Features Agronomy
A tree’s life depends on proper pruning

Knowing when to prune a tree and how to do it properly as well as understanding why it must be done can make a significant difference in its lifespan and its structural integrity.

Ian Bruce, president and senior consulting arborist with the Bruce Tree Expert Company in Toronto, shared the dos and don’ts of tree pruning with an audience of landscape contractors attending January’s Landscape Ontario Congress in Toronto.

“When you look at the condition of a tree, and you say it’s in good condition, the condition reflects two things that are connected but totally different,” he said. “There is a health or biological condition, and there is a structural integrity condition.”

A tree can be perfectly healthy, Bruce added, yet it can be a “structural basket case.”

Throughout his presentation, he showed photographs of trees challenged by structural issues that had been remedied through cabling and other measures. Proper cabling, for example, can save an otherwise healthy tree that is simply structurally weak.

The relationship between the diameter of the branch and the trunk it’s attached to should be about 30 per cent, Bruce recommended.

“As you go over that and reach 50 per cent, that’s a very weak structure.”

If a branch is shortened, stunted and held back, the trunk will get bigger and the branch will never become a problem, he said.

If someone doesn’t understand how to properly prune or when best to do it, he’s likely going to do more harm than good. A physiological balance exists between the crown and root system.

“If you interrupt that balance, the tree is automatically under stress, period. There are no ands, ifs or buts about it.”

If a tree’s crown is reduced or roots are cut, it will become stressed. If the tree is already suffering from other types of stresses such as diseases or insect pests, it will be less able to respond to those.

“Know why you’re pruning. Understand how much you can take out and minimize how much you remove.”

Technique more important than timing
Bruce said proper technique is generally more important than timing. If the tree is healthy, timing is less important while technique will always be important. The negative consequences of improper pruning technique will, in most cases, last a lifetime, he added, especially when improper pruning is done to a mature tree. Young trees are apt to survive, but older trees can be damaged for life.

The negative consequences of improper timing, on the other hand, are short-lived, usually lasting no more than a year or two before it becomes a non-issue.

Bruce recalled a case of a tree that was improperly pruned and that subsequently died, costing $15,000 to remove.

“Don’t prune without good reason and select the right tools.”

Every branch counts, he said, especially if they are larger than two inches in diameter. If a cut is made in the wrong location or if the branch is not cut properly, it will allow an entry point into the tree for disease and decay to occur in the trunk. Improper pruning, therefore, can artificially shorten the life of the tree.

“All pruning cuts should be made at a growing point.”

Bruce said long stubs shouldn’t be left behind. For best long-term results, pruning should maintain the natural habit of a plant, allowing the tree to respond better.

“It will cost you more if you try to make the tree something it isn’t. It will end up costing you more, and every two or three years that you go back and re-prune it the price will go up because costs are going up. That goes back to selecting the right tree in the first place.”

More branches can be safely removed from a young tree than from an older tree, Bruce said.

Attempts to help nature regain the balance between a tree’s crown and its root system makes sense, he said, but if the tree if topped it is already expending energy to defend pruning cuts and to regain its equilibrium, he said.

“If you then cut the roots, you’re putting it under more stress because it also has to expend energy to defend the damage to the roots.”

Worst time to prune
From bud break to full green condition is the worst time to prune because the tree is expending energy to break the buds and is also undergoing shoot elongation, secondary thickening and annual ring. When trees are flowering, they are expending further energy.

Injury or damage caused by pruning cuts “put it right on the edge.”

The general rule of thumb in pruning is to avoid removing more than 25 per cent of a tree’s living crown. Bruce said the standard was once 30 per cent, but the number was eventually deemed too much.

When shortening a branch – especially a larger limb – it should be cut back to a point where there is a living branch that is 25 to 30 per cent of the size and diameter of the branch being removed. Anything less and the smaller branch may not support the cut, he said.

The smaller the cut, the greater the opportunity the impacted surface has to close over, decreasing the chances of decay.

Bruce recalled that when he first began in the industry in the 1970s, he was instructed to make flush cuts as close to the trunk as possible because it was believed the wound would close over faster and, once it had healed over, it was difficult to see a cut had ever been made.

The science today suggests that cuts are made only into the branch tissue and not into trunk tissue.

“If you cut the branch off and stay outside the protective zone, then any decay that develops from that cut will only involve branch tissue in the trunk.”

Branch tissue in the trunk started at the pith the first year the annual ring was put on outside the pith and may have been one-sixteenth of an inch in diameter. Every year a successive annual ring gets put on that branch and gets bigger, leaving a cone-shaped branch rising out of the trunk and getting bigger towards the outside of the trunk. When decay occurs, cone-shaped decay is visible if only branch tissue is involved.

As a tree trunk grows up and divides into two co-dominant leaders – or equal-sized branches growing vertically near the top of the tree – one branch should be cut to allow the other to become dominant. Leaving the co-dominant leaders unattended could initiate perforation at the V-junction, subjecting one of the branches to split or tear in a heavy wind, jeopardizing the tree’s health.

Water sprouts, or shoots that occur along branches usually where pruning has been done, should be managed or thinned so that they don’t become a problem themselves, Bruce said. Leaves on water sprouts are photosynthesizing and sending energy locally first before energy is sent to the roots.

Pruning at planting should be limited to broken, dead, diseased or interfering branches because the tree is already stressed. The first three times a tree is pruned after planting are the most critical, he added.

“You are setting the framework and once that framework has been developed, there is a lot less significance in the pruning that goes beyond that. So the first three prunings in a tree’s life – if they’re done properly – are the absolute most important.”

Dormant season is generally the best time for pruning, Bruce said, because the tree is being disturbed at a time when it is free of such other disturbances as weather, drought and the sun. He said pruning is done year-round by arborists because “it’s the nature of the beast,” but he suggested caution be exercised because some types of trees prefer to be pruned at different times of the year.

He said he occasionally tells clients that certain trees should not be pruned at a particular time of year. Evergreens, for example, should not be pruned in the fall because their foliage will turn brown.

Trees susceptible to profuse “bleeding” from pruning cuts should be pruned in the fall or early winter or spring when leaves have fully formed, Bruce said. This will help avoid putting a tree under stress and adversely affecting its health.

Pruning into the branch collar or branch bark ridge cuts into trunk tissue, leaving the tree’s life expectancy at a fraction of what it might have been because a trunk cavity will be created.

If the objective is to cut a one-inch branch on a tree whose trunk is one foot in diameter and the cut is made in the correct location, a one-inch cavity will have been made in branch tissue and not trunk tissue. If a flush cut was made to the same tree, the cavity will be made into trunk tissue and runaway decay will likely result because many tree species don’t compartmentalize well.

That’s the significance of making the right cut in the right place, Bruce said.