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  • Peter LaRue

Pruning Mature Trees


Last week, I posted a link on our FaceBook page to an article about Tree Topping and why it is bad for trees. I've been thinking about that article a lot, and looking at some improperly pruned trees as my family and I drive around, when my wife asked me the million dollar question. "How would you do it differently?". There are multiple approaches to tree pruning, and it all depends on the tree, its situation, and the clients needs. Tree Pruning can range from very basic clearance pruning, to a more elaborate deadwooding, thinning, subordination pruning. I don't want to dwell on the workers in our industry who arent properly informed or educated, or who will do any type of work on a tree for the right price. It's my mandate to provide as much information as possible to tree owners so that the trees in their care are safe, beautiful, and an asset to their property. (I've even heard it said that properly pruned and cared for trees and shrubs can add up to 20% on to your property value...). So, today I would like to introduce you to one style of pruning, the Reduction Prune. This is the most common type of risk-mitigation pruning that LaRueTree does. It involves selective weight reduction off the ends of select branches. Select is the key word here. Very very rarely do Arborists reduce the height of entire mature trees. Height reduction pruning is usually reserved for shrub containment and fruit tree pruning, and not large shade trees. So here it is, an introduction to Subordination/Reduction Pruning. Enjoy!

Reduction Pruning

Reduction is the selective removal of branches to decrease the height and/or spread of a tree. It requires the use of reduction cuts which remove larger branches back to smaller side branches.

Just being tall does not indicate that a tree is structurally weak and prone to storm damage!

Reduction is a method to reduce potential wind loading on large trees with structural defects. Reducing and thinning both decrease potential failure from snow loading. However, researchers are questioning the effectiveness of overall tree reduction. Depending on growth rates, the tree may simply regrow the removed branches in a few years. Current thought in reducing storm loading is that selective structural pruning on weak secondary trunks will be more effective than general tree reduction.

Important to note, not all trees can be reduced without predisposing the tree to decline and death!

Crown reducing requires the extensive use of reduction cuts which can predispose the branch/trunk to internal decay. On older trees showing stress or decline, heading cuts can accelerate decline and death.

Not every tree should be reduced!

In a proper reduction cut, the side branch pruned back to will be at least one-third the diameter of the trunk/parent branch removed. Under ANSI pruning standards, if the side branch is less than one-third, it is considered a heading cut, which is unacceptable.

It is very difficult to use crown reducing to permanently maintain a tree at a small size without causing tree decline. Ideally, trees were selected with adequate space for their mature size. Where size control is necessary, it is best to begin reduction pruning as the tree reaches acceptable size, rather than when the tree becomes overgrown.

In crown reducing, first visualize the new outer edge of the smaller canopy. Then prune the tree back to appropriate branch unions for a proper reduction cut or removal cut. Some branches will be left taller than the visualized outer edge while others will be cut back below the visualized canopy edge. Don’t make heading cuts and avoid rounding off the tree canopy.

Just because a tree is tall does not indicate that it is structurally unsound. Potential risk of failure should be evaluated by an experienced Arborist based on branching structure, branch union integrity, signs of internal decay, and previous damage.

Written specifications for reduction pruning should include the following:

  • Clarify the desired reduction in height/spread.

  • Specify criteria for reduction cuts. For example, “All cuts should be made on branches less than two inches in diameter. Diameter of the side branches pruned back to shall be at least one-third the diameter of the branch removed.”

  • Percentage of foliage to be removed. For example, “Pruning should not exceed 10% of the total live wood/foliage.”

Restoration

Restoration is the selective removal of branches, sprouts, and stubs from trees that have been damaged by improper pruning, vandalism, and storms. The objective is to restore the trees structure, form and appearance to the extent possible. Restoration generally requires annual pruning over a period of years.

Actual pruning procedures vary with the situation. When dealing with situations of excessive watersprouts, a rule to thumb is to remove one-third and reduce one-third with each annual pruning. Removing all of the watersprouts at one time often stimulates the growth of more watersprouts.

Pollarding

Pollarding is a training system that involves creating “heads” on secondary branches were small tertiary branches arise. The small tertiary branches are all removed back to the head every one to three years (depending on growth rates).

Pollarding started as a method to produce shoots for fuel, shelter, and products made from the young shoots. Today, it is used as an art form. Pollarding is common in some parts of Europe to keep tree small and shaped as living screens. Pollarding is not topping and should not be considered a routine method to keep large trees small. Due to annual labor involved, it is uncommon in Northern Alberta.

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Peter LaRue
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