剪定の目的で、仕上がりは変わる──育てる・保つ・縮める・作り直す、植木職人の四つの手入れ|樹木医が図解(Pruning to a Purpose: Growing, Maintaining, Reducing and Restoring a Tree)

English: Pruning to a Purpose — Growing, Maintaining, Reducing and Restoring a Tree

Pruning is decided less by which branch to cut than by what you want the tree to become. Will you let it grow, keep it at its present size, make it a size smaller, or rebuild a disordered form? A different purpose means different cuts and a different finish. This illustrated article explains the work and the finish for each of the four purposes, with reference to Japanese public pruning standards.

This is the second special edition of our series on the pruning techniques of Japanese gardeners (niwashi and uekiya). The first described what a gardener watches while thinning. This time the subject is the purpose decided before the shears go in.

Before cutting, decide the target crown

What we decide before pruning is the tree’s target crown: the outline we want to keep it at, drawn in the mind. Then we decide how far to take it back this time (the crown at pruning). Allowing for next year’s growth, we go back a little inside the target, so that a year later the crown returns to just the target size (Saitama Prefecture, Street Tree Pruning Manual).

Target forms fall broadly into two lines: the natural form, kept as it is or reduced in similar proportion, and the artificial form, made by heading cuts or shearing. For street trees, it is said that one must first be clear whether the model is the natural form or an artificial one (Saitama Prefecture).

From our experience, work begun without a target form shifts its cuts a little every year, until the tree quietly becomes a “cut” shape. Among Japanese gardeners it is said that the first thing to do in front of a tree is not to take out the shears, but to decide what it should look like ten years from now.

Growing: decide the framework, remove only unwanted branches

For young trees, or trees that may still grow larger, the work is “growing”. The aim is to spread the branches while keeping a natural form and to increase the volume of green. Because the size is not reduced, cutting is limited to removing unwanted branches at their origin, such as water sprouts, crossing, inward-growing, parallel and upright branches, and heading back the tips is not done as a rule (Saitama Prefecture).

What matters is deciding the framework branches early. If a tree grows large with dense branches, in zelkova and similar trees the inner branches die and the crown shifts upward. Removing thick branches after the tree has grown lets decay in or disorders the form (Saitama Prefecture). One small decision while the tree is young prevents a large cut twenty years later.

Maintaining: keep size and softness by switching

Much of the work on garden trees is keeping the present size. Its basis is kirikaeshi, switching a long branch to a shorter one. The outline stays almost the same size, while the softness of the tips and the gaps within the crown are kept (Saitama Prefecture).

When there is no branch to switch to, you head back at a bud and wait for a new shoot; once a suitable one has grown, you switch to it. “Maintaining” is planned work that proceeds while raising the next branch to switch to (Saitama Prefecture).

Cut an old branch with no buds partway, and several thin shoots sprout around the cut. Left alone, they become a source of crowding. The next year, decide the one or two to grow on and remove the rest at their origin (Saitama Prefecture).

Another point in maintaining is the direction of the branch you keep. Keep an outward branch and the form spreads outward; keep an inward one and it stays inside (Saitama Prefecture). We use the direction of the kept branch according to where we want the tree to spread and where to hold it in.

Reducing: a size smaller, in the same natural form

When making an overgrown tree smaller, the aim is still to keep its natural form. Rather than lopping the top to a height or heading branches partway, the approach is to reduce the original form in similar proportion (Saitama Prefecture).

For spreading trees such as zelkova and cherry, you look at the forks of the limbs spreading in all directions for a short branch that fits the reduced outline, and switch to it. If there is none in the right place, switch at the fork just before or beyond, never cutting an old branch off partway (Saitama Prefecture).

For narrow trees whose trunk runs to the top, such as ginkgo, you find a young upward branch at the height you want and switch the leader to it at its origin. Upper branches are kept short and few, lower ones longer and more numerous, keeping a slender cone (Saitama Prefecture).

Reducing also involves more cuts to thick branches. The cut is made just outside the swelling at the branch base, and thick branches are cut in stages to prevent tearing (the branch collar).

