Concave cuts and cut paste: the two-thirds of wound care that people skip

Most wound problems in bonsai are not caused by failing to seal a cut. They are caused by making the cut in the wrong place, at the wrong angle, or at the wrong time of year. Cut paste helps — but it cannot rescue a bad cut.

Concave versus flush: why the shape of the cut decides the scar

A branch cut flush with the trunk leaves a flat circular plane. The callus forms around its rim and grows inward, but because the surface is flat, the new wood grows level with the bark rather than over it, leaving a permanent disc of different texture. A concave cut — made with a curved cutter that scoops the wound into a shallow dish — allows the callus to roll inward and over, sealing the wound flush with the surrounding bark. On a 20 mm branch the difference at year three is a visible disc versus a faint line.

The branch collar, and cutting outside it

Where a branch meets a trunk there is a slight swelling of tissue, the branch collar, which contains the tree's own chemical defences against decay. Cutting into the collar removes that defence and lets decay into the trunk. Cutting too far outside it leaves a stub that dies back and takes two extra seasons to heal. The correct cut is just outside the collar, at the point where the branch begins to swell into the trunk, and the angle follows the collar rather than being perpendicular to the branch.

When paste helps and when it does not

Cut paste main job is to stop the wound surface from drying and cracking, which slows callus formation. On a vigorous deciduous tree making a cut in spring, paste makes a measurable difference of about one season on a 20 mm wound. On a conifer in dormancy, sap pressure is low and a sealed wound can trap moisture against the wood; many growers leave conifer cuts unsealed in winter and seal them in the growing season instead. On deadwood, paste should never be used.

The bigger decisions that paste cannot fix

Three decisions matter more than any seal. First, the size of the cut relative to the tree: a 30 mm wound on a 20 mm trunk will never heal cleanly and is better made as a deadwood feature or avoided altogether. Second, the number of cuts in one session: more than about four large cuts on a small tree in a single season leaves it unable to close any of them. Third, the timing: cuts made in early spring, as growth starts, heal fastest, while cuts made in late summer on a deciduous tree often sit open until the following June.