Growth and fruiting habits of walnut trees related to pruning

  1. Bud heterogeneity and branching vigor. Bud heterogeneity refers to the differences in growth vigor and other characteristics among buds at different positions on the same shoot, caused by different environmental conditions during their development and differences in the internal nutritional status of the shoot. Because buds at different positions on a shoot form at different times, their nutritional status, hormone supply and external environmental conditions differ, resulting in differences in their quality, which is called bud heterogeneity. For example, the buds at the base of a shoot form in early spring, when the shoot is just beginning to grow, leaf area is small and temperatures are low, so bud quality is relatively poor. If the shoot stops growing in time, the terminal bud has the best quality. The quality of axillary buds depends mainly on the size of the leaf at that node and its ability to supply nutrients, because the nutrients and energy for bud formation come mainly from the leaf at that node, so buds at the base and at the slender tip of the shoot are generally of poorer quality. The selection of the pruning cut bud mainly makes use of bud heterogeneity.

  Increased branching in walnut disperses nutrients and moderates growth, which favors nutrient accumulation, thereby promoting flower bud differentiation and laying the foundation for early fruiting and high yield. With more branches, leaves also increase relatively, producing more organic nutrients and promoting nutrient accumulation; however, too many branches result in poor light, which needs to be regulated through training and pruning. When young trees have more main branches, the opening angle of the main branches also becomes larger.

  In walnut, the polar growth of inclined branches is weakened and growth vigor shifts to the two sides, so the lateral branches on both sides tend to expand, increasing the amount of branches and leaves and enabling earlier flower formation and fruiting; therefore, the scaffold branches of walnut trees are required to have an open angle. Fewer main branches with a small opening angle, or more main branches with a large opening angle, both favor early fruiting and high yield. Fewer main branches with a small angle can also obtain more light and can bear fruit. Many main branches that are upright will inevitably be crowded, resulting in poor light and reduced fruiting; however, if the opening angle is large, even with many main branches, the light is still improved and fruit can still be borne. This is the self-regulating effect of walnut trees.

  2. Apical dominance of shoots. Generally, arbor species have relatively strong apical dominance, and the degree of dominance often varies with cultivar type, tree age and vigor, and planting method. Generally speaking, apical dominance weakens successively from upright-growing, horizontal-inclined and drooping shoot types; in other words, shoots with a large angle have weaker apical dominance. Using these characteristics, during the training period of young trees, in order to expand the canopy as early as possible, vigorous shoots should be headed back less, and the apical dominance of large terminal buds should be used to promote strong long shoots.

  Walnut has strong apical dominance, and its plump terminal buds easily produce vigorous shoots. The lateral buds below the terminal bud have a very small proportion of shoot growth and fruiting; especially the lateral buds in the middle and lower parts of the shoot mostly dry up and fall off on their own, forming a bare zone. Therefore, the canopy of walnut trees is mostly relatively sparse.

  Apical dominance means that in an upright-growing shoot, growth at the tip is dominant and strongest, decreasing progressively downward. In inclined shoots, the tip becomes lower, so dominance is also weakened; tip growth slows somewhat, while growth in the middle and rear parts becomes stronger, shown by an increase in sprouted shoots. Horizontal shoots have no towering tip and thus no particular dominance; growth vigor is even from front to back, growth is moderate and branching is uniform, which favors flower formation and fruiting.

  3. Layering. Due to the effect of apical dominance, the top few buds of a long shoot develop into long shoots, the upper and middle buds develop into medium and short shoots, and the lower buds do not sprout and form a bare zone. This pattern of growth occurs every year, so after growing into a large tree, the main branches are distributed in layers on the tree; this is layering. Using layering for training and pruning allows young trees to take shape in time, and large trees to have distinct layers and good light, with high, stable yield and quality. For fruiting branch groups, heading back long shoots promotes short shoots in the middle and lower parts of the shoot, changes the bare phenomenon and makes the tree form compact. During the training stage, when selecting and retaining main branches, attention should be paid to appropriate spacing within each layer's training zone, so as to avoid adjacent main branches in large trees causing a "choking" phenomenon, which is manifested as overly vigorous lower main branches and upper main branches that cannot grow, resulting in an imbalanced tree vigor. At the same time, the distance between layers must be opened up according to standards to improve ventilation and light conditions throughout the tree. Adjacent arrangement of main branches not only easily causes "choking" but also makes the structure unstable, while neighboring arrangement can avoid this shortcoming.

  4. Clear hierarchy and balanced tree vigor. Because the distribution of branches on the tree is layered due to apical dominance and layering, for the whole tree or a certain scaffold branch, the lower part (formed earlier) and the upper part (formed later) must maintain a hierarchical relationship of size and height. That is, the earlier-formed ones should be larger and higher, as the dominant; the later-formed ones should be smaller and lower than them, as the subordinate. In terms of tree structure, this means the central leader is stronger than the main branches, the main branches are stronger than the lateral branches, and so on. Such a structure has a clear hierarchy, characterized by large branches below and small branches above; small branches block less light, large branches receive more light, ventilation and light are good, fruiting positions are stable, the tree structure is firm, and yield is high and stable.