Pruning with Dr. Alex Shigo

Who is Alex Shigo? I would like to introduce all of you to a man, a scientist, a mover and a shaker within the industry of arboriculture. He is a man that changed my life 25 years ago. Dr. Shigo took the art of tree care out of grandpa’s wood-shed and put in into the laboratory where it belongs. For too long we have tortured, contorted and contrived our trees and shrubs into every form imaginable and filled them with every substance known to man. Too often we have done our trees a disservice rather than listening to them and responding accordingly. In 1907 John Davey, founder of the Davey Tree Expert Company, in his book, The Tree Doctor, pleaded with tree men to stop removing valuable, health giving tops from trees, claiming it was a disastrous, maiming practice from which trees would never recover. Here we are 97 years later doing better, while many professionals continue “topping” and “tipping” our beautiful, priceless shade trees.

Dr. Shigo began his studies of how woody plants respond to the wounding process after talking to an old black walnut veneer-log buyer who warned him against buying logs that had been “flush cut”. With peaked interest, Dr. Shigo had to know why. Thirty years and approximately 15,000 tree dissections later, Dr. Shigo published his findings, changing the world of tree care forever.

Dr. Shigo has blessed our industry with myriad books on woody plant chemistry and biology. He calls it Tree Biology. One in particular is my favorite. He starts his book with the 7 most important practices pertaining to the well-being of the urban tree. It is my desire to share these practices with you which may answer some common questions we hear all the time. The book is called: Tree Pruning, a Worldwide Photo Guide for the Proper Pruning of Trees. Author, Alex L. Shigo, copyright 1989 by Shigo and Tree Associates, Durham, New Hampshire 03824 USA. The ideas in this article are not mine, but I have used them since Dr. Shigo so graciously signed my copy of his book so many years ago. If you are a professional in the tree care industry and do not own this book, I recommend you buy it and consume it before making one more cut. As Dr. Shigo states in the book on page 180: "In time, the medical professors put down their text books, climbed down from their big chairs and touched the body. That was the beginning of modern medicine.  IT IS TIME TO DO THE SAME FOR TREES."

  1. Wrong tree, wrong place. If you have to prune your tree out of your way every year, you have the wrong tree in the wrong place. Know what size and shape of woody plant you need, and know what your choice will become. Dr. Shigo calls trees obligatory organisms. In other words, trees must get bigger every year, and when they do not they will soon die. Select trees that are appropriate for your site and if you are not sure what that is, GET HELP from a professional. Getting off on the right foot is critical in planting your new tree.
  2. Flush cuts. When you flush cut, (cut too close to the parent stem), you destroy a tree’s major defense system. We must never cut into a tree’s branch collar or recklessly remove the collar. The collar belongs to the parent stem, not the branch being removed. There are trunk collars and there are branch collars. They are different and perform different functions, even as much as they function together. When we understand the roles they play the clearer and more precise the function of pruning itself becomes.
  3. Stub cuts. “Stubs are food for organism that start rot and cankers”, quotes Dr. Shigo. Never leave living or dead stubs. Dr. Shigo calls stubs “sugar sticks” that provide an avenue where pathogens can enter the system. Removing all stubs allows the tree to perform closure and seal wounds caused by pruning. Pruning is an opportunity to do the right thing in the right place for the tree. Pruning that leaves a stub is deleterious to the tree while pruning stubs correctly at each branch collar is an exquisite procedure that will reap future rewards.
  4. Topping and tipping large trees. This is not an article for Nike. Instead, JUST DON’T DO IT! Could it get any simpler? Topping is difficult, dangerous for those performing the work, time consuming and kills trees. Wouldn’t you think those who top trees would prefer not to, if given a choice? Learn the difference between “sub-dominating” young trees whose systems are all dynamic mass rather that topping older trees going through the geriatric portion of their lives. Pruning older trees verses younger trees is a discussion in itself that must be understood. The two organisms are not the same and should be treated accordingly. Older trees cannot react to wounding as can younger trees. This is what our pruning should reflect. Tipping and topping does not enhance safety, health or the dignity of earth’s largest and most complex organisms. Let’s prune our trees with respect and assist them in becoming what they could most naturally.
  5. Over pruning. The point Dr. Shigo makes is one I learned later in my career. An older and wiser consulting arborist once explained the destruction of over pruning. He said when we over prune older shade trees we are simply preparing them for removal. Over pruning strips precious photosynthetic material from the tree, not allowing it to produce the food it needs for survival. Again, tipping and topping create this situation as does removing too much inner growth from the crown. Over pruning can lead to elevating tree risk, root problems, excessive sprouting and invites unwanted insects, including borers to attack its system. Pruning generally includes well placed cuts throughout the entire tree. Specifications that call for a 10% crown thinning include the trees’ entire crown.
  6. Improper Pollarding. In as much as this practice is more often done on old world trees, the United States does practice it on a smaller scale as well. This may very well be an answer to topping or tipping but as Dr. Shigo stresses; it must be done by professionals who adhere to the strict rules of Pollarding. Pollarding, when appropriately applied, is not tree mutilation. You must start with a tree that lends itself to the practice, establish the desired framework when the tree is young and small, remove sprouts every year and never cut into the pollard heads. Pollarding is the careful reduction of a tree’s crown to the exact same point each and every year. This will cause a swelling at the original cutting point, where after only water sprouts will be removed annually to their exact point of origination. Great care should be taken during this process, and should take place under the supervision of certified arborists.
  7. Wound Dressing. The question asked more than all others is: do I paint or not paint my pruning cuts? Dr. Shigo seemed to research this topic quite diligently. I will simply touch upon his findings. I urge you to dig deeper in his writings if this interests you. He says wound dressings do not stop decay. He warns against applying dressing over rotted or infected wood. Some dressing actually stimulates rot. In addition, I remember Dr. Shigo discussing wound closure in one of his labs I attended. After a cut is made the phenomenon of discoloration occurs. This serves as a reaction defense to stall decay entering the wound site. His research, if my memory serves me correctly, showed that some dressings slowed this discoloration process or narrowed the “strands of discoloration” making entry into the wound more accessible. All in all we are surely blessed that we do not have to carry “Paint Pots” any more.

Again, everything we discussed here was straight from Dr. Alex Shigo’s writings and almost fifty years of research. Twenty-five years ago he changed my life and career by giving what I was doing a great deal of meaning, structure and foundation. At The Pruning School we preach what he taught us with a touch of Monet in every cut. Learn your tree biology and practice your artwork. Pruning will help create your own beautiful little section of an already bountiful natural world.

Copyright 2005 by Peter Deahl. All rights reserved.

 
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