透かし剪定をもう一歩深く──植木職人が見ている「切り替え」「上下の加減」「手を止めるとき」|樹木医が図解(Sukashi in Depth: What a Japanese Gardener Watches While Thinning)

English: Sukashi in Depth — What a Japanese Gardener Watches While Thinning

Sukashi, Japanese thinning, is more than removing crowded branches. It means switching long branch tips to shorter ones to keep the crown a size smaller, leaving the top light and the bottom full, and protecting the softness of the branch tips. This illustrated article explains the three things a Japanese gardener (niwashi or uekiya) watches during thinning, and when to stop, with reference to Japanese public pruning standards.

This is a special edition of our series on the pruning techniques of Japanese gardeners. Our first article on sukashi drew many responses, so here we go one step deeper. The basics, which branches to remove and where to cut, are in earlier articles (eda-nuki and the three levels of sukashi). This time I try to write what goes on in a gardener’s head while the hands are working.

Sukashi is a series of switches

Thinning is often thought of as taking out crowded branches from inside. Much of what actually happens up in the tree is switching branch tips: replacing a tip that has grown long and outward with a shorter tip coming from the fork just behind it. The longer branch is removed at the fork, and the shorter one is kept as the new tip. This is called kirikaeshi (cutting back to a lateral), and it is the basis of pruning for both street trees and garden trees (Saitama Prefecture, Street Tree Pruning Manual).

Each switch is a small job, but repeated across the whole crown, the outline shrinks by a size while keeping its original shape. The next year, buds on the remaining tips grow and bring the crown back to about its former size. How far back to go is decided by allowing for that year of growth. This is where sukashi differs from karikomi, shearing that traces the outline with a blade.

The tip to switch to is one that faces outward. If you keep an inward-facing branch, it grows into the crown the next year and becomes a new source of crowding. Another thing often said among Japanese gardeners: do not keep cutting in the same place. It is listed among the basics of pruning (Japan Greenery Research and Development Center), and repeated heading cuts at the same point make a knuckle there (Saitama Prefecture). When you find last year’s cut, choose the fork just before or just beyond it this year.

Light at the top, full at the bottom: keep branches in the ratio 1:2:3

Trees naturally grow most vigorously at the top, where the sun is strongest, while lower branches weaken in the shade above. This is apical dominance. If you thin the whole tree evenly without thinking, the top soon crowds again and the bottom grows thinner year by year.

So we thin the top more strongly and the bottom more lightly. For street trees, the guide is to divide the crown into upper, middle and lower thirds and keep branches in the ratio 1:2:3 from top to bottom (Saitama Prefecture, citing the Japan Landscape Contractors Association’s Street Tree Pruning Handbook). The tree gains a settled form with its weight low, and because the top catches less wind, it also makes sense for stability.

The idea is the same for garden trees. From our experience, on the ladder you tend to be absorbed by the branches in front of you and thin the top and bottom equally. After finishing the upper third, climb down once and compare the density of the three tiers before moving to the middle.

Once lower branches die back, they rarely return (Saitama Prefecture). Lightening the top is less for the top’s sake than to send light to the lower branches.

Keeping the branch tips soft

On a well-thinned tree, branches become thinner step by step from base to tip, and the tips look soft. Keeping to the basics of pruning preserves this natural branch form (Saitama Prefecture). Cut a branch off partway instead, and thin, straight shoots sprout in a bundle around the cut, giving the tip a stiff, broom-like look.

Repeated heading cuts at the same point turn the cut into a knuckle (sentei-kobu). The knuckle itself is healthy tissue, so rather than cutting it off, keep one or two well-placed shoots from it and remove the rest at their origins (Saitama Prefecture). Over several years the remaining shoot takes over as the branch tip.

Among Japanese gardeners it is said that skill in thinning shows at the branch tips. Two trees may look alike from a distance, but up close the tips reveal where the cuts were made. If the cuts are hidden in the shadow of the forks and the tips end thin and pointed, no one notices the tree has been cut.

Thinning differs by species

Zelkova (keyaki) is broom-shaped, with several branches rising in a fan from low on the trunk. Even when reducing it, you switch to shorter branches at forks among the limbs spreading in all directions, and never cut old branches off partway (Saitama Prefecture). Cherry is regarded as a species into which decay enters easily from cuts (Saitama Prefecture). In our own work, large branches are left uncut wherever possible and the tree is shaped by thinning finer branches.

