2. Low temperature is the dominant factor to induce flower bud differentiation of onion, and most varieties can complete vernalization at 2-5℃. Second, the seedlings must grow to a certain size before they can be induced by low temperature and pass the vernalization stage. It is generally believed that when the stem at the base of leaf sheath of seedlings is thicker than 0.6 cm, flower buds can be formed by vernalization at low temperature, but when the stem is less than 0.5 cm, flower buds can not be formed by vernalization even at low temperature.
3. The sowing date is early, and the seedlings are larger in winter. After vernalization, bolting is caused.
4. Sometimes the sowing date is not early, but choosing big seedlings for planting will also cause the seedlings to be bigger in winter and cause bolting.