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Concave Cutter: An Indispensable Bonsai Tool

July 14, 2022Beginners, General, Tools & Pots

concave cutter

In this article, we’ll unpack details about the concave cutter, explore buying considerations, discuss how to use it, and cover how it differs from similar tools.

Ask any bonsai enthusiast what they consider to be the most important bonsai tool they own, and chances are good that the answer will be the concave cutter. 

The concave cutter is a tool unique to the practice of bonsai, and for which there is really no substitute. It resembles a plier, but has oval-shaped, slightly rounded, sloping jaws with sharp edges. This tool removes branches flush at the trunk, creating an elongated, tapered, slightly concave cut which heals smoothly and evenly with the surrounding bark. 

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Selecting a Concave Cutter

If you don’t already own a concave cutter, choosing one boils down to the questions of size, material and quality.

Buy the right size concave cutter. 

When choosing a cutter, start with the size that best fits your first trees, then add bigger or smaller tools as your collection grows. Those new to bonsai generally start with smaller trees, for which cutters sized 6.5″-7” work well. Large trees might require a tool sized 11” or more.

When deciding what size to buy, an 8″ concave cutter probably hits the “sweet spot” size-wise. Perfect for medium-sized trees, this size could be used with care on smaller ones as well.

 Avoid trying to remove thick branches in one cut with an undersized cutter, as this can damage the tool. However, you may be able to use a smaller tool to remove the branch safely by making two or more smaller cuts.

Stainless concave cutter vs carbon steel  

While stainless steel tools cost more and have a classier look, they are not necessarily superior to carbon steel. Stainless resists corrosion, however, many hobbyists prefer carbon steel tools because they are easier to keep sharp. Carbon steel also tends to be less expensive than stainless. Both types work well, so the decision largely depends on personal preference. Look for well-reviewed tools such as these 6.5″ stainless steel or carbon steel models.

Don’t forget the oil.  

Every bonsai practitioner needs a small bottle of quality lubricating oil for proper tool care. Japanese bonsai practitioners traditionally use camellia oil, however many modern hobbyists opt for machine oil or even WD-40. No matter what oil you choose, make it a practice to clean your tools after each use and oil them several times a year.

Using the Concave Cutter

These simple tips will help you use the concave cutter easily and effectively to shape your trees:

Disinfect the concave cutter. 

Its good practice to always disinfect the cutting edges of the tool before and after working on a tree. Since you will be making wounds in the tree with the concave cutter, a dirty tool could introduce disease or fungus that could harm your bonsai. If you are working on multiple trees in one session, be sure to disinfect all cutting tools before moving to the next tree. I keep alcohol pads in my bonsai tool kit to give my cutting implements a quick wipe-down as needed.

Cut in the direction of growth. 

Always position the concave cutter tool so that the wound you create in the trunk or branch is parallel to the direction of growth. For example, if you are removing a branch from a trunk growing straight up, the wound should be vertical in relation to the ground.

Why is this important?  Tree respond to wounds by sealing off the damaged area, then growing a layer of living callus tissue over top of it. The new layer grows in from the sides – not top to bottom – in thick, elongated rolls.

The bowl-shaped wound left by the concave cutter helps facilitate the growth process of the new tissue. Also, the depression in the middle of the wound from the concave cut accommodates the extra thickness of the new growth. This eliminates or reduces scarring and produces a smoother final look that blends with the surrounding bark.

Use cut paste. 

Cut paste is a compound used to cover tree wounds after trimming. It promotes faster healing by sealing the wound to retain moisture and prevent infection. Dark in color, it blends with the bark of the tree for a more aesthetic appearance after trimming work. While cut paste is not critical to the healing of tree wounds, using it is a practice that most bonsai hobbyists follow to ensure the optimum health of their trees.

Similar Tools

Those new to the world of bonsai can easily be confused about the purposes of various tools and the differences between them. Regular concave cutters are similar to a couple of other common tools, such as the knob cutter. 

Knob Cutter vs Concave Cutter

The knob cutter’s jaws are rounded and fit together straight, not angled like the concave cutter. While the concave cutter is perfect for leaving a slightly concave wound, the knob cutter’s design allows for deeper bowl-shaped cuts.

The choice of which of the two tools to use often depends on the type of tree and the work that needs to be done. The concave cutter is sleeker and easier to maneuver in dense foliage. It also works better for younger trees and those with thinner bark. 

concave and knob cutters
Concave Branch Cutters (left) and Knob Cutters (right)

Hobbyists often use the knob cutter to clean up and smooth out the area around larger cuts to promote smooth wound healing. They also use the knob cutter to trim off thicker branches, to perform sculpting work on tree bark, and to cut through large, unwanted roots with ease.  

Cutter Confusion

Be aware that it’s not uncommon to find bonsai practitioners and tool retailers who refer to the knob cutter as a “concave cutter”, while calling the tool highlighted in this article a “branch cutter”. Since both tools produce concave wounds, it seems understandable, albeit confusing, that the name could be used interchangeably.

To make matters even more complicated, there is a third tool that is something of a cross between the concave cutter and the knob cutter.  This hybrid “rounded” or “spherical” concave cutter cuts a bit deeper than the regular cutter and can handle somewhat thicker branches. Many hobbyists find it a handy addition to their tool kits, but it definitely qualifies as a “down the road” acquisition. Some practitioners opt to skip the hybrid tool, preferring to work on trees with the concave branch cutter and knob cutter combination.

The bottom line? No matter which of the three tools you might choose to use, you’ll be producing a concave cut on the tree. The difference lies in how deep you need the cut to be to achieve your goal as well as the characteristics of the particular tree you need to cut. Chances are, the concave branch cutter will be the tool you’ll reach for most often.

Interested in learning more about bonsai tools? Check out our articles on the five basic tools and the pros and cons of multi-tool kits.

Most bonsai hobbyists consider the concave cutter the most important implement in their tool kit. This article highlighted the importance of the concave cutter, considerations when buying one, how to use and care for the implement, and how it compares to related tools. 

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