When is the best time to prune a walnut tree? Why can't you prune in winter?
To achieve high walnut yields, in addition to basic management work, you also need to prune the branches to promote lateral branch growth. However, pruning a walnut tree is subject to timing restrictions - you can't just prune whenever you want, and you cannot prune in winter. So when is the best time to prune a walnut tree? Why can't you prune in winter?
Spring pruning: From bud break to full bloom of the walnut tree. Spring pruning helps moderate tree vigor and promotes flower formation and fruiting; it is mostly used on young trees.
Autumn pruning: From White Dew to Frost's Descent (early September to early October, from after fruit harvest to half a month before leaf fall). Autumn pruning helps strengthen tree vigor and promotes the differentiation of mixed buds.
Summer pruning: From the Beginning of Summer to Grain Full (from after flower drop to before physiological fruit drop). Summer pruning controls excessive vigor; it is suitable for pruning excessively vigorous trees that do not bear fruit.
Do not prune in winter: Pruning cannot be done in winter (the dormant period). This is because wounds made in winter are very likely to cause bleeding (sap flow), leading to a massive loss of water and stored nutrients from the tree, which weakens tree vigor and can even cause branches to die, affecting normal growth and fruiting.
Pruning Methods
1. Saw wound to force branches
On some walnut trees, the latent buds are at the base of the branch section, while on others they are present in both the middle and the base of the branch section. Therefore, for areas that have already become bare (without branches), you need to saw above the latent bud at the bare area, deep into the xylem, to force out new branches. The growth of the forced new branches varies depending on the thickness of the parent branch and the depth of the saw wound; the long ones can reach 1-2 m or more (this method can be used on bare areas). If the saw cut is very shallow, only small branches less than 30 cm long can grow. Using this method, you can develop fruiting branch groups or cultivate new scaffold branches on bare areas. On early-bearing walnuts, the new shoots forced out can flower and fruit the following year as long as they grow vigorously. On some early-bearing trees, some latent buds are mixed buds and can flower and fruit in the same year they sprout, while new shoots sprouted on late-bearing trees need 3 years to flower and fruit.
2. Ring sawing
For larger non-scaffold branches, sawing a ring above a latent bud deep into the xylem can both weaken the growth vigor of that branch and cause small branches to grow below, filling the space in bare areas. Its effect is similar to saw wound forcing, but stronger. Spiral staggered ring sawing is safer. Generally, for branches 5-10 cm thick, saw one ring; for 10-15 cm, saw two rings, with a spacing of about 15 cm between the two rings. If the spacing is too narrow and the sawing too deep, the branch will die.
3. Bending branches
Before the new shoots in the inner canopy become lignified (in June), those growing in unsuitable directions can be bent into spaces that have room. Because walnut branches are very flexible and the leaves are large and thick, for branches under 5-10 cm in diameter, bending them downward in summer can open the angle to the required degree without needing stakes or ropes.
4. Thinning branches
When thinning large walnut branches, it is best to leave a slanted stub from the base, low on the inside and high on the outside, so that small branches can grow on the outside to fill the space. Do not leave a residual stub, otherwise the stub will die and form a hole.
5. Pinching off the terminal bud
Walnut terminal buds are large and fat, with strong apical dominance, often forming a large terminal bud with a central shoot and a low branching rate. Pinching off the terminal bud before germination or pinching off the tender tip of young shoots after germination can both increase the branching rate and form compact fruiting branch groups.
6. Cutting tender shoots
Pruning non-lignified tender shoots in June can cause axillary buds to sprout immediately. Using this method to promote branching and cultivate fruiting branch groups is much better than heading back lignified branches.
7. Changing the leader and heading back
Walnut pruning mainly uses the method of changing the leader and heading back, and heading cuts are rarely used. If heading back a large branch would create too large a wound, leave a cut-stub branch above the target branch to protect the wound, or leave a latent bud below the cut.
8. Heading back one-year-old branches with a protective bud
Walnut branches have a large hollow pith. After heading back a one-year-old branch, the area below the cut often shrinks and dries back a section, causing poor growth of the bud at the cut. Therefore, when heading back a one-year-old branch, leave one protective bud above the target bud and cut close to the protective bud, so that the protective bud can grow into a small branch. This both avoids the cut drying back and ensures the vigor of the target bud.
9. Three methods for rejuvenating small branches
After the full bearing period, the annual yield of large trees mainly depends on vigorous fruiting branch groups bearing fruit continuously. Weak fruiting branches cannot continue to form mixed buds, so timely rejuvenation of fruiting branches is one way to strive for high yields year after year. After fruiting branches have borne fruit for several consecutive years, their vigor will weaken. Currently in production, weak ones are often simply cut off, but on the basis of improving fertilizer, water and light, adopting appropriate pruning measures can rejuvenate weak branches. There are mainly three methods:
(1) Heading back the parent branch: If the parent branch on which the small branch is borne is slender and weak, cutting the parent branch off above the point where the small branch is attached can rejuvenate the small branch.
(2) Notching the parent branch: If the parent branch is thick, sawing a notch above the small branch deep into the xylem can rejuvenate the small branch.
(3) Heading back small branches that extend in a single axis: Some small branches extend forward in a single axis and form relatively long whip-like branches, and their vigor becomes weak. If the base of the small branch is more than 1 cm, you can leave a latent bud at the base and make a heavy heading cut to regenerate new shoots. If the base of the small branch is a thin branch less than 1 cm, after heading back, new shoots usually cannot regenerate from the base, so you can only use the methods of heading back the parent branch or notching the parent branch to rejuvenate it.