Flush Pruning Cuts, the Inside Story
By John M. Phillips

In a previous article, it was reported that trees pruned with “flush” cuts will produce better quality wood than those whose branches are pruned outside the branch collar.  While this may happen once in awhile, most of the time the opposite is true.  Flush cuts can, and usually do, lead to a myriad of defects, including radial cracks, circumferential cracks, discolored wood and wood decay.   None of these defects enhance wood quality.

Because flush pruning wounds can stimulate abundant callus, it follows that woundwood will be produced quicker. This enables the wound to close faster with subsequent production of wood sooner than would be done with the other type of cut.

On small diameter cuts (less than two inches across), this may be so.  With larger cuts, the possibility for the callus to curl in on itself increases.  We call this the “ram’s horn” effect. When this happens, the tree can wound itself creating radial cracks that can lead to wood fracturing later in the tree’s life.

But even with the small cuts, the potential for defect exists.  Branches have a core attachment into trunks. This core helps to contain any infection to a minimum volume of tissue.  When branches are pruned too flush, this protection feature is violated allowing the succession of tree decomposers deeper into the tree than if the pruning cut is made outside the branch collar or the branch bark ridge.   Flush pruning cuts wound the trunk and the branch tissues, rather than just the branch, increasing the chances for loss of sound wood in the trunk. 

The tree can respond to trunk wounds by building walls that resist the spread of infection.  One of these walls follows the current growth increment circumferentially. This is the tree’s attempt to contain the infection so it will not spread to wood produced after the time of wounding. This wall often leads to circumferential cracks, commonly know as “ring shake”.

This process of containment is called compartmentalization and is far more critical than the rate of wound closure. There are many factors that affect this process, the least of which is an adequate supply of energy.

So while it may appear on the surface that closed wounds make for more wood, the internal effects of flush cutting are what matter the most.  In order to see what happens on the inside of trees, it is necessary to do longitudinal dissections, before and after the treatment. This work has been done extensively by Dr. Alex Shigo.  He has cut over 15, 000 trees with a chainsaw and his experiments are documented in over 270 publications.  His work has been brought together in several books that can be previewed at ShigoandTrees.com.

(This was written for Thembi Borras’ Tree Tips newspaper column).

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