大透かし・中透かし・小透かしとは──枝の太さで分ける、透かし剪定の3つの段階|樹木医が図解(Three Levels of Sukashi: Large, Medium and Fine Thinning)

English: Three Levels of Sukashi — Large, Medium and Fine Thinning in Japanese Pruning

O-sukashi, chu-sukashi and ko-sukashi (large, medium and fine thinning) are the names Japanese gardeners (niwashi and uekiya) give to sukashi, thinning, according to the thickness of the branches removed. The framework of the tree is adjusted with the thick branches, crowding is loosened at the medium branches, and the finish is made with the small twigs at the branch tips. In this illustrated article, a Jumokui (tree doctor) explains the order and the amount, with reference to Japanese public pruning standards and sources from abroad.

This is the sixth article in our series on the pruning techniques of Japanese gardeners. So far I have written about the idea of sukashi (thinning) and about two kinds of cut, eda-nuki (removal cuts) and kirimodoshi (cutting back). This time the subject is the level at which thinning is done: at which thickness of branch.

Thinning is divided into three levels by branch thickness

A tree’s branches become thinner step by step: thick main scaffold branches from the trunk, secondary scaffolds from the main branches, and laterals and twigs beyond them. Sukashi (eda-nuki, thinning) keeps this framework and adjusts the amount and density of branches and foliage by removing crowded branches at their forks.

The name changes with the level at which branches are removed. Removing thick branches, from the outer part of the main scaffolds to the secondary scaffolds, at their forks is o-sukashi; removing branches at the forks of the secondary scaffolds is chu-sukashi; removing only the laterals at the branch tips is ko-sukashi (NILIM, Guide to Preventing Street Tree Failure; Saitama Prefecture, Street Tree Pruning Manual). Cutting a main scaffold off at the trunk is called eda-oroshi and is treated separately from thinning.

The three are not separate techniques so much as a sequence used within the care of one tree, from thick to thin. In regular yearly care, a combination of chu-sukashi and ko-sukashi is the usual practice (Saitama Prefecture manual). O-sukashi is used on trees too crowded for that to keep up.

O-sukashi: adjusting the framework

A tree that has not been thinned for years is packed with foliage right into the inside of the crown, and from outside it looks like a heavy, rounded mass. The inner branches weaken for lack of light, branches rub against each other, and the wind cannot pass through.

On such a tree, the large-scale crowding is loosened first with the thick branches. Thick branches heading into the crown and those overlapping other thick branches are removed at their origin on the main scaffold. The cut is made just outside the swelling at the base of the branch (the branch collar) (on cutting at the branch collar).

After o-sukashi, the outline hardly changes, but large gaps open inside the crown and the way the trunk divides into main scaffolds becomes visible. The fine crowding at the branch tips is still there. From here, the tree is refined with chu-sukashi and ko-sukashi.

Removing thick branches means larger cuts. As a guide in our own work, o-sukashi on a garden tree is done once every few years, for example when rebuilding a tree that has gone a long time without care. From our experience in the field, the cuts of o-sukashi are less noticeable from outside if positions are chosen where they will be hidden in the shadow of the remaining branches.

Chu-sukashi: loosening crowding at the secondary forks

Chu-sukashi is thinning at the level of the secondary scaffolds, removing crowded branches at their forks. Overlapping branches, branches heading inward and branches running parallel to others are removed, so that the remaining branches stand at intervals on either side of the main branch. The way of identifying branches to remove is the same as in our imi-eda article.

The Tokyo Metropolitan Government’s standard specification for street trees also uses the term “chu-sukashi” for thinning overcrowded parts in removal pruning. In garden care, too, chu-sukashi is the core of yearly thinning.

Ko-sukashi: finishing with the twigs at the branch tips

Ko-sukashi removes only the laterals and twigs at the branch tips. Crowded overlapping twigs, downward-facing twigs and weak thin twigs are removed at their origins, so that the remaining twigs spread like a fan and small gaps open between the clumps of leaves.

Because the branches are thin, they are removed one at a time with ueki-basami (Japanese garden scissors). The gardener lifts the branch tip lightly with one hand and checks the origin of the twig behind the leaves before the scissors go in. It is this handwork of ko-sukashi that finally decides how the tree looks from outside.

Order and amount: thick to thin, top to bottom

There is an order to thinning: from thick branches to thin. The City of Yokohama’s specification says the usual order is eda-oroshi, removal cuts, cutting back, and heading. In terms of height, work proceeds from the upper branches to the middle and then the lower branches, thinning the upper foliage more strongly and the lower foliage more lightly (NILIM guide; Tokyo standard specification). The upper part of a tree grows more vigorously, while the lower branches tend to weaken for lack of light.

There are guides for the amount, too. When evergreens are thinned in leaf, the amount removed is kept to one-third of the whole or less (NILIM guide; Tokyo Metropolitan Government, Street Tree Maintenance Plan). The American pruning standard ANSI A300 and the USDA Forest Service guide state that no more than one-quarter of the living crown should be removed at a time. The older and larger the tree, the less is removed.

