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Why Prune?

This is the $100,000 cost saving question.

Maureen Sexsmith-West


Certified Arborist PR-4600A


Once you cut if off – you can’t glue it back on. Pruning cuts should be made thoughtfully and deliberately.

Why and what you prune can increase or decrease your maintenance requirements and ultimately the cost. Concepts of pruning have evolved for centuries through study of past practices and the affects they have on the longevity of trees. Pruning standards through scientific study are published (ANSI A300) and are used world wide in the preservation and care of trees by professional, trained arborists. The person you hire should follow these guidelines and will not do anything to your tree to adversely affect its health – which in turn will cost your money. The ISA has available Best Management Practices on this and many other topics.

Below are simplified explanations. I welcome the opportunity to provide on-site classes for those who want someone to tell you, show you and supervise you as you practice on your own trees and shrubs.

We ask – If trees are not pruned in nature, why in an urban setting? Quite often the location, the species, tree age and growth rate, and the number of competing plants will greatly influence your need for pruning. Some trees should be pruned annually (such as apples), most should be routinely kept free of dead wood, and some are season specific (Elm).

Here are the reasons to prune. What you need to accomplish is:

To maintain health and vigor

To reduce risk of failure

To manage and direct growth

To train a young tree

To provide clearance

To enhance fruit/flowering production

To improve a view or aesthetics

To affect privacy, noise, shade values and wind resistance

To restructure

Before you make cuts, it is important to understand WHY you are pruning and what you want to achieve. For the best results you need to know what species of tree you are working on. Understanding that birches/maples are best pruned in late July or early August, that there is a ban on Elms in Alberta is important. Understanding what is normal for the particular species of tree or shrub, how much it typically grows by examining the twig elongation for the past two or three years all influence the success of your pruning decisions. Understand that removing more than 25% of live tissue will subject your tree to stress. All pruning cuts should be made to the branch collar and a suitable sized lateral. It is important to have a vision for the future. Here is the most effective order of pruning:

1. DEAD, DISEASED OR BROKEN BRANCHES: Insects, fungus, bacteria all enjoy the benefits of decomposing wood whether for food, egg laying sites or hiding spots. When their population gets high enough, they can spread into otherwise healthy parts of the tree.

2. HAZARDS: In order to identify ALL hazards it is necessary to be IN the tree. Many flaws are not visible from the ground. Large dead tops (referred to as stags) should be eliminated. Weak or separating branches should be removed as early in the tree’s life as possible. Just because a tree is tall does not make it a hazard. Cracks, cavities and other structural weaknesses should be evaluated by a Certified Arborist on their potential to result in a limb or tree failure.

3. SIZE MANAGEMENT: Trees get big (some very big) and that is okay. What matters is that they are growing in their natural form there are no pre-existing defect or hazards that would make it unsafe. We see far too often deciduous and spruce trees topped to reduce their size – this actually causes a great deal of stress and the reactionary re-growth presents a more likely liability than the tree did in the first place (For more reading see Post from November 14, 2012). ANY tree can be managed to some extent through a pruning technique called CROWN REDUCTION. The canopy can be reduced in height and volume through removal of carefully selected branches. Pruning is made to a suitable lateral lower in position that can handle the movement of vascular fluids and maintain the role of leader. It preserves the basic form of the tree – allowing for continued enjoyment of shade, habitat and wind break properties.


Avoid Rounding Over of Trees

4. STRUCTURE AND FORM: As branches add new wood each year they require room to grow – they don’t move up the tree – they just get thicker. Prevent damage by eliminating crossing and rubbing limbs. Each species has a typical distance between limbs. The larger the tree the greater the spacing as they age. This does not mean removing all but those branches as they provide food for the tree – the smaller, temporary branches should be culled out as the tree matures. Strive for both vertical and horizontal branch spacing. A method I would discourage is the ‘rounding over’ of deciduous trees – this technique is similar to topping – only a lot more stub cuts.

5. DIRECTIONAL PRUNING: In an urban setting we see trees planted in the worst possible spot – cedars which grow 20 feet directly under the roof eave, trees too close to walks, fences and drives or trees under service lines. What do you do as the tree grows? Short of picking it up and moving it over, directional pruning can solve a lot of long term problems. You can encourage branches to grow over, around and along anything over time.


Photo Credit Unknown

My first suggestion is make better choices when you are landscaping. The proper pruning cut will direct growth away from structures or use areas. This technique if referred to as PROVIDING CLEARANCE.

It provides long term solutions requiring less frequent pruning which ultimately saves you money.

You should anticipate growth in three-five years when deciding what and where to cut. This method is ideal to address conflicts between trees and overhead services lines. By V-ing out the tree to grow around/beside or under the lines, the tree can maintain a more natural form without causing damage.

6. CROWN THINNING: If a tree has become to heavy headed (too much weight on the ends of the branches) it is beneficial to thin the OUTER canopy. Trees pruned on the inside only result in smaller branches and a tuft at the end. This removes the food source for that portion of the branch thereby weakening the structure. It no longer gains girth at the same rate in response to the tip or heavy end. These branches are subject to damage from winds and particularly snow. Thinning should balance the weight of a limb in relation to the tree. It helps to create a more proportional shape and allows sun light to filter through to the understory and fight against fungal/bacterial growth. It will also allow a branch to sit up higher.

Photo Below: Before and After – applying 1-6 WHY prune techniques. The lower limbs were removed to allow for mowing maintenance, a weak branch was removed and we eliminated a competing leader. A few more crossing limbs and presto – a great looking, balanced tree.


7. CROWN RESTRUCTURING: This is sometime required when a tree has been previously topped, damaged by a storm, has substantial deadwood or improperly pruned. It is possible to restructure the tree into a more natural form. This usually takes several growing seasons to accomplish and is best done by or in consultation with a Certified Arborist.

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