Trees do not heal, they seal
This is the idea everything else follows from. A tree cannot repair damaged tissue the way an animal does. What it does instead is compartmentalise: it walls the wound off chemically and physically, and grows new tissue over the top from the edges.
The tissue that does that walling-off sits in the branch collar — the slightly swollen ring of wood where a branch joins the parent stem. Respect the collar and the tree seals the wound. Remove it and the decay goes straight into the trunk.
The two ways people get it wrong
The flush cut. Cutting hard against the trunk to leave a neat, flat face removes the collar entirely. It looks tidy for a season and then produces a long vertical column of decay in the trunk above and below the cut. It is the most damaging common pruning error.
The stub cut. Cutting too far out leaves a length of branch beyond the collar. The stub has no living tissue to seal it, so it dies back and rots, and the decay works inward past the collar anyway.
The correct cut is just outside the collar, at the angle the collar itself suggests. On most branches you can see the line.
The three-cut method
Any branch heavy enough to tear needs three cuts, not one. An undercut a short way out from the final position, a top cut slightly further out to remove the weight, and then the final cut at the collar on the now-light stub. Skipping this is how a falling branch strips a metre of bark off the trunk on the way down, which is a far worse wound than the pruning cut.
How much can come off
AS 4373, the Australian Standard for pruning amenity trees, sets out the recognised pruning types and the limits. As a working rule, no more than about a quarter of the live crown of a mature tree in one operation, and less on anything already stressed. A tree is a solar collector; removing too much of it at once starves it and triggers a mass of weak epicormic regrowth.
Where a bigger change is wanted, it is staged across two operations a year or more apart.
Spikes
Climbing spikes puncture the bark and the wounds do not close. They are appropriate on a tree that is being removed and inappropriate on a tree that is being kept. If someone is spiking up a tree you intend to keep, stop them.
In practice
Good pruning is slower and less dramatic than bad pruning, and the tree looks much the same afterwards — which is the point. See the pruning service page for what we cover, why topping fails for the opposite approach, and ring (02) 9090 4915 for an assessment. More on the blog.
Frequently asked questions
Should I paint or seal a pruning wound?
No. Wound dressings were standard practice for decades and research has since shown they trap moisture against the wound and can encourage decay rather than prevent it. A correctly placed cut at the branch collar lets the tree seal the wound itself with its own defensive tissue, which is far more effective than anything from a tin.
Can I prune my own small trees?
Small formative pruning on a young tree with your feet on the ground is reasonable if you understand where the collar is. Anything requiring a ladder, a chainsaw or a rope is not a homeowner job — ladder falls and chainsaw kickback cause serious injuries every year. And on a protected tree, significant pruning may need council consent regardless of who does it.