What are the main pruning methods for walnut trees?
1. Heading back. Heading back refers to cutting off part of a one-year-old branch. On young walnut trees, the method of heading back vigorous vegetative branches is commonly used to increase the number of branches. The targets for heading back are vigorously growing vegetative branches sprouting from first-order and second-order lateral branches; the purpose is to promote new shoot growth and increase branching. The cut length is 1/4 to 1/2 of the branch length; after heading back, generally about 3 longer branches can sprout. Through heading back, the apical dominance of the bud at the cut is changed, the new shoots at the cut grow vigorously, which can promote branching and improve the branching ability. Medium-length or weak branches on walnut trees should not be headed back, otherwise it stimulates the lower part to send out thin, weak short branches, the tissue is not firm, and in winter sunscald easily occurs and they dry up, affecting tree vigor.
2. Leaving unpruned (long release). That is, no cutting is done to the branch at all; its effect is to moderate the branch's growth vigor, increase the number of medium and short branches, favor the accumulation of nutrients, and promote fruiting in young vigorous trees. Except for upright vigorous branches on the back (which can be bent flat and then left unpruned), the effect of leaving other branches unpruned is relatively good.
After leaving unpruned, bending the branch can produce many short branches; if not bent, only a small number of long branches sprout at the branch tip, easily causing the rear part to become bare and the fruiting position to move outward.
After leaving unpruned, different cultivars and different branches respond differently. For upright-growing cultivars and upright-growing branches, only a small number of branches sprout at the branch tip and grow vigorously, while the middle and rear parts do not send out branches, easily causing the rear part to become bare and the fruiting position to move outward; whereas for branches with a more open tree posture and open angles, a large number of branches can sprout, the fruit-setting rate is high, and the fruiting position is compact.
3. Thinning. Removing branches from the base is called thinning. The targets for thinning are generally male flower branches, diseased and insect-damaged branches, dead and dry branches, useless water sprouts, overly dense crossing branches, and overlapping branches, etc. When there are too many male flower branches, a large amount of nutrients is consumed during flowering, thereby causing the tree to weaken; during pruning they should be properly thinned to save nutrients and strengthen tree vigor. Dead branches are places where diseases and insects breed and should be thinned in time. When the density of branches inside the canopy is too great, follow the principle of removing the weak and keeping the strong, and thin overly dense branches at any time to facilitate ventilation and light penetration. When thinning branches, cut close to the branch base; never leave a stub, so as to facilitate healing of the cut.
Walnut trees have large compound leaves, so too many branches cannot be retained; upright vigorous branches, crossing branches, overlapping branches, overly dense branches, thin and weak branches, male flower branches, and large branches on the back should be thinned in time to solve the ventilation and light conditions in the inner canopy, rejuvenate fruiting branches, and improve fruiting capacity. For cultivars with an upright tree posture, a large number of vigorous branches easily occur in the upper part and should be thinned in time. Otherwise, lower branches easily die and the fruiting position moves outward.
4. Renewal pruning (retrenchment). Cutting back a perennial branch is called renewal pruning or retrenchment. The effect of retrenchment varies with the location of the retrenchment: one is a rejuvenating effect, and the other is a suppressing effect. In production, the rejuvenating effect is applied in two aspects: one is local rejuvenation, for example, retrenching and renewing fruiting branch groups, perennial overlong drooping unpruned branches, etc.; the other is whole-tree rejuvenation, mainly retrenchment and renewal of old and declining trees. In production, the suppressing effect is mainly used to control vigorous auxiliary branches, suppress strong scaffold branches in trees with unbalanced vigor, etc. When retrenchment creates an excessively large wound, it has the effect of weakening the growth vigor of the first branch below the wound; retrenching a vigorous tree too heavily easily stimulates vigorous branches, so in production the location and severity of retrenchment should be well controlled.
Using retrenchment of different degrees all has the effect of rejuvenating the lower part, but the location of the retrenchment and the thickness of the branch differ in their rejuvenating effect. The location of the retrenchment is related to the growing position and growth vigor of the first branch below the cut; generally, when the first branch below the cut grows vigorously and is positioned higher, its rejuvenating effect is good; otherwise the effect is poor.
5. Pinching and removing sprouts. Pinching is removing the tip portion of the current year's new shoot, which can promote the formation of secondary shoots and increase branching. Pinching the extension shoots of the main and lateral branches of young trees promotes branching and accelerates the shaping process. Pinching upright branches in the inner canopy can promote the formation of horizontal-slanting branches, moderate growth vigor, and lead to early fruiting. It is commonly used in the shaping and pruning of young trees.
Response to pinching and scoring: During the shaping stage of young trees, many walnut new shoots have plump terminal buds and very strong dominance, with weak ability to sprout lateral branches and short branches. In summer, when new shoots are 60-80 cm long, pinching can be done to promote 2-3 branches, which can strengthen the shaping effect of young trees and lead to earlier canopy formation. For perennial single-axis extension branches, especially bare-legged branches about 1 cm in diameter, scoring above the annual ring scar, deep to the xylem, can promote latent buds to sprout new branches and promote fullness of branch groups.
After winter pruning, especially after thinning large branches, latent buds below the wound are often stimulated to sprout, forming many vigorous shoots; therefore, removing excess sprouts in time in the early part of the growing season is beneficial for tree shaping and saving nutrients, and promotes healthy branch growth. During the shaping process of young trees, useless branches also often sprout; when they first sprout, it is best to rub them off by hand, as they are then less likely to sprout again; if they grow larger and are thinned with shears, they will sprout again.
Starting from the second year, young walnut trees whose new shoots grow vigorously without pinching can produce shoots 1.5-2 m long; summer pinching should be carried out, which can promote branching, increase branch number, expand the canopy as quickly as possible, increase the number of fruiting mother branches, lead to earlier fruiting and earlier high yield. For vigorously growing branches about 60 cm long, pinch them at the part above semi-lignification.
6. Opening the angle. By propping, pulling, weighting and other methods, the branch angle is enlarged to moderate growth vigor; this is a common method during the shaping period of young trees to regulate the growth vigor of each scaffold branch, improve light conditions, and promote flower bud differentiation.