Correctly pruning a tree.
There are three ways to prune a tree. One way allows the tree to recover using its natural defences. The other two ways prevent those natural defences. Guess which one is correct?
A treeworld “superstar” called Alex Shigo discovered that “trees don’t heal, they seal”. The phrase was a pithy way of communicating one of the biggest arboricultural1 revelations of modern times. He identified, researched and brought to the world the process of how trees seal in response to wounding. It changed the way that pruning was taught and communicated. His efforts have probably done more for the health of trees than any other human in the field of arb. The process is called the “Compartmentalisation Of Decay In Trees” (CODIT). When a tree is pruned correctly (which is technically wounding with a little more finesse and mindfulness) the CODIT process activates and the tree recovers and perhaps even becomes a little more vital2.
There are three broad approaches to pruning a tree and two of them (called flush cutting and stump cutting) cause a wound thats prevents the CODIT process from activating. The third (correct