Onion bolting, or premature flowering, is often triggered by environmental factors such as temperature and plant growth conditions. When onion seedlings have a stem diameter greater than 0.6 cm, they can complete flower bud differentiation under low temperatures (2°C to 5°C) within 60 to 70 days. As the stem diameter increases beyond 0.9 cm, the onion becomes more sensitive to cold, reducing the time needed for vernalization. When temperatures rise and daylight increases, the onions may begin to bolt.
Different onion varieties respond differently to low temperatures, and the number of days required for vernalization varies accordingly. Southern varieties typically require about 60 days at 2°C–5°C to complete vernalization, while northern varieties may need 100–130 days under the same conditions. Factors like fertilizer availability, soil moisture, and sunlight also influence the plant's sensitivity to cold. Deficiencies in nutrients, drought, and insufficient light can promote early flower bud formation and increase the risk of bolting.
To prevent premature bolting, it's important to choose varieties that are winter-hardy and less prone to bolting. Northern onion varieties are generally more resistant to early flowering. Red onions tend to bolt less frequently than white onions. When introducing new varieties, it’s usually safer to avoid moving high-latitude varieties to lower latitudes, as they may bolt too early. Conversely, low-latitude onions introduced to higher latitudes may be more likely to twitch due to extended cold periods.
Choosing the right sowing and planting dates is one of the most effective ways to prevent bolting. Sowing too early may result in large seedlings that are more likely to go through vernalization before spring, increasing the risk of bolting. On the other hand, late sowing can lead to weak seedlings with poor cold resistance, which negatively affects yield. The ideal approach is to sow on time so that seedlings remain small and healthy, reducing the chance of premature flowering.
Controlling fertilization and irrigation during the seedling stage is crucial. Over-fertilizing can cause excessive growth, making the plants more vulnerable to bolting. Instead, applying small amounts of fertilizer and keeping watering moderate helps develop strong, compact seedlings. For larger seedlings, reducing water and fertilizer can help manage growth. After spring, proper management of nutrients and water becomes even more important. If bolting has already occurred, techniques like lifting the bulbs can help reduce the impact.
Maintaining an appropriate seeding density is also key. A single plant should have about 4–5 square centimeters of space in the seedbed. This prevents overcrowding, which can lead to weak, thin seedlings, and also avoids the formation of overly large seedlings that are more prone to bolting.
Another method to control bolting is the use of plant growth regulators. Spraying 0.25% ethephon or 0.16% cyanine during the seedling stage or when flower buds appear can help suppress premature flowering. These treatments can be an effective tool in managing onion bolting, especially in areas where environmental conditions favor early flowering.
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