透かし剪定とは──日本の庭師が「抜いて整える」剪定の考え方|樹木医が解説(Sukashi: Japanese Thinning Pruning)

English: Sukashi — The Thinning Principle at the Heart of Japanese Pruning

Sukashi (透かし), or thinning pruning, removes crowded branches inside the crown at their point of origin, letting light and air through while keeping the tree’s form. This article explains this basic technique of Japanese gardeners (niwashi and uekiya) as a Jumokui (tree doctor), a qualification certified by the Japan Greenery Research and Development Center, comparing it with pruning standards in Tokyo and Japan and with arboricultural research abroad.

This is the first article in a series introducing the pruning techniques of Japanese gardeners, one at a time. Each article is written in Japanese with a full English version, so that gardeners and arborists outside Japan can read it as well. We begin with sukashi, the foundation of all the other techniques.

What sukashi is: removing branches from within, not shearing the outside

When people think of pruning, many picture a hedge being sheared along its outer surface. Japanese gardeners also do another kind of work: reaching into the tree and removing branches from the inside. That is sukashi.

In Japanese public standards it is called eda-nuki sentei (branch removal pruning). The Tokyo Metropolitan Government’s Street Tree Maintenance Plan (December 2021) defines it as pruning that removes unnecessary branches that disturb the form, together with crowded branches, to adjust the balance and density of the foliage, carried out while keeping the framework of main, secondary and lateral branches. The second edition of the guide to preventing street tree failure by Japan’s National Institute for Land and Infrastructure Management (NILIM, 2019) lists branch removal pruning together with the term “eda-sukashi” (branch thinning). The street tree management manual of the Tokyo National Highway Office of the Ministry of Land, Infrastructure, Transport and Tourism (February 2022) states that heavy pruning should be stopped and that sukashi and shortening cuts should be the basis of pruning.

Tokyo’s standard specification for street tree maintenance (April 2013) describes branch removal pruning as “naka-sukashi” (thinning the interior) of overcrowded parts and cutting unnecessary branches at their point of origin. This is the essence of sukashi: rather than cutting a branch partway along its length, you go back to where it divides and remove it, one branch at a time.

The NILIM guide distinguishes “o-sukashi” (large thinning), in which thick branches from the tips of main branches to secondary branches are removed at their forks, from “chu-sukashi” (medium thinning), in which branches are removed at the forks of secondary branches. In our own work, we also include finer work on small twigs, and we choose the level of thinning according to the size of the tree and how crowded it is.

The National Gardens Association, which manages the Kyoto Gyoen National Garden, writes in an article on pine care that, following “gosho-sukashi” (the thinning style of the Imperial Palace gardens), the trees are shaped to their own natural form without showing a marked change before and after the work. Pruned, yet not looking pruned: I think this sums up what sukashi aims for.

Sukashi and karikomi: both are cuts, but in different places

Japanese gardens also have trees shaped by karikomi (shearing). Rounded shrubs (tamamono) and hedges are sheared with hedge shears to create a surface. Yokohama City’s common specification for street tree maintenance (February 2026) classifies shearing as a type of kirizume (shortening cut), in which branches are cut partway along their length.

The difference between shearing and sukashi lies in where the cut is made. Shearing cuts branches partway along and makes the surface denser with the new shoots that grow from there. Sukashi removes branches at their origin, so the outline of the crown hardly changes and only the density inside is reduced.

From our experience in the field, evergreens that have been sheared and nothing else for many years develop a thick outer layer of leaves; light no longer reaches the interior, and the inner branches die. It is not unusual to find a tree that looks lush from outside but is full of dead branches when you look inside.

Shearing is not wrong. It is essential for keeping the shape of trained trees. Tokyo’s specification also describes a sequence for medium and low trees in which the interior is thinned first and the outer surface is then sheared. We also combine interior thinning with shearing on trees whose shape is maintained by shearing.

