Chapter 15 Branch Pruning Physiology

This essay is a compilation of two articles written in 2000 and 2003 by Erv Evans, NCCE Consumer Horticulturist.

15.1 A Tree’s Wound Response

Woody plants heal injuries by producing rolling sheets of callus that gradually grow over the wound. Callus is made up of a generalized type of cell found in all plants. You can think of them as the plant version of embryonic stem cells. The big difference is, plants have a large number of these cells that stand ready to migrate and form new structures as soon as tissues are damaged, or the plant gets a signal to grow.

Callus is not just on the outside of the tree either. Internally the callus cells wall off the damaged tissue from healthy wood. These responses to wounding occur more rapidly just prior to the onset of new growth in the spring and just after maximum leaf expansion in June. It is just my opinion, but it seems more than coincidence that these are the same times a tree is most likely to lose branches “in the wild”; during late season ice and snow storms, and at the start of thunderstorm season.

Trees form four types of walls to compartmentalize the area thus preventing the spread of decay organisms (see figure below). Wall 1 is formed by callus plugging the vertical vessels of the vascular system above and below the injury. This is the weakest wall but can slow the vertical spread of decay. Wall 2 is formed boundary of a growth ring. It is a stronger barrier and provides some resistance to the spread of decay inward towards the center of the trunk. Looking in at the cross–section of a trunk, we can see it is also split into wedge–shapted compartments (like a pie) by the rays. Rays provide resistance to lateral spread (Wall 3) by way of a maze of physical obstacles and a chemical barrier. Wall 4 is formed by callus cells growing in from the cambium after an injury. This is the strongest of the four walls. Internally, it separates the wood present at the time of injury from overlying new wood formed after the injury. Externally, the wood that develops around the injury eventually covers it entirely. The decay or injury remains but now is sealed off and does not increase in size so long as the callus walls are intact.

When we cut off a branch, callus again rolls over the wound, just like in a wound on the trunk. The major difference is that callus moves in radially from the cambium to cover the end of the stump. Internal walls still block loss of water out from the stump, and prevent disease organisms from moving inward along the xylem and phloem.

Walling off a large injury on the trunk. An injured area can be sealed off by the rolling cambium, but the dead tissue remains. As long as the walls remain intact, microorganisms in the wound cannot spread to the rest of the tree.

15.2 How Does This Affect Pruning Technique?

The traditional practice in horticulture has been to cut tree branches flush with the tree trunk (just as we do in bonsai). However, as Figure 2 shows, flush cutting a branch along the imaginary line of the trunk breaks through the branch collar where groups of callus–forming cells reside, AND injures the main trunk. Using concave cutters to excavate into the trunk will make the wound even worse. The result is a MUCH larger wound that takes longer to repair. Meanwhile, decay organisms have additional time to infect the main trunk of the tree. The current recommended practice from a horticultural standpoint is to prune back large branches to just outside their branch collar (small sucker branches and bud sprouts can still be cut flush). There is an easy visual cue to use: properly pruned branches will have a circular face of cut wood, and the in–rolling cambium will form a circle closing in around the wound. If the branch was cut too close to the trunk, the closure will be oval or distorted.

Positions for pruning cuts. A flush cut will remove the area marked “A” that contains the cambium callus cells needed for repairing the wound. Using a concave knob cutter incorrectly removes wood from the area marked “B” and interferes with sealing of the trunk wound. The preferred cut line is the one marked “C.”

15.3 But, But, But…What About Tradition?

Sorry, but despite all we were taught, research indicates the Japanese got this one wrong. It is safer for the tree to cut a branch further out, beyond the collar. This does not mean you cannot continue to cut into the trunk as we were all taught. Just realize that you are stressing the tree beyond what it normally tolerates. So do not be afraid to leave the branch collar in place if the stump will be towards the back of a tree.

One counter–argument is that most of us use cut paste or other sealants. The general consensus among arborists though is that sealing a wound on a tree is counterproductive. While sealants can stimulate callus growth, most also trap excess moisture and decay–causing microorganisms. Taken together, the benefits and problems seem about equal. Don’t toss out the sealants just yet though. The colored pastes do hide open wounds nicely until they seal over on their own, and retard black mildew stains on the open ends of stumps.

If they are not beneficial to the trees, where did the Japanese tradition of concave cuts originate? It is not horticultural but aesthetic. One of the goals in traditional Japanese styles is to “leave no evidence of the artist.” Clean, unbroken trunk and branch surfaces are a priority, and these are produced by concave cutters PLUS careful follow-up attention to ensure the cambium keeps rolling in to conceal all such cuts.

So what to do? I am not trying to overturn hundreds of years of styling tradition. After all, bonsai artists have been concave cutting branches successfully since the late 1800s. However, I do think that the practice of cutting branches flush or concave rather than just beyond the branch collar is more about aesthetic ideals, not horticultural best practices. As a distinctly American style of bonsai continues to develop, I hope that we will look critically at the training methods we use, and come to place equal emphasis on both plant health and aesthetics.