What are the technical measures for flower and fruit retention in walnuts?
In view of the causes of flower and fruit drop in walnuts, flower and fruit retention should focus on prevention, combining "prevention, treatment and management". Strengthening soil management is the main approach, combined with spraying microelement fertilizers and growth regulators to keep walnut growth in a moderate state. For external catastrophic weather and uncontrollable factors, in principle prevention should be the main approach, improving the tree's ability to resist adverse environments by enhancing tree vigor. In addition, excellent varieties with late flowering, cold resistance, moisture tolerance and a short growth period should be bred. Specifically, the following measures can be taken.
1. Strengthen comprehensive orchard management and properly arrange pollinizer trees. Walnut orchards should apply more organic fertilizer to enhance tree vigor, raise the level of stored nutrients in the tree, and promote flower bud differentiation. At the same time, attention should be paid to the combined use of nitrogen, phosphorus and potassium fertilizers, which helps increase the proportion of robust flowers, thereby reducing flower and fruit drop. For young vigorous trees, apply less nitrogen and more phosphorus and potassium fertilizer; for mature trees, use nitrogen, phosphorus and potassium fertilizers in a reasonable combination and apply more organic fertilizer. For trees with a large number of flowers, apply fertilizer during the flowering period, and spray 0.3% urea on the leaves after flowering.
Adopt a method based mainly on summer pruning and supplemented by winter pruning to reduce canopy shading and improve light conditions. During summer pruning, cut out overly weak and overly dense flower-bearing branches, and thin the buds and flowers of the remaining flowers so that nutrients are concentrated, which is beneficial for fruit set.
2. Keep more flower buds. For "off-year trees" and strongly vigorous trees with few flower buds, retain as many flower buds and young fruits as possible when pruning, and if necessary "keep every flower you see" so that they bear more fruit, set fruit steadily and bear large fruit, thereby increasing yield. Therefore, when shaping and pruning walnuts, do not overemphasize tree form and prune away too many flower buds.
3. Thin flowers and fruits. Carry out flower and fruit retention and flower and fruit thinning in a timely manner so that the trees bear fruit reasonably and avoid bearing too much or too little fruit. This is an important measure for making walnut trees high-yielding, stable-yielding and high-quality.
Walnut flower and fruit thinning and flower and fruit retention serve one common purpose. Flower and fruit retention directly adopts various measures to retain flower buds and young fruits, while flower and fruit thinning uses the means of "thinning" to achieve the goal of "retention".
4. Spray boron and growth-regulating substances during the flowering period. Boron is an indispensable microelement for fruit trees. It can promote pollen germination, pollen tube growth and ovary development, increase the fruit-setting rate and improve fruit quality. Therefore, spraying walnut trees once at full bloom with 300~350 times borax plus honey or brown sugar water can not only meet the tree's boron requirements but also increase the sticky fluid on the stigma, enabling pollen grains to absorb more water and nutrients, thereby increasing the fertilization rate and fruit-setting rate. Note, however, that borax is insoluble in cold water but soluble in boiling water, so before spraying it must first be dissolved in boiling water and then diluted with water for spraying.
Spraying boric acid, rare earth and gibberellin during the flowering period can also significantly increase the fruit-setting rate of walnut trees. According to experiments by the Shanxi Provincial Forestry Research Institute, the optimal concentrations for spraying gibberellin, boric acid and rare earth at full bloom are 54 mg/kg, 125 mg/kg and 475 mg/kg respectively.
The order of the degree of influence of the 3 elements on the fruit-setting rate is gibberellin>rare earth>boric acid. After spraying, yield can be increased by 55%. In addition, spraying 0.5% urea and 0.3% potassium dihydrogen phosphate 2~3 times during the flowering period can improve the tree's nutrient status and promote fruit set.