The finish is hard to put into words, but Tokyo’s Street Tree Maintenance Plan says, for evergreen oaks and Machilus, that crowded branches are thinned until the sky shows slightly through. Foliage remains both inside and outside the crown, and sky and branches show evenly between the clumps. That is the form after the three levels.

Now, in October, is the right season for thinning evergreens (September to October in the Yokohama specification). Deciduous trees are thinned around November to February, after leaf fall, while looking at the branch framework.

The craftsman’s order: decide from afar, finish up close

Among Japanese gardeners it is said that thinning goes from large to small and from far to near. First the gardener steps back, looks at the whole tree and decides which thick branches to remove. Next, closer to the tree, the medium branches are arranged, and finally, on the ladder, the branch tips within arm’s reach are finished. The viewing distance and the thickness of the branches removed always go together.

Reversing this order does not work. From our experience in the field, if the fine work at the branch tips is done first, removing one thick branch afterward leaves a large hole just there, spoiling the tips that had been so carefully arranged. Deciding the thick branches first is not a way of saving effort, but a way of not ruining the finish.

Where to stop is also decided by the craftsman’s eye. As a guide in our own work, we come down from the ladder and look up from under the tree; when the sky shows slightly through the leaves, that face of the tree is finished. Among gardeners, some lay the cut branches out on the ground to see by eye how much has been taken. The pine care at the Kyoto Imperial Garden (Kyoto Gyoen) is known as gosho-sukashi, and the Ministry of the Environment’s description explains it as thinning so subtle that one only just notices the tree has been cared for. The three levels of sukashi are, I believe, the wisdom of order that leads to such a restrained finish.

How sukashi corresponds to crown thinning: for readers outside Japan

Sukashi corresponds to crown thinning in arboriculture. ANSI A300 defines thinning as selective pruning to reduce the density of live branches, and requires that branches be left evenly distributed on individual limbs and throughout the crown. Lion’s tailing, in which too many inner branches are removed and foliage is left only at the tips, is regarded as unacceptable.

As far as I could confirm, English-language standards have no division that names levels by the tier of branch at which thinning is done, as o-, chu- and ko-sukashi do. The standards specify work by type of pruning, amount (percentage) and distribution of branches. The three levels of Japanese sukashi may be seen as building the same aim around a measure that suits the gardener’s hands and eyes: the thickness of the branch.

One of the pillars of our approach, GREEN MANAGEMENT, is the integration of scientific knowledge (arboriculture) with field experience. How much to remove is decided by how the tree works; which tier of branches to start from is decided by the gardener’s eye. The three levels of sukashi, I believe, are the procedure that connects the two.

Frequently asked questions

Q1. What are o-sukashi, chu-sukashi and ko-sukashi?

A. They are names for sukashi (thinning, eda-nuki) divided by the thickness of the branches removed. Removing thick branches, from the outer main scaffolds to the secondary scaffolds, at their forks is o-sukashi; removing branches at the forks of the secondary scaffolds is chu-sukashi; removing only the laterals at the branch tips is ko-sukashi.

Q2. In what order is thinning done?

A. From thick branches to thin: o-sukashi, then chu-sukashi, then ko-sukashi. In terms of height, work goes from the upper branches to the lower, thinning the upper foliage more strongly and the lower more lightly. If the tips are finished first, removing a thick branch afterward leaves a large hole.

Q3. Is o-sukashi needed every year?

A. Regular yearly care usually combines chu-sukashi and ko-sukashi. O-sukashi is done when rebuilding a crowded tree that has gone a long time without care, and because the cuts are larger, on garden trees it is limited to once every few years.

Q4. How much can be thinned at one time?

A. For evergreens in leaf, the amount removed is kept to one-third of the whole or less. American standards and guides state no more than one-quarter of the living crown at a time. The finish should let the sky show slightly through the leaves.

Q5. When is thinning done?

A. For evergreens, around September to October is the right season, so October is just the time for thinning evergreens. Deciduous trees are thinned around November to February, after leaf fall, while looking at the branch framework.

Sources

– National Institute for Land and Infrastructure Management (NILIM). Guide to Preventing Street Tree Failure, 2nd edition. Technical Note No. 1059, 2019.
– Saitama Prefecture, Road Environment Division. Street Tree Pruning Manual. December 2020 (citing the Japan Landscape Contractors Association, Street Tree Pruning Handbook).
– Tokyo Metropolitan Government, Bureau of Construction. Street Tree Maintenance Plan. December 2021.
– Tokyo Metropolitan Government, Bureau of Construction. Standard Specification for Street Tree Maintenance (Green Space Management). April 2013.
– City of Yokohama, Road and Highway Bureau. Common Specification for Street Tree Maintenance Contracts. February 2026.
– Ministry of the Environment, Kyoto Gyoen. Gosho-sukashi (Japan Tourism Agency multilingual commentary database).
– American National Standards Institute. ANSI A300 (Part 1) Pruning (copy published by the City of Long Beach).
– 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