Many growers find their onions start developing flower heads instead of the bulb swelling as they want. This is called bolting. Bolting reduces yield and usually produces a thick neck so the onion bulbs will not store well.
In this article we look at what causes onions to bolt and what gardeners can do to prevent onions bolting.
Onion flower head that will produce seeds on the stalk of a bolted onion
Onions – Biennial Plants
Onions are classified as biennial plants, which means they have a two-year life cycle.
Year One
After planting an onion it spends the first year focusing on growing its bulb. This is the part we typically eat. During this stage, the plant is in what’s called its vegetative phase. It grows leaves and stores energy in the bulb, which swells in response to the change in day length following the solstice.
Year Two
If left in the ground over winter, the onion enters its reproductive phase in the second year. Instead of focusing on growing the bulb, the plant uses the energy stored in the bulb to produce a flower stalk and eventually seeds.
Most gardeners harvest onions at the end of the first year when the bulbs are mature and tasty. But if you want to collect seeds, you’d leave some onions in the ground to complete their second year of growth.
What is Bolting?
Onions bolt when they prematurely start to produce a flower stalk instead of focusing on forming a large bulb. This usually happens when onions experience stress, especially from temperature fluctuations.
Common reasons onions bolt:
1. Temperature Stress
Onions are sensitive to temperature. If they’re exposed to a cold period followed by warm weather (often in the spring), they can get tricked into thinking they’ve gone through two growing seasons, prompting them to bolt.
2. Planting Time
Planting too early or too late can also lead to bolting. Onions planted too early may experience cold stress, while those planted too late might rush to bolt before fully developing.
3. Water Stress
Inconsistent watering can stress the plants and trigger bolting. Onions prefer consistent moisture levels.
Once an onion bolts, the bulb stops growing, and the energy goes into producing the flower and seeds. While bolted onions are still edible, they don’t store well and might have a more pungent flavour.
Avoiding Onions Bolting
To avoid bolting, plant at the right time and try to keep growing conditions as stable as possible. It is difficult to control the weather though!
N.B. Onions grown from seed are more resistant to bolting but not immune.
Many growers prefer to grow from onion sets, which are more likely to bolt when stressed as described above.
Heat Treated Onion Sets
Heat-treated onion sets are onion bulbs that have been specially treated with heat to reduce their tendency to bolt. This process involves exposing the onion sets to a specific temperature (usually around 77-82°F or 25-28°C) for several weeks before they are sold.
Benefits of Heat-Treated Onion Sets
1. Reduced Bolting
The primary benefit of heat-treated sets is a significant reduction in the likelihood of bolting. Heat treatment helps stabilise the onion sets, so they’re less likely to react to temperature fluctuations that would otherwise trigger premature flowering.
2. Better Bulb Formation
By preventing bolting, the energy of the plant is directed towards growing a larger and more robust bulb, resulting in a better harvest.
3. Longer Growing Season
Since heat-treated sets are less prone to bolting, they can be planted earlier in the season without the risk of cold-induced bolting, allowing for a longer growing period.
When to Use Heat-Treated Onion Sets
These are particularly useful if you live in an area with unpredictable weather, where temperature swings could cause regular onion sets to bolt. They’re also great for gardeners who want to start their onions early in the season.
Heat-treated sets are usually a bit more expensive than regular sets, but they can be well worth the investment if bolting has been an issue in your garden.
What to do if an onion does bolt?
If you spot an onion starting to develop the central flower head stalk, snap or cut it off. This often allows the bulb to continue developing and will, at least, produce an edible bulb, albeit smaller than otherwise.
Be aware that this bulb will not store well, so use quickly after harvesting.
Information on Growing Onions
How to Grow Onions – Guide
- Harvesting and Storing Onions & Shallots
- How to Grow Onions from Seed – Allium Cepa
- How to Grow Onions from Sets – Allium Cepa
- Onion Pests, Diseases & Problems
- Onions Bolting – Cause & Prevention
Growing Onions for Show
- Growing Onions for Show – Selecting & Propagating Onions
- Growing Onions for Show – 250 gram Exhibition Onions
- Growing Onions for Show – Starting Large Onions from Seed
- Growing Onions for Show – Cultivating Exhibition Onions
