Causes of Spring Yellowing in Winter Onions and Control Methods

For some time now, the author has continuously received letters or phone calls from the public asking about spring management of winter onions. Some people report that winter onions which grew well under low-temperature conditions developed yellowing leaves, root aging, and even death after temperatures rose in early spring, and that cold-resistant winter onion seedlings actually grew worse than local winter onions. Why?

Through field observation, the author learned that in spring 2005, winter onions in most regions nationwide showed similar conditions. On the Jiaodong Peninsula, which has the largest planting area in the country, many onion seedlings died in large numbers due to improper management. The cause was mainly the abnormal weather from winter 2004 to spring 2005, when temperatures fluctuated up and down. In early winter 2004, temperatures nationwide were relatively high, and winter onions grew very well. Around the Spring Festival of 2005, the weather remained cold, and Zhengzhou experienced its lowest temperatures in over a decade. The leaves of ordinary giant onions had already turned yellow and dried up, while winter onions remained firm and dark green, demonstrating superior low-temperature tolerance. At the same time, because soil temperatures were relatively low, the roots of winter onions grew slowly, with few new roots developing. Since aboveground growth was slow, aboveground and belowground parts were basically able to grow in coordination. After early spring, however, temperatures fluctuated up and down, and in February the highest temperature in Zhengzhou reached over 20°C. The sudden rise in temperature caused the aboveground parts of winter onions to grow rapidly, with vigorous transpiration, and the plants needed large amounts of water and inorganic salts. But during the same period, soil temperature increased slowly, root activity was not vigorous, and few new roots sprouted, so they could not promptly supply sufficient water to the aboveground parts. A conflict arose between aboveground and belowground growth, causing the plants to develop yellowed leaves and root aging, and in severe cases root rot and leaf death. Ordinary giant onions or Welsh onions, however, have small and few aboveground leaves, so a sudden temperature rise has little effect on their leaf growth and transpiration. As air and soil temperatures rise, the plants gradually turn green again, and their growth condition is actually better than that of winter onions. Experienced growers should focus on the "three earlies" in spring management of winter onions, namely early planting, early watering, and early cultivating. Well-managed winter onions still grow noticeably better than ordinary giant onions. If a plant cell division agent is added during watering, and Jianzhibao is sprayed when temperatures are high, even better results will be achieved. In some areas, growers manage winter onions by transplanting them into small arched sheds before winter or covering them with plastic film for overwintering, and the winter onions also grow very well.