“Make a concavity when finishing pruning cuts.”

This is what we’re taught. And, for most part, it’s not bad advice. We use ball cutters or a chisel to hollow out an area where a branch was.

The callus and resulting woundwood that grows over the cut has some meat to it, so a depression makes room for that and allows the area to cover over without becoming a bump.

One situation calls for a different approach. A very large wound. 

Our example tree, a Styrax japonicus, Japanese Snowbell. This spring we took off an air-layer, which left a large, trunk-sized cut. The bottom half of the tree still had promise, which is not always the case. But we still had this huge cut. 

Our team—Arturo stabilizing and Ted braving the surgery. Using a fine-toothed Japanese pull saw helps with such exacting work.

Notice that our cut is not a simple straight cut. It has two facets. The rise between the two will be smoothed over. A cut line in chalk is still visible.

The main thing is that this is not a flat cut, but one with a convex surface. 

With a very large wound, of 2” or more, outward curvature gives a future advantage. If we did a concavity here, the wound might still close, but we’d have a depression. For a wound 1” and smaller, a depression might be perfect for the depth of the woundwood creation. 

Trees make thin or thick callous. Apple is thick. Pine is thick. Satsuki is thin. There is enough variation that they become variables along with the size of the wound in determining what shape our surgery arrives at—concave, flat, or convex.

Small wounds are usually concave. Large might be convex, like this Styrax wound. We use Kirikuchi for wounds like this, for its strong activation of callus growth giving a better chance at closing a big wound.

The final tree. It has a long way to go. A big wound like this on a Satsuki azalea might need another approach, like deadwood, for they rarely close a wound this big. But Styrax are likely to. Eventually, and with some cajoling.

The shoots above the wound have also been left to run. We may let them run a year or two to help close the wound. The design of the tree has a long way to go as well, so there’s no rush there.

We have a little tree from the air-layer that is fun, and now have another possible bonsai from the lower part. From one, two—a good spring.