A Detailed Guide to the Pruning Characteristics of Walnut Trees at Different Ages
1. Pruning timing. The pruning timing for walnuts differs from that of other fruit trees. Because pruning walnut during the dormant period after leaf fall causes "bleeding" (sap flow) from the wounds, leading to nutrient loss, weakened tree vigor, and even dead branches, it is not suitable to do it in winter. The walnut bleeding period generally runs from after leaf fall in autumn to before bud break the following year (that is, from mid-November to late March of the following year). For this reason, walnut pruning should avoid the bleeding period. The suitable pruning period should be after fruit harvest but before the leaves turn yellow, and after leaf expansion in spring. However, spring pruning loses more nutrients and easily damages young twigs and leaves, so mature trees should be pruned in autumn before the leaves turn yellow, before October; autumn pruning does not cause bleeding, and wounds heal quickly. Young trees can be pruned in spring, summer, or autumn. In recent years, some people have also pruned during the low-tide period between the two peaks of bleeding (mid-December to mid-March of the following year), and no bleeding occurred or only slight bleeding occurred. The response after winter pruning is better than that after spring pruning and autumn pruning, so many experts advocate winter pruning, and the best pruning period is before bud break in spring.
2. Training and pruning of young trees. Walnut grows very quickly at the young tree stage. If it is allowed to develop freely, it is not easy to form a good high-yielding tree shape, especially for early-bearing walnuts, which have strong branching ability and bear fruit early, and easily send out secondary shoots that cause the tree shape to become disordered, which is not conducive to normal growth and fruiting. The pruning task during the young tree stage is to cultivate a strong framework and a high-yielding tree shape. Attention should be paid to regulating the subordinate relationships among branches at all levels, maintaining the dominance of the central leader and the vigorous growth of the main branches. Use the terminal bud shoot as the extension shoot for the central leader, and leave more auxiliary shoots on it to strengthen the growth of the central leader. On the main branches, competing shoots and back shoots should be controlled.
Walnut shaping usually has two forms: the delayed-open central leader form and the natural open-center form, which can be chosen according to differences in cultivar characteristics, soil quality, fertilizer and cultivation management, and adapted to the tree and local conditions.
(1) Delayed-open central leader form. It has an obvious central leader and 5 to 7 main branches; the crown is large and the yield is high. It is suitable for walnut trees of strongly upright cultivars, on fertile and deep soil, and under relatively good cultivation and management conditions.
(2) Natural open-center form. It has no central leader, fewer main branches, and a smaller crown. It is suitable for walnut trees with an open crown, on barren soil, or under poor management conditions. According to differences in the number of main branches retained, there are 3 types: the two-main-branch open-center form, the three-main-branch open-center form, and the multi-main-branch open-center form.
3. Pruning of fruiting trees. When walnut trees enter the fruiting stage, the crown is still continuing to expand, and the fruiting positions constantly increase, so the contradiction between growth and fruiting easily appears. Ensuring that walnuts reach high and stable yields is the main task of this period. Therefore, in pruning, attention should always be paid to cultivating good branch groups, making use of auxiliary shoots and water sprouts, and promptly handling back shoots and drooping shoots.
From the early fruiting stage onward, strong fruiting branch groups should be cultivated in a planned way, increasing fruiting positions, expanding the fruiting area, and raising the yield of young trees. When entering the full fruiting stage, with the expansion of the crown and the increase in fruiting positions, the contradiction between growth and fruiting easily appears. The pruning task in this period is mainly to adjust the relationship between vegetative growth and reproductive growth and improve the ventilation and light conditions inside the crown, thereby achieving the goal of high and stable yields. In pruning, the main aims are to balance tree vigor, moderately release or shorten vigorous branches to moderate their growth, increase branch number, and promptly retract and renew fruiting branches to rejuvenate them and prevent early decline. Continue to cultivate fruiting branch groups, make good use of auxiliary shoots and inner-canopy water sprouts, and prevent the inner canopy from becoming empty and the fruiting positions from moving outward. Promptly handle back shoots, drooping branches, and crowded branches to prevent closure.
(1) Pruning of trees in the early fruiting stage. At this stage, the tree structure has initially formed, and tree vigor should be kept balanced. Remove and renovate upright upward water sprouts, thin out dense outer branches and ineffective branches with long internodes, retain sufficient effective branches (thick, short, strong), control strong branches so they develop toward moderate vigor (by summer pinching, bending, and changing the leader), and fully use all usable fruiting branches (including drooping branches), to achieve the goal of early fruiting and early high yield.
(2) Pruning of trees in the full fruiting stage. For walnut trees in the full fruiting stage, most of the crown is close to closure or has already closed, the number of outer branches gradually increases, and most of them become fruiting branches. Due to insufficient light, some small branches dry up, and bare zones appear at the rear of the main branches. Fruiting positions move outward, and alternate bearing easily occurs. Therefore, the main task of pruning at this time is to adjust and improve the ventilation and light conditions inside the crown and constantly renew fruiting branches, so as to achieve the goal of high and stable yields. The key points of pruning are: thin out diseased branches, let sunlight through, retract the outer parts, promote the inner canopy, raise branch angles, conserve nutrients, cultivate branch groups, and increase yield.
4. Pruning of aging trees. After walnut enters the aging stage, branch tips and large branches often die, and yield declines year by year. They should be renewed and rejuvenated in time to prolong the fruiting years.
(1) Trunk renewal (major renewal). Saw off all the main branches so that they send out new branches again and form main branches. There are the following two specific methods.
1) For plants with too tall trunks, the entire crown can be sawed off at an appropriate part of the trunk, so that latent buds below the saw cut sprout new branches, and then 2 to 4 branches with suitable direction and vigorous growth are selected from the new branches and trained as main branches.
2) For open-center plants with suitable trunk height, they can be sawed off at the base of each main branch. If it is a central-leader-form plant, the crown can first be sawed off above the first tier of main branches, and then sawed off from the base of each main branch, so that latent buds at the base of the main branches sprout and send out branches.
(2) Main branch renewal (moderate renewal). Retract at an appropriate part of the main branch so that it forms new lateral branches. The specific pruning method: select a vigorous main branch, retain 50 to 100 cm of length, and saw off the rest, so that it sends out branches near the saw cut on the main branch. After branching, select 2 to 3 vigorous branches with suitable orientation on each main branch and train them into first-order lateral branches.
(3) Lateral branch renewal (minor renewal). Retract the first-order lateral branch at an appropriate part to form a new second-order lateral branch. Its advantages are that the new crown forms and yield increases relatively quickly.