Branches to remove: start with those that cause crowding

Which branches should be removed? Tokyo’s specification lists dead branches, weak branches, diseased or infested branches, obstructing branches and hazardous branches, as well as branches that are unnecessary for the form and growth of the tree: superfluous branches, root suckers, epicormic shoots, water sprouts, crossing branches, inner branches and upright branches. These overlap with what Japanese gardeners call “imi-eda” (branches to be avoided). The illustration marks four of them in red:

– a branch growing inward toward the trunk
– a branch crossing another branch
– one of two branches growing close together and parallel in the same direction
– a branch shooting straight up from the top of a thick limb (a water sprout)

Left in place, each of these rubs against other branches or competes for light, and becomes a source of crowding a few years later. How to identify imi-eda will be covered in detail in the next article of this series.

Where to cut: at the branch origin, leaving the branch collar

Because sukashi removes branches at their origin, the accuracy of the cut directly affects the health of the tree. Tokyo’s Street Tree Maintenance Plan states that the cut should be made just outside the branch collar, the swelling on the underside of the branch base, leaving the collar intact. This is because a protective zone that helps stop decay forms in the branch collar, and cutting it off may lead to decay. The plan also calls for avoiding stub cuts, as decay in a stub can reach the trunk.

If a large branch is cut in one go, its weight tears the bark below the branch base. The branch is first shortened further out to reduce its weight, and then cut again at the correct position (Tokyo’s Street Tree Maintenance Plan).

Inserting the pruning shears into the crown and removing branches one by one while looking at each branch base takes far more time than shearing from outside. In my view, the quality of sukashi is decided less by how much is cut than by which branches are left.

Why we thin: light, air, and the harm of over-thinning

NILIM’s guide to street tree regeneration (March 2016) lists the purposes of pruning as improving resistance to wind load, preventing dead branches from falling, removing obstructing branches such as those crossing a boundary, improving the landscape, and controlling pests and diseases; for pest and disease control, it says branches are thinned to improve air flow. Yokohama’s specification also includes securing sunlight and ventilation within the crown among the purposes of pruning.

Sukashi is also a way to reduce the load on trees exposed to strong winds. Tokyo’s plan states that when the crown is large relative to the root spread, or when branch ends are crowded, the crown should be reduced or the crowded branches thinned by branch removal pruning.

Over-thinning, however, is counterproductive. When inner twigs are stripped and leaves remain only at the branch tips, the result is called a “lion’s tail.” Tokyo’s plan states that the risk of breakage rises when the length of a branch is 40 or more times its diameter at the fork (L/D ≥ 40). The USDA Forest Service guide “How to Prune Trees” also notes that lion’s tailing leads to sunscald, excessive sprouting, weak branch structure and branch failure. Stripping the interior and leaving only tufts of leaves at the tips is not sukashi.

How much and when

There is also a guideline for how much to remove at one time. Tokyo’s plan and NILIM’s guide on street tree failure state that summer is the right season for thinning warm-climate evergreens, but because the trees are in leaf, the amount of foliage removed should be no more than one third of the whole.

As for timing, deciduous trees are basically pruned for structure in the dormant season (late autumn to early spring), with only light pruning in summer. Warm-climate evergreens such as camphor tree (Cinnamomum camphora) and Machilus thunbergii are sensitive to cold and may die back from cuts, so pruning in the coldest part of winter is avoided (Tokyo’s plan). NILIM’s guide lists pruning during the spring flush of new leaves, heavy pruning in summer, early autumn pruning, and pruning warm-climate broadleaf evergreens in the coldest season as things that must be avoided. Timing for each species will be covered one by one in the species articles of this series.

Japanese sukashi and Western arboriculture

The idea of sukashi overlaps closely with pruning in Western arboriculture. The USDA Forest Service guide “How to Prune Trees” (first published 1995, revised 2012) describes crown thinning as selectively removing branches to let light and air into the crown while keeping the existing structure and form. It recommends removing no more than one quarter of the living crown at a time, spreading larger removals over several years, which is an even more cautious guideline than Tokyo’s one third.

