Where The Cut Is Made

A tree can’t repair damaged wood. It seals the damage off instead, forming a barrier around the injury while new wood grows over the outside. Rot that gets past that barrier moves into the trunk, so the position of the cut determines how well the tree contains it.

The barrier is strongest at the branch collar, the slightly swollen ring of wood where a branch meets the trunk or a larger limb.

A cut made just outside that ring leaves the collar intact, and the wound closes over a few seasons.

A cut made flush with the trunk removes the collar along with the branch and gives rot a direct path inward. A long stub creates a problem of its own, since it dies back to the collar anyway and becomes a pocket of rot the tree has to seal off.

Heavy limbs come off in stages, with the weight taken down before the final cut is made. A limb removed in a single cut tears as it falls and strips bark down the trunk, leaving a wound much larger than the cut itself.

Wound paint and tar-based sealers don’t stop rot. They hold moisture against the wound and slow the tree’s own sealing, and research has shown this for decades. Cuts are left open. The one exception is oak wilt, covered below.

How Much Growth Should Come Off

The ANSI A300 standard is the reference that certified arborists work from. Older versions advised removing no more than 25 percent of a tree’s crown in a single growing season, and that figure is still quoted as a fixed limit.

The current standard puts the emphasis on the purpose of the pruning, along with the species, age, and condition of the tree.

Many trees should lose considerably less than a quarter. A tree already under stress from drought, construction damage, or a recent transplant should lose no live growth at all, because it needs every leaf it has. A young tree being shaped for good structure can take more.

A quote that gives a percentage without explaining the purpose behind it is incomplete.

Crown Thinning

Thinning removes some live branches to reduce the density of the crown, spread evenly across the tree rather than concentrated in one area. Light reaching the interior keeps the inner growth alive, and that inner growth is part of what makes a limb strong.

Thinning is often sold as storm protection. Research at the University of Florida found that thinning and crown reduction both reduced how much a trunk moved in high wind, though the effect fades as the canopy fills back in. It’s a reasonable service with modest benefits rather than an answer to storm risk.

Better airflow through a thinned tree may also help with some leaf diseases, though removing infected wood does considerably more.

Stripping the inner and lower branches from a limb and leaving the leaves bunched at the end is called lion-tailing, and it’s the most common way mature trees are damaged during pruning.

The effects are well documented:

• The limb stops thickening toward the trunk, which makes it more likely to break under wind or snow.
• The weight is concentrated at the tip, where it puts the most strain on the joint at the trunk.
• Bark that grew in shade is exposed to full sun, which scalds and cracks it.
• The bare sections produce watersprouts, thin vertical shoots that are poorly attached.
• The tree loses the steadying effect that inner branches provide when a limb moves in wind.
• The tree gains no protection from wind, since the same Florida research found lion-tailing ineffective at reducing how much a trunk moves.

Thin vertical shoots along the inside of a tree a year or two after pruning indicate it was over-thinned.

Pruning And Disease

Pruning controls disease mainly by removing infected wood at the right time of year.

Black knot is the most common example on properties around Orillia and Barrie, affecting cherries and plums. It appears as hard black lumps that wrap around a branch and eventually kill it. The fungus extends further along the branch than the visible lump, so cuts are made at least 15 to 20 centimetres below it, back to a side branch or the trunk rather than leaving a stub.

The work is done between late fall and early spring, while the tree is bare and the lumps are easy to identify. Tools are disinfected between cuts, and the removed branches are destroyed rather than piled, since a cut knot continues to release spores for months.

Pruning Restrictions For Oaks And Elms

Oak wilt was confirmed in Canada for the first time in 2023, at three sites: Niagara Falls, Niagara-on-the-Lake, and the Township of Springwater, which is in our service area.

The fungus blocks the channels a tree uses to move water up from its roots. Red oaks are affected most severely and often die within a single season, while white, bur, and swamp oaks hold up better. Beetles that feed on sap carry the fungus from infected wood to fresh cuts on healthy trees.

Ontario’s guidance is to avoid pruning oaks from the start of April to the end of July, which is the highest-risk period, and preferably to leave them alone through November.

A limb broken by a storm during that window should be treated immediately with a thin coat of wound paint, the one case where sealing a cut is correct. Larger storm damage on oaks needs the same care, since the wounds left by a broken crown are far bigger than any pruning cut.

Suspected oak wilt is reported to the Canadian Food Inspection Agency. Firewood shouldn’t be moved between properties, since that’s how the disease travels long distances, and the Springwater case is believed to have arrived that way.

Elms are covered by a national restriction. The CFIA advises against pruning elms anywhere in Canada between April 1 and September 30, because the beetles that spread Dutch elm disease are drawn to fresh cuts throughout the growing season.

For other species, winter is the default.

Problems are easier to identify without leaves, and the tree isn’t spending stored energy on new growth. Fall works as well once the leaves are down, and dead wood or storm damage can be handled at any time of year.

Late spring, while leaves and shoots are expanding, is the period to avoid, though it’s a good time to check trees for winter damage and book the work for later in the year.

Safety And What An Arborist Looks For

Dead limbs have no flexibility left and come down in the first strong wind or heavy wet snow, both of which are common in central Ontario. Removing them is one of the jobs worth booking before the snow arrives.

Beyond the obvious dead wood, an arborist assessing a property looks for problems that are harder to spot.

One is a pair of stems of similar size growing from the same point on the trunk with bark pinched in the joint between them, something that never forms solid wood and often splits under load. Others include limbs that have grown long and heavy at the far end with little growth closer to the trunk, cracked joints, and rot around old wounds or previous cuts. Branches approaching a roof or a hydro line are noted at the same time.

When a limb needs to be shorter or lighter, reduction is the correct method.

The cut goes back to a side branch large enough to take over as the new end of the limb. Topping isn’t a substitute, because a cut made at an arbitrary point leaves a stub that rots, and the tree responds with fast shoots attached to that stub. Those shoots are poorly anchored and become a hazard as they gain size and weight, usually above the same spot the original limb hung over.

Appearance And Natural Shape

Pruning done well for appearance is difficult to notice. The crown stays even, the tree keeps the natural shape of its species, and no individual cut stands out.

Poor work is visible from the ground, and it usually takes one of three forms.

• A white pine stripped up the trunk with growth left only at the top.
• A maple thinned until the canopy is transparent.
• A crown cut back to a uniform outline unrelated to how the species grows.

Most of these begin with a request to open the tree up, with no agreement on what that means.

Young trees offer the best return. Pruning for structure during a tree’s first fifteen to twenty years corrects competing main stems and sets good branch spacing at a fraction of what the same corrections cost once the tree is mature.

Questions To Ask Before Hiring A Pruning Contractor

• What is the purpose for each tree: cleaning, thinning, raising, or reduction? A quote that says only “trim” isn’t a plan.
• How much live growth is coming off, and why that amount on this tree?
• Will the cuts be made at the branch collar, with no flush cuts and no stubs?
• What is the plan for any oaks or elms on the property, given the seasonal restrictions?
• Are the climbers ISA certified, and does the company carry current liability insurance and WSIB coverage?

Pruning a mature tree properly is slow work, and the results take years to show. The trees around Orillia, Oro-Medonte, Ramara, Severn, Springwater, and Kawartha Lakes still standing strong at eighty and a hundred years old got there through a long series of good decisions rather than any single visit.

Woodland Tree Service’s ISA-certified arborists can assess the trees on your property and tell you which ones need work this season and which are better left alone.