What causes flower and fruit drop in walnut trees

  For most walnut varieties or types, flower drop is relatively mild and fruit drop is more severe, but there are also cases where flower drop is quite serious. The main reason why existing large walnut trees have low and unstable yields is severe flower and fruit drop. According to research by Wang Genjiu and others, the flower and fruit drop rate of walnuts is generally 40%-90%. Walnut flower and fruit drop has 3 peaks within a year. The first is 2-3 weeks after flowering, accounting for 50%-70% of the total flower and fruit drop; the second is 4-6 weeks after flowering, accounting for 30%-40% of the total flower and fruit drop; the third is 6-10 weeks after flowering, accounting for 10%-20% of the total flower and fruit drop. The causes of walnut flower and fruit drop are mainly shown in:

  1. Low management level and poor tree nutrition. When walnut trees are planted on barren mountainous soil, or when post-planting management is very extensive, fertilizer and water are insufficient, pruning is improper, and pest and disease damage is relatively severe, all of these can cause insufficient nutrient accumulation in the tree. The level of tree nutrition, especially stored nutrients, directly affects the process of walnut flower bud differentiation. Due to insufficient tree nutrition, the development of floral organs is affected, and only a small amount of fruit is set on the outer and lower parts of the canopy, while almost all fruit in the inner canopy drops. In addition, when fertilizer and water are sufficient, especially when there is too much nitrogen fertilizer, branches grow excessively, leading to an imbalance between reproductive growth and vegetative growth, which can also cause a large amount of flower and fruit drop.

  2. Poor pollination and fertilization. Walnut is monoecious with separate female and male flowers on the same tree and is wind-pollinated. On a single tree, the opening periods of female and male flowers mostly do not coincide. In some varieties on the same tree, the female and male flowering periods can differ by more than 20 days, and this is even more serious in scattered walnut trees. Natural pollination is limited by natural conditions, and fruit set varies greatly from year to year. Moreover, spring temperatures in northern regions change drastically. Once a cold wave intrudes, the temperature drops sharply below 0°C, sometimes accompanied by strong winds or overcast rain, and the floral organs will freeze, thereby losing the ability to pollinate and fertilize. In addition, the distance between the main cultivar and the pollinizer cultivar should be within 300 m. When it exceeds 300 m, pollination and fertilization are poor or pollination cannot occur. Also, some newly built walnut orchards are not equipped with pollinizer trees, and young walnut trees only bear female flowers, with male flowers appearing only after 3-4 years. If artificial supplementary pollination is not carried out, a large amount of flower and fruit drop will also occur.

  3. Excessive fruiting on the tree. In walnut trees with too many female flower buds, if too many female flower buds are retained during pruning, the nutrients accumulated by the tree will be insufficient, forcing flowers or young fruit to compete with each other for nutrients, thus producing a large number of ineffective flowers and self-thinned fruit, resulting in "a tree full of flowers but only half a tree of fruit," which consumes nutrients and yet cannot produce a high yield.

  4. Severe weather conditions. The severe weather that causes flower and fruit drop mainly includes late spring cold, strong winds, heavy rain, hail, and dusty weather. From bud break to the flowering period of walnut flower buds, it is exactly the time of late spring cold in northern production areas. Prolonged low temperatures make walnut flower buds highly prone to frost injury or even freezing to death, causing catastrophic losses. During the walnut flowering period, if there are continuous overcast days or blowing sand and dust weather, the pollen dispersal rate can be reduced, obstructing the process of pollination and fertilization. In addition, strong windy weather during the flowering period is also one of the factors causing flower and fruit drop.