The cutting position is the same as well. The USDA guide also asks for cuts that do not damage the branch collar or the branch bark ridge, the raised ridge of bark on the upper side of the branch base. Behind this lies the research summarized in 1977 by Dr. Alex L. Shigo and colleagues at the USDA Forest Service on how a wounded tree confines decay within compartments (CODIT). The American pruning standard ANSI A300 also treats lion’s tailing as an unacceptable practice.

Sukashi itself has become a subject of research. A paper published by the Japanese Institute of Landscape Architecture compared the form of trees in a Japanese garden before and after sukashi pruning using 3D point cloud data (Yahagi et al., 2021).

One of the pillars of GREEN MANAGEMENT, the approach of our company, is the integration of scientific knowledge (arboriculture) and field experience. We decide where to cut according to how trees work, and choose which branches to leave with the eye that Japanese gardeners have passed down. I see sukashi as the technique in which these two meet within a single act of pruning.

Frequently asked questions

Q1. What is sukashi pruning?

A. It is pruning that removes crowded or unnecessary branches inside the crown at their origin, adjusting the density of the foliage while keeping the tree’s framework and form. In Japanese public standards it is called branch removal pruning (eda-nuki sentei), and Tokyo’s specification defines it as thinning overcrowded parts of the interior and cutting unnecessary branches at their origin.

Q2. How does sukashi differ from shearing (karikomi)?

A. Shearing cuts branches partway along to create an outer surface, while sukashi removes branches at their origin to reduce the density inside. Yokohama’s specification classifies shearing as a type of shortening cut. Tokyo’s specification describes thinning the interior of medium and low trees first and then shearing the outer surface.

Q3. How much can be thinned at one time?

A. Tokyo and NILIM documents state that when thinning warm-climate evergreens in summer, no more than one third of the foliage should be removed. The USDA Forest Service guide recommends removing no more than one quarter of the living crown at a time, spreading larger removals over several years.

Q4. When is the right time for sukashi?

A. For deciduous trees, the dormant season in winter (late autumn to early spring) is the basis, while summer is considered the right season for thinning warm-climate evergreens. Cold-sensitive evergreens such as camphor tree should not be pruned in the coldest part of winter. NILIM’s guide lists pruning during the spring flush, heavy summer pruning and early autumn pruning as things to avoid.

Q5. What happens if a tree is over-thinned?

A. If too many inner branches are removed and leaves remain only at the tips, the branches become lion’s tails and break more easily. Tokyo’s plan states that the risk of breakage rises when branch length is 40 or more times its diameter, and the USDA Forest Service guide also cites sunscald and excessive sprouting.

Sources

– Tokyo Metropolitan Government, Bureau of Construction, Parks and Green Spaces Division. Street Tree Maintenance Plan (Gairoju Iji Kanri Keikakusho). December 2021.
– Tokyo Metropolitan Government, Bureau of Construction. Standard Specification for Street Tree Maintenance (Green Space Management). April 2013.
– National Institute for Land and Infrastructure Management (NILIM). Guide to Preventing Street Tree Failure, 2nd edition. Technical Note No. 1059, February 2019.
– NILIM. Guide to Street Tree Regeneration. Technical Note No. 885, March 2016.
– MLIT Kanto Regional Development Bureau, Tokyo National Highway Office. Street Tree Management Manual. February 2022.
– Yokohama City Road Bureau. Common Specification for Street Tree Maintenance Contracts. February 2026.
– National Gardens Association, Kyoto Gyoen. “Pine needle plucking and pruning: toward the year’s end.” 19 December 2018.
– Bedker, P.J., O’Brien, J.G., Mielke, M.E. How to Prune Trees. USDA Forest Service, NA-FR-01-95, 1995 (revised 2012).
– Shigo, A.L., Marx, H.G. Compartmentalization of Decay in Trees. USDA Forest Service, Agriculture Information Bulletin 405, 1977.
– Tree Care Industry Association. ANSI A300 Part 1 – Pruning.
– Yahagi, T., Kato, A., Kato, T., Mitani, T. Changes in tree form by sukashi pruning in a Japanese garden using 3D point cloud data. Landscape Research Japan 84(5): 531–534, 2021.

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