Anything we can do to reduce the size of the wound and to facilitate this sealing will reduce the chance of disease organisms invading the plant.

An article by D.W. Robinson in Chronica Horticulturae points out that standard text books have traditionally recommended that when a tree is pruned, branches should be cut flush to the trunk or to a larger branch. After pruning, the cut surface should be painted with some substance to prevent rot. When decay develops, the wound should be cleaned out and the cavity filled. Research in recent years has refuted these recommendations.

Dr. Alex Shigo of New Hampshire has shown that flush cuts remove the tree's natural defense system providing a starting point for many problems, such as decay, cavities, cracks, and cankers.

When a flush cut is made (as along line C to E in Figure 1), callus tissue often grows strongly on both sides of the wound giving what appears to be satisfactory healing. The serious damage done by flush cuts went unrecognized until Dr. Shigo cut a tree along its length with a chain saw. He found the strong callus growth, which is often assumed to mean that the wound has healed well, occurred only at the sides of the wound made by the flush cut and not at the top and bottom (Figure 1, Diagram 2). Pockets of decayed wood usually developed rapidly above and below this type of wound.

Importance of Branch Collars - By examining trees in nature, Dr. Shigo identified the branch collar as the area of wound healing. He showed that most wound problems can be avoided if living and dying branches are cut as closely as possible to the branch collars. The collar is usually seen as a small swelling where the branch meets the main stem or trunk. The branch collar is clearly seen as the slight swelling at E to B in Figure 1. Instead of flush cutting the branch from C to E, as recommended in the past, it is now established that the correct way to remove the limb would be to cut from B to A. It is essential to remove most of the branch first by stub-cutting. This is done by cutting upward first at F and then downward at G.

The small protrusion left when the branch is removed, by cutting from B to A at the branch collar, is not a focus for infection, as was previously thought. Instead the tree's natural branch-protection zone will form a strong defensive barrier in this region and prevent attack by bacteria, fungi, and pests.

It would be helpful if one could recommend a set angle for a proper cut. This is impossible because every tree species is different. In Figure 2, the pruning cuts A, B, C, and D are made at different angles, but all are proper cuts. In many trees, like European mountain ash Sorbus aucuparia, the branch collar is very obvious.

Wound Paints and Sealants - While most horticultural books recommend the use of paints or sealants on tree wounds, Dr. Shigo's research shows that such treatments have no long-term value. Provided pruning is done properly by cutting as closely as possible to the branch collar, there is no need to paint wounds regardless of their size.

Bad pruning is often the result of planting the wrong tree in the wrong place. When the tree starts to grow too large for its site, it is then mutilated. The best way to reduce the damage done by pruning is to select plants for their shape and ultimate size, as well as their aesthetic merits.

(Reference: "Tree Biology and Tree Management," by D.W. Robinson in Chronica Horticulturae 31(1). Originally published as "Minimizing Pruning Wounds," by Diane Relf, Extension Specialist, Consumer Horticulture, Virginia Tech, in The Virginia Gardener Newsletter, Volume 11, Number 1.)