How to Prune a Large Tree Branch Safely (and When to Call a Pro)

Where to cut: the branch collar

Every branch grows out of a slightly swollen ring of tissue where it meets the trunk or a larger limb. This is the branch collar, and it holds the tree's natural defences against decay. A good pruning cut is made just outside the collar, leaving the ring intact. The tree can then seal the wound over time.

Two cuts cause most of the damage we see. A flush cut, made right against the trunk, removes the collar and leaves a larger wound that is slower to close. A stub cut, which leaves a long piece of branch sticking out, cannot seal at all, and the stub rots back into the tree. Cutting at the collar avoids both.

The three-cut method for a large branch

A heavy limb that is simply cut from the top will fall before the cut is finished and peel a strip of bark down the trunk. That tear can run well below the cut and is far more harmful than the pruning wound. The three-cut method prevents it:

1. Undercut. Cut a third of the way up into the underside of the branch, about 30 to 60 cm (1 to 2 feet) out from the trunk. This stops the bark from tearing downward.
2. Top cut. A little further out from the undercut, cut down from the top until the branch breaks away. The weight comes off, and the stub that remains is light.
3. Final cut. Cut the stub just outside the branch collar, from the top, with the saw held steady and the stub supported by your free hand.

Wound paint and sealants are not needed. Research on tree wounds has found they do not help the tree heal.

What you can safely do yourself

Small jobs are reasonable for a homeowner with the right tools: hand pruners or loppers for twigs and branches up to about the thickness of your thumb, and a sharp pruning saw for limbs up to a few centimetres across that you can reach with both feet on the ground.

Keep to these limits:

• Stay on the ground. A ladder and a saw together are a common cause of serious falls.
• Do not cut above shoulder height with a chainsaw.
• Do not remove a branch that is bigger than you can control as it falls.
• Wear eye protection, gloves and a hard hat, and keep people and pets well clear of the drop zone.
• If anything about the job makes you hesitate, stop.

When a branch is too long, high or heavy

Searches such as "pruning long branches" usually describe a limb that has reached over a roof, a driveway or a neighbour's yard. These are the cases that go wrong most often. A long branch carries its weight far from the trunk, so it acts like a lever, and cutting it in one piece can send it somewhere you did not plan.

A crew handles these by taking the limb off in sections, lowering heavy pieces with ropes, and controlling where each piece lands. A branch over a house, a car or a fence needs rigging, not a bigger saw.

Call a professional if:

• The branch is thicker than your forearm, or you cannot reach the cut from the ground.
• It hangs over a roof, a vehicle, a play area or a neighbour's property.
• It is near a service line or power line.
• It is cracked, hanging, or partly torn from the trunk.
• It is part of a tree that already looks unhealthy.

Power lines: stop and call

Never prune a branch that is touching or near a power line, and never use a pole saw or ladder near one. Electricity can travel through a wet branch, a pole or a ladder. Keep well back and call the utility. Work near lines is done by qualified line clearance crews, not by homeowners.

Common pruning mistakes

Most pruning damage comes from a short list of habits:

• Topping. Cutting the main stems back to stubs. The tree answers with a thicket of weak shoots that break easily.
• Flush cuts and stubs. Both slow healing and invite decay, as described above.
• Removing too much at once. Heavy cutting stresses the tree and can leave it open to pests.
• Tearing the bark. Skipping the undercut on a heavy limb.
• Dull or dirty tools. A dull saw crushes tissue and leaves ragged cuts. Wipe blades between trees if you have been cutting diseased wood.
• Cutting the wrong branch. Take the time to look at the whole tree before cutting, so the cut you make is the one the tree needs.

Pole saws and reaching high branches

A pole saw can reach branches that are a few metres up, and it is a sensible tool for small limbs. It also has limits. A heavy branch cut on a pole saw falls toward you, the saw is hard to hold steady, and the final cut at the collar is hard to place. Pole saws and metal ladders are also the tools most often involved in contact with power lines. If the branch is much thicker than your wrist, or if you cannot see exactly where the cut will land, leave it for a crew.

How much to take off

A tree can lose some of its crown and recover, but heavy cutting stresses it. A common guideline is to remove no more than about a quarter of the live crown in a season, and less from older trees. Never top a tree, which means cutting main stems back to stubs. Topping produces weak, fast-growing shoots and often shortens the tree's life.

Remove the right branches, not the most branches. Dead, broken, diseased and crossing branches come first. Our guide to warning signs of a dangerous tree shows what to look for.

When to do it

Late winter, while the tree is dormant, suits most trees in Ottawa, but oaks, maples, birches and spring-flowering trees have their own timing. Dead or hazardous branches can be removed any time of year. Our guide to when to prune trees in Ottawa gives the season-by-season detail.

A quick checklist before you cut

1. Identify the branch collar and decide where the final cut goes.
2. Check overhead and around the tree for lines, roofs and people.
3. Plan where the branch will fall and clear the area.
4. Check that you can make all three cuts from the ground.
5. Use a sharp, clean saw.
6. Make the undercut, the top cut, then the final cut at the collar.
7. Do not apply wound paint.
8. Clean up. Branches can be chipped into mulch: see what to do with wood chips.

Storm-damaged and hanging branches

After wind, ice or heavy snow, branches may be cracked, partly torn from the trunk or caught up in the crown. These are hazards, and they behave unpredictably. The wood is under tension, and a cut can release it suddenly. Keep people away from the area under the branch and call for help instead of climbing up or reaching under it. Our guide to dangerous tree warning signs explains what to check afterwards, and our crews offer 24-hour emergency service.

What to expect after a branch comes off

A correct cut at the branch collar looks like a ring of tissue that slowly rolls over the wound. This is normal, and it can take a few years on a large cut. Watch for signs of trouble such as oozing, a cavity forming or fungus growing at the cut, and have the tree looked at if you see them. Nothing needs to be painted on, and extra fertilizer does not help.

Permits and City trees

Under Ottawa's Tree Protection By-law, work that maintains a tree's health and follows good arboricultural practice does not need a permit, but heavy cuts or damage can still count as injuring a protected tree. A tree on the City boulevard belongs to the City: call 3-1-1 rather than working on it yourself.

Bisson Services handles tree pruning across Ottawa and Prescott-Russell, including long, high and heavy limbs, with service in English and French. Call (613) 558-4806 for a free estimate. If the branch is part of a tree that should come down, we also do tree cutting and removal.

Common questions

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