Causes of flower and fruit drop in walnut trees

Most varieties or types of    walnut have light flower drop and heavy fruit drop, but there are also some serious flower drop. The main reason for the low and unstable yield of existing walnut trees is the serious drop of flowers and fruits. According to Wang Genjin and other studies, the flower and fruit drop rate of walnut is generally 40% to 90%. Walnut flower and fruit drop has three peaks in a year, the first time in 2-3 weeks after flowering, accounting for 50%-70% of the total fruit drop; the second time in 4-6 weeks after flowering, accounting for 30%-40% of the total fruit drop; and the third time in 6-10 weeks after flowering, accounting for 10% and 20% of the total fruit drop. The main causes of walnut flower and fruit drop are as follows:

   1, low level of management, lack of tree nutrition, planting walnut trees in mountainous areas with poor soil, or extensive management after planting, lack of fertilizer and water, improper pruning, serious damage caused by diseases and insect pests and so on will lead to insufficient nutrition accumulation. Tree nutrition, especially the level of storage nutrition, directly affects the process of flower bud differentiation of walnut. Due to the lack of nutrition in the tree, the development of floral organs will be affected, only a small amount of fruit will be produced in the periphery and lower part of the crown, and almost all the fruits in the inner chamber will fall off. In addition, in the case of adequate fertilizer and water, especially too much nitrogen fertilizer, branches grow too long, resulting in reproductive growth and vegetative growth is not coordinated, but also lead to a large number of flowers and fruits.

   2. Poor pollination and fertilization of walnut are monoecious and wind-pollinated. Most of the female and male flowers in a tree do not meet during the blooming period. In some varieties of the same tree, the difference in florescence between female and male can be more than 20 days, and the walnut trees planted sporadically are more serious. Natural pollination is limited by natural conditions, and fruit setting varies greatly from year to year, and the temperature in spring in the north changes violently. Once the cold current invades, the temperature drops sharply below 0 ℃, sometimes accompanied by strong wind or rain, the flower organs will be frozen, thus losing the ability of pollination and fertilization. In addition, the distance between the main varieties and pollinated varieties should be less than 300 meters, when pollination is more than 300 meters, pollination is poor or can not be pollinated, and some newly built walnut orchards are not equipped with pollination trees, while young walnut trees only have female flowers, and male flowers appear only after 3-4 years. If there is no artificial auxiliary pollination, there will be a large number of flowers and fruits.

   3. Walnut trees with too many female flower buds, if they leave too many female flower buds during pruning, will make flowers or young fruits compete for nutrients with each other because of the lack of nutrients accumulated by the tree, resulting in a large number of ineffective flowers and self-thinning fruits, resulting in "full flowers and half-tree fruits", which not only consumes nutrients, but also can not produce high yield.

   4. The main bad weather caused by bad weather conditions are cold in late spring, strong wind, rainstorm, hail and sand and dust. The flower bud of walnut from germination to flowering is in the cold weather in late spring in the northern producing area. The low temperature for a long time makes the flower bud of walnut easy to frostbite or even freeze to death, resulting in catastrophic losses. In the walnut blossom stage, if the weather is continuous cloudy and dusty, the pollination rate of pollen can be reduced and the process of pollination and fertilization will be blocked. In addition, the windy weather during the flowering period is also one of the factors causing the drop of flowers and fruits.