Onion bolting, or premature flowering, is often triggered by the plant's exposure to low temperatures when the seedling stem diameter exceeds 0.6 cm. Under conditions of 2°C to 5°C, onions can complete flower bud differentiation within 60 to 70 days. As the stem diameter increases beyond 0.9 cm, the onion becomes more sensitive to cold, which shortens the vernalization period required for flowering. When temperatures rise and daylight hours increase, the onion may begin to bolt and produce flowers.
Different onion varieties respond differently to low temperatures, and the number of days needed for vernalization varies accordingly. Southern varieties typically require about 60 days at 2°C to 5°C to complete vernalization, while northern varieties may need 100 to 130 days under the same conditions. Additionally, factors such as fertilizer availability, soil moisture, and sunlight play a role in the plant’s sensitivity to cold. Deficiencies in nutrients, drought, or insufficient light can accelerate flower bud formation and lead to early bolting.
To prevent premature bolting, it is recommended to use varieties that are winter-hardy, less responsive to cold, and resistant to twitching. Northern varieties are generally more suitable for this purpose. Red onions tend to bolt less frequently than white onions. When introducing new varieties, it's important to note that bolting from high latitudes to low latitudes is rare, but the opposite—bolting from low to high latitudes—is more common.
Choosing the right sowing and planting dates is one of the most effective ways to prevent bolting. Early sowing can result in large seedlings that are more likely to go through vernalization before winter, increasing the risk of bolting in spring. On the other hand, sowing too late may lead to weak seedlings with reduced cold tolerance, which negatively affects yield. The ideal approach is to sow on time so that the seedlings remain small and less prone to bolting.
During the seedling stage, it's important to control both water and fertilizer. Over-fertilizing can cause excessive growth, while moderate feeding helps develop strong, healthy plants. For larger seedlings, reducing irrigation and fertilization can help avoid bolting. After spring, increasing nutrient and water supply supports better growth. If bolting has already occurred, techniques like exhumation (removing the bulb) can be used to manage the issue.
Maintaining an appropriate seeding density is also crucial. Keeping the single plant area in the seedbed between 4 to 5 square centimeters helps prevent overcrowding, which can lead to weak, thin seedlings. It also avoids the formation of overly large seedlings due to excessive space.
Lastly, using plant growth regulators such as 0.25% ethephon or 0.16% cyanine during the seedling stage or when flower buds appear can help suppress premature bolting. These treatments have shown some effectiveness in controlling early flowering.
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