Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: THURSDAY, March 5, 1992 TAG: 9203050062
SECTION: CURRENT PAGE: NRV-6 EDITION: NEW RIVER VALLEY
SOURCE: Joe Hunnings
DATELINE: LENGTH: Medium
In animal wounds, damaged cells are replaced by new, healthy cells. In plant wounds, the damaged area is covered over by callus tissue.
In other words, animal wounds heal, plant wounds seal.
Anything we can do to reduce the size of the wound and hasten the sealing process will reduce the chance of disease organisms invading the plant.
So let's look at the pruning techniques you should be using to maintain healthy trees and shrubs:
Twigs, small branches
When pruning twigs and small branches, always cut back to a vigorous bud or an intersecting branch.
When cutting back to a bud, choose a bud that is pointing in the direction you wish the new growth to take. Be sure not to leave a stub over the bud or cut too close to the bud.
When cutting back to an intersecting (lateral) branch, choose a branch that forms an angle of no more than 45 degrees with the branch to be removed.
Also, the branch that you cut back should have a diameter at least half that of the branch to be removed.
Make slanting cuts when removing limbs that grow upward; this prevents water from collecting in the cut and expedites healing.
Thick, heavy branches
Large branches should be removed flush with the collar at the base of the branch, not flush with the trunk.
The collar is an area of tissue that contains a chemically protective zone. It is the slight swelling at E to B in figure 1.
In the natural decay of a dead branch, when the decay advancing downward meets the internal protected zone, an area of very strong wood meets an area of very weak wood.
The branch then falls away at this point, leaving a small zone of decayed wood within the collar. The decay is stopped in the collar.
This is the natural shedding process when all goes according to nature's plan. When the collar is removed, the protective zone is removed, causing a serious trunk wound. Wood-decay fungi can then easily infect the trunk.
Even if the pruned branch is living, removal of the collar at the base still causes injury to the tree.
For more than a half-century, the recommendations for pruning have been to flush-cut and paint. These recommendations have no basis in scientific fact.
The flush-cut increases the tree injury, which the paint hides. The paint is primarily cosmetic, a psychological treatment for the person doing the pruning to show that he or she has done something "help" the tree.
In fact, paints or wound dressings may trap moisture and increase disease problems.
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.
On trees where the collar cannot be seen easily, the correct line of cut is found (as shown in illustration) as follows:
Draw a line in your mind from A to E.
Find point D, which is the end of the bark ridge H (this is the prominent ridge of raised bark that forms within the branch crotch).
The correct line of cut is found by making the angle EAD approximately the same as the angle EAB.
This may seem complex, but standing beside a living tree, there will be no difficulty in seeing the bark ridge and deciding on the position of the branch collar.
Joe Hunnings is the Virginia Cooperative Extension Service agent for agriculture in Christiansburg. If you have questions call the Montgomery County extension office at 382-5790.
by CNB