing season.
Most gardeners will harvest their onions after their first growing year to collect as many produce as possible. Because bolting, hurts the bulb quality and storage life, it is not recommended to grow your onions longer than one year. If your goal is to collect seeds for the next growing seasons it is, of course, recommended to leave a few onions in the ground after the first growing year.
Are you planting onion seeds, transplants, or sets?
You can either start with onion seeds, transplants, or with small onion sets. Onion sets are smaller bulbs and onion transplants are the seedlings of onions, both can be bought by commercial growers. By starting with an onion bulb you skip the whole seed/seedling- stage of growing onions. You just put the onion sets in the ground, make sure to leave the top of the bulb (the papery bit) above soil level, and water them as you would normally.
How are these onion sets grown?
By planting onion seeds after the last frostdate super closely they don’t have enough space and stay small. You harvest the small onion bulbs in July and store them over winter at 37 – 39 °F until March. When the danger of frost has passed you plant the bulbs and grow them until they are ready for harvest again.
Pros of planting onion sets or transplants
An onion set or transplant generally will produce a bigger bulb, grow faster, are less likely to bolt, and can be harvested earlier than when growing onions from seed. The onion set or transplant already has one growing season in the pocket plus it has energy stored in its bulb. This is why it has a head start over onions planted from seeds.
The main onion varieties explained
There are three main onion varieties available. You have short-day varieties on one hand, intermediate-day varieties in the middle, and long-day varieties on the other hand. It will depend on your growing region which variety will perform the best in your garden. Let’s dive a little deeper into these varieties to expand our knowledge before we start on why onions are bolting prematurely.
Short-day onions or short-day onions are a type of onion that is specifically adapted to grow and form bulbs under relatively short day-length conditions and a lower temperature. This type of onion is typically grown in regions with mild winters and relatively short daylight hours. They require approximately 10 to 12 hours of daylight to initiate bulb formation.
These onions are often planted in the fall and harvested in the late spring or early summer, the onion matures in about 110 days. They are well-suited for southern and coastal regions with warm climates where the winter days are shorter.
This type of onion will start bulb formation, the formation of the onion, when the plant receives 12-14 hours of daylight.
Long-day varieties or long-day onions, on the other hand, require more than 14 hours of daylight and warm temperatures to form bulbs and are typically grown in regions with longer daylight hours during the growing season.
Choosing the right type of onion variety for your specific location and daylight conditions is crucial to a successful onion harvest, as it can help prevent issues like premature bolting. You can read more about these onion varieties here.
What are causes of onion bolting?
Onions are highly sensitive to temperature changes. Prolonged exposure to cold or warm weather can trigger bolting as the plant mistakenly believes it has completed a year of growth and should proceed to seed production. Temperatures below 50 °F (10 °C) or above 75 °F (24 °C) increase the likelihood of bolting.
Onions come in several varieties. An onion can be categorized as a long-day or a short-day variety, depending on their response to the number of daylight hours.
Long-day onions require 14-16 hours of daylight, while short-day onions need 10-12 hours. Planting the wrong type for your region’s day length can be an implication for premature bolting.
Stress and environmental factors
Can onions experience stress? Yes, they can! Stressors like insufficient water, poor soil fertility, overcrowding, or physical damage can prompt the onion plant to bolt prematurely as a survival mechanism.
Can you still eat a bolted onion?
Bolting adversely affects the taste and storage qualities of onions. The bulbs become smaller and may develop a woody texture, resulting in a stronger, more pungent flavor.
While still edible, bolted onions are no longer tasty for direct consumption and are better suited for immediate use rather than long-term storage.
Managing onion bolting 7 tips
Follow these 7 tips to minimize the occurrence of bolting and maintain healthy onion bulbs!
1. Select resistant varieties
Choose wisely and choose a type of onion that is known for its resistance to bolting in your climate and day length requirements. Seek advice from local nurseries, gardening resources, or experienced growers for suitable choices.
2. Make sure to plant early and provide adequate space per onion
Plant onions early in the season, ensuring they have enough time to establish before extreme temperatures set in. Provide sufficient spacing between plants to encourage proper air circulation and reduce stress from competition for nutrients.
3. Maintain consistent soil moisture
Keep the soil adequately moist through regular, deep watering. However, avoid overwatering, as it may lead to bulb rot. Well-hydrated plants are less prone to bolting due to stress.
4. Provide the optimal soil texture
Onions prefer loamy or sandy soils. These soil types provide good drainage and aeration while retaining enough moisture for the onions to grow. Avoid heavy clay soils that can become waterlogged and hinder root development.
Onions have fibrous roots, also known as shallow root systems. These fibrous roots are fine and numerous, originating from the base of the onion plant and extending horizontally into the soil. This root system helps anchor the plant and absorb water and nutrients from the soil. The majority of onion roots are concentrated near the soil’s surface, typically within the top 6 to 8 inches of soil.
4. Utilize organic mulch
Apply organic mulch around onion plants to regulate soil temperature and moisture levels, providing insulation during temperature fluctuations and reducing the risk of bolting.
5. Optimize soil fertility
Prior to planting, enrich the soil with organic matter like compost to ensure your onions receive the necessary nutrients for healthy growth and resilience to stress-induced bolting.
6. Monitor weather and provide protection
Keep a close eye on weather conditions and shield onions from extreme temperatures using shade cloth, row covers during hot periods, or protective covers during cold snaps.
7. Harvest promptly
If you notice signs of bolting in your onions, promptly remove the flowering part. Wait to harvest onions until the stalks become brown and tip over, just like non-bolting onions.
By understanding the causes and consequences of onion bolting you’re able to take proactive measures in preventing this issue. By selecting appropriate varieties, providing optimal growing conditions, and managing environmental factors, you can enjoy a bountiful harvest of flavorful, healthy onions.