Why a concave cutter is worth the money
Why a concave cutter is worth the money
Ordinary secateurs cut a branch flush, leaving a small flat stub. On a shrub nobody looks at, that is fine. On a bonsai it produces a permanent lump on the trunk, because a flat cut heals by rolling callus over the top and the callus stands proud.
A concave cutter exists to solve that one problem, and it is the reason it is the most expensive tool most people buy for this hobby.
What the hollow blade does
The blades of a concave cutter are curved so that the cut face is dished inwards, below the surface of the trunk.
The tree then heals by growing callus from the edges of that hollow towards the centre. Because the wound starts below the surface, the new tissue fills the depression and finishes level with the surrounding bark instead of standing above it.
Give it two or three seasons and a properly made concave cut becomes close to invisible. A flat cut in the same place is a bump for the life of the tree.
Concave, knob and spherical
Three related tools, and the differences matter.
Concave cutter — blades curved in one plane. For removing a branch flush with the trunk. The general-purpose one, and the one to buy first.
Knob cutter — blades curved in two planes, cutting a rounded hollow. For removing a stub or a knob that is already there, and for hollowing out a wound so it heals flat. Also the tool for taking a lump off a root base.
Spherical knob cutter — a deeper, rounder version of the same idea.
Most people buy a concave cutter first and add a knob cutter when they have a stub to deal with. A set containing both is usually better value than either alone.
Using one without tearing bark
Four things, and the first is the one everybody gets wrong.
- Do not twist. The blades are ground to meet precisely and are relatively thin. Twisting to finish a cut springs the joint out of alignment permanently, and a sprung cutter never cuts cleanly again. This is how most concave cutters die.
- Do not exceed the rated capacity. A cutter sized for half-inch branches will be damaged on an inch-thick one. Use a saw for anything beyond the rating and clean up with the cutter afterwards.
- Cut in one closing movement. Position, squeeze through, release. Partial cuts crush the tissue at the edge.
- Angle the blade to sit against the trunk, not perpendicular to the branch. The hollow should be aligned with the trunk surface, which is the whole point.
Steel and price
Two broad grades.
Carbon steel takes and holds a keener edge, and it rusts if you look at it wrongly. In south Florida humidity that is a real maintenance commitment, not a theoretical one.
Stainless resists corrosion and does not hold quite as fine an edge. For a humid climate and an occasional user, it is the sensible choice.
Where a maker states the steel — Yasugi steel and high-carbon steel are both stated on tools we stock — the listing repeats it. Where they say nothing, neither do we.
What you are paying for
The difference between a cheap concave cutter and a good one is almost entirely in the joint. A good cutter has blades ground to meet along their whole length and a pivot that holds them there under load.
A cheap one starts acceptable and develops a gap. Once the blades no longer meet, the tool crushes and tears instead of slicing, and a torn cut heals badly — which is the exact problem the tool was bought to avoid.
Test it by holding it up to a light closed. Any light between the blades means it will never cut cleanly.
After the cut
Cut paste — a putty sealing the wound — is standard practice in Japanese bonsai and debated elsewhere. The argument for it is that it keeps the cambium from drying at the edges, which is where healing starts. The argument against is that trees seal wounds perfectly well on their own.
In this climate, where a wound can dry hard in an afternoon and where fungal spores are plentiful, sealing large cuts is the more cautious choice. On small cuts it makes little difference.
Care, and it matters more here
Humidity in south Florida is the enemy of every tool on this bench.
Clean sap off after every session — sap is acidic and etches steel. Wipe dry. Then a light film of camellia oil or any light machine oil on the blades and into the joint.
Never leave a cutter outside overnight. Dew is enough to start rust on carbon steel by morning.
Store in a roll bag or a wooden box rather than loose in a drawer where the edges meet other metal.
Where to look
Concave cutters, knob cutters and trunk splitters are on the cutters and shears page. Sets that include a concave and a knob cutter with scissors and wire cutters are under bonsai tool sets, which is usually the better first purchase. Wire is at training wire, and the repotting tools at tweezers, rakes and repotting.