Restoring: rebuilding a disordered form over several years

Trees with knuckles from repeated heading cuts at the same point, or trees grown too large and disordered, need “restoring”. With the future form (five to ten years ahead) in mind, you cut hard once and then raise the shoots that follow to rebuild the form. The management in the years after the cut is therefore especially important (Saitama Prefecture).

The manual describes the sequence like this. Cut hard in year one, and a dozen or more shoots appear from the cut, so decide three or four to grow on and remove the rest. In year two, do not prune; just watch. In year three, the strength and direction of the kept shoots become clear, so narrow them to one or two (Saitama Prefecture).

A tree with pruning knuckles has two paths: keep the knuckles and maintain it as an artificial form, or cut back past the knuckles and return it to a natural form. If the branch at the base of a knuckle is thicker than an arm, decay enters easily from a cut, so keeping the knuckle is considered safer (Saitama Prefecture). The knuckle itself is healthy tissue, so we never cut through it.

From our experience, a difficult part of restoring is year two, the “year of doing nothing”. You want to tidy the shoots around the cuts early. But which shoots will grow strong cannot be known for another year. Let the tree give its answer first, then choose. Whether you can keep that order decides the finish of the restoration.

Evergreens: combine cuts branch by branch

Evergreens are covered in leaves all year and their framework is hard to see. The work proceeds branch by branch, combining removal, switching and heading. Remove one long branch at its origin, switch another to a short branch along it, and head back only the tip that has still grown too far.

The depth of thinning can also be chosen. Removing medium branches from a thick limb opens the crown widely; keeping the medium branches and removing the twigs beyond spreads small gaps throughout. For street trees, thinning with large branches is sometimes done, but keeping to thinning with medium branches is considered better (Saitama Prefecture).

Sharing the purpose: the gardener’s work and the people who entrust a tree

Among Japanese gardeners it is said that skill in pruning shows in whether you can continue work that suits the purpose for years. Even a fine finish in one visit will, if the cuts drift from the purpose, leave the tree’s form broken within a few years. Conversely, a tree being restored looks sparse for a while.

From our experience, that in-between stage is more easily conveyed by showing a drawing first to the people who entrust us with the tree. The Saitama manual also includes an example of a notice board that shows residents, in drawings, how a street tree will look over the years of a major rebuild. Which purpose, over how many years, toward what form: sharing that before the shears go in is, I believe, the start of caring for a tree over the long term.

Frequently Asked Questions (Q&A)

Q1. What kinds of pruning “purpose” are there?

A. Street tree standards divide them into four: growing, maintaining the present size, reducing by a size, and restoring a disordered form. Each has two lines, aiming at a natural form or an artificial one.

Q2. How should I cut to keep the present size?

A. The basis is kirikaeshi, switching a long branch to a short one at a fork. The outline stays about the same size and the tips stay soft. Where there is no branch to switch to, head back at a bud to raise a new shoot, and switch to it at the next visit.

Q3. Can an overgrown tree be made smaller in its natural form?

A. Yes, by switching to short branches at the forks with the aim of reducing the original form in similar proportion. Lopping the top or cutting branches off partway loses the natural form. Large reductions are spread over several years.

Q4. How many years does restoring a disordered tree take?

A. Street tree standards plan three to five years of management. The sequence shown is: cut hard in year one and choose three or four shoots to grow on, watch in year two, and narrow to one or two in year three.

Q5. Should pruning knuckles be cut off?

A. The knuckle itself is healthy tissue, so we never cut through it. Choose either to keep the knuckles and maintain an artificial form, or to cut back before them and return to a natural form. If the branch at the knuckle’s base is thicker than an arm, keeping the knuckle is considered safer.

Sources

– Saitama Prefecture, Road Environment Division. Street Tree Pruning Manual. December 2020 (citing the Japan Landscape Contractors Association, Street Tree Pruning Handbook; Public Works Research Institute and JLCA, Street Tree Pruning Guide; Fujita, Pruning Street Trees; and others).
– Bedker, P.J., O’Brien, J.G., Mielke, M.E. How to Prune Trees. USDA Forest Service, NA-FR-01-95, 1995 (revised 2012).

Masatoshi Nakamura
Jumokui (Tree Doctor), Toshi Landscape Co., Ltd., Tokyo