Evergreens are covered in leaves all year, and their framework cannot be seen from outside. On each branch, you remove at their origin the branches heading inward, those overlapping others, and those hanging down, and keep those heading outward and sideways. For street trees, thinning is best kept to the level of chu-sukashi, using medium branches rather than large ones (Saitama Prefecture).

Major work on evergreens is best around early to mid April, as new buds begin to move. From summer into early autumn, as new shoots harden, it is kept to lightly shortening overgrown shoots and thinning a little (Saitama Prefecture). Heavy thinning late in autumn leaves the remaining leaves easily damaged by cold.

Step inside the crown and look outward

When thinning evergreens, what we value is putting our body inside the crown. From outside, you see only the surface of the leaves. Put your head into the tree from the ladder, look up from beside the trunk toward the bright outside, and the branches blocking the light appear as overlapping shadows.

From our experience, branches to remove are decided one at a time. Taking out one branch changes how the next one looks, and a branch you meant to remove may no longer need to go. When in doubt, leave it. A branch once cut cannot be put back. After removing a few, gently shake the branches to feel the density with your hand, and climb down from time to time to look at the whole tree.

When to stop: reading the finish by light and shade

What makes thinning difficult is knowing when to stop. For evergreen oaks, the guide is to thin until the sky shows slightly through (Tokyo Metropolitan Government, Street Tree Maintenance Plan). From our experience, we also look at the shade beneath the tree. Before thinning, one dark shadow falls on the ground. With the right amount of thinning, small spots of komorebi, sunlight filtering through leaves, scatter within it.

If the spots of light grow too large, too much has been removed. Over-thinning that leaves leaves only at the branch tips and empties the inside is called lion’s tailing abroad, and is treated as pruning to avoid (ANSI A300).

The real check comes the next year. From our experience, if thin shoots have sprouted in a bundle from a cut, that is a sign it was cut too far partway. If single, well-behaved shoots grow from the forks, last year’s thinning has been accepted by the tree. Seeing that answer each year, you decide where to switch next. I believe thinning is not work finished in one go, but handwork continued over years with the same tree.

How sukashi differs from crown thinning: for readers outside Japan

In English, sukashi corresponds roughly to crown thinning, and switching tips to the reduction cut (cutting a branch back to one that can take over its role). Western standards express the work in numbers, such as the proportion of branches removed or the ratio of branch thickness (ANSI A300; USDA Forest Service). Japanese gardeners have judged the amount by visible cues: the gradation from top to bottom, the softness of the tips, and how the dappled light falls.

Frequently Asked Questions (Q&A)

Q1. What is “switching” in sukashi?

A. It is a cut that replaces a branch tip grown long and outward with a shorter tip from the fork just behind it. The longer branch is removed at the fork and the shorter one is kept as the new tip. Repeated across the whole crown, it makes the tree a size smaller while keeping the same shape.

Q2. Why thin the top and bottom by different amounts?

A. Because trees grow most vigorously at the top while lower branches weaken in the shade. For street trees, the guide is to thin the top more strongly and the bottom more lightly, keeping branches in the ratio 1:2:3 from top to bottom. Once lower branches die back, they rarely return.

Q3. What should be done with a branch that has a pruning knuckle?

A. Do not cut off the knuckle itself. Keep one or two well-placed shoots from it and remove the rest at their origins. Over several years the kept shoot takes over as the tip. To avoid knuckles, do not keep cutting in the same place.

Q4. When should thinning stop?

A. For evergreen oaks, the guide is until the sky shows slightly through. The right amount is when small spots of sunlight scatter in the shade beneath the tree. If the spots grow too large, too much has been removed.

Q5. How can you tell whether the thinning was good?

A. From the next year’s growth. If single, well-behaved shoots grow from the forks, the thinning has been accepted. If thin shoots sprout in a bundle from a cut, it was cut too far partway.

Sources

– Saitama Prefecture, Road Environment Division. Street Tree Pruning Manual. December 2020 (citing the Japan Landscape Contractors Association, Street Tree Pruning Handbook, and others).
– Tokyo Metropolitan Government, Bureau of Construction. Street Tree Maintenance Plan. December 2021.
– Japan Greenery Research and Development Center. Pruning Techniques for Landscape Trees.
– 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