What are the technical measures for flower and fruit thinning in walnut trees?
Flower and fruit thinning is a major technical measure for improving the yield and quality of walnut trees. It can save a large amount of nutrients and water, which not only benefits the development of the tree in the current year, improving the current year's nut yield and quality, but also benefits new shoot growth and flower bud differentiation, ensuring the yield of the following year.
1. Reasons for flower and fruit thinning. For walnut trees that produce too many female flowers, carrying out a moderate amount of flower and fruit thinning to control the number of flowers and fruits can reduce the ineffective consumption of nutrients, increase effective flowers, raise the fruit-setting rate, and coordinate the conflict between reproductive growth and vegetative growth, so that sufficient nutrients are supplied for tree growth and flower bud differentiation, ensuring that fruiting, tree growth and flower formation are all accomplished. In addition, early-bearing walnut trees mainly bear fruit on lateral flower buds, have a relatively large number of female flowers, and after reaching full bearing age, in order to ensure a relative balance between vegetative growth and reproductive growth and maintain high quality, high and stable yield, flower and fruit thinning must be carried out; otherwise, too much fruiting will cause the fruit to become smaller and the quality to deteriorate, and in severe cases lead to premature tree decline, extensive dieback of branches and even the death of the whole tree.
2. Principles of flower and fruit thinning. When thinning flowers and fruits of walnut, the key lies in whether the number of flowers and fruits retained is appropriate. Because under certain conditions, yield is positively correlated with the number of fruits retained, the number of flowers and fruits retained must be just right. If too many fruits are retained, the fruit will be small, the single fruit light, the quality poor, and more fruits will not mean higher yield, let alone higher value; if too few fruits are retained, the fruit will be large and the single fruit heavy, but the number is too small to increase yield. The number of fruits retained in walnut is generally determined by the number of compound leaves or the thickness of the fruiting branch. Generally, a fruiting branch bearing twin fruits needs more than 5-6 normal compound leaves to ensure normal development of the branch and fruit. A fruiting branch with 1-2 compound leaves can hardly form flower buds, and even if it sets fruit, the fruit develops poorly, so fruits on such fruiting branches should be thinned. Walnut bears fruit on strong, vigorous branches; branches with a thickness above 1.0 cm can generally set 2-3 fruits; those with a thickness of 0.8-1.0 cm can set 1-2 fruits; fruiting branches with a thickness below 0.7 cm can hardly set fruit at all.
When thinning flowers and fruits, production should be determined by the tree, the quantity by the branch, and the flowers retained by the quantity; thin more when there are many flowers, and thin less or not at all when there are few flowers; thin more on weak trees and weak branches, and thin less on vigorous trees and vigorous branches; do not thin vigorous trees with few flower buds, so as to regulate tree vigor and stabilize yield. Generally, under moderate or better site conditions and moderate to fairly vigorous tree vigor, the number of fruits retained per square meter of crown projection area is 60-80; for walnut trees under superior site conditions and with very strong vigor, the number of fruits retained per square meter of crown projection area is 80-100.
Flower and fruit thinning should be based on the site, the orchard and the tree, and should not be applied mechanically or taken too far. In areas with late frost damage and orchards with severe disease and insect damage, the number of flowers and fruits retained should be 30%-40% more than the actual required amount, and fruit thinning should be carried out after the fruit has set firmly, so as to make up for losses caused by frost damage or disease and insect damage.
3. Timing of flower and fruit thinning. Neither flower thinning nor fruit thinning of walnut should be done too late; if too late, it not only increases the ineffective consumption of nutrients in the early stage, but also affects fruit development and fails to achieve the desired effect. Fruit thinning can be carried out after the physiological fruit drop, generally 20-30 days after fertilization of the female flower, that is, when the ovary has developed to a diameter of 1-1.5 cm.
4. Methods of flower and fruit thinning
(1) Manual thinning method. Special attention should be paid: fruit thinning is limited to early-bearing walnut cultivars with a high fruit-setting rate, especially weak trees bearing many fruits. First thin the young fruits on weak trees or slender weak branches, or cut them off together with the weak branches; when each inflorescence has more than 3 young fruits, depending on the strength of the fruiting branch, 2-3 may be retained, and the positions of the retained fruits should be evenly distributed within the crown, with more thinning in the shaded inner canopy.
(2) Chemical thinning method. Manual flower and fruit thinning has a stable effect but is labor-intensive and time-consuming; chemical flower and fruit thinning saves labor and effort, but agents and methods suitable for local application should be selected, and small-scale trials should be carried out before large-scale application. The agents and concentrations suitable for walnut flower and fruit thinning still require further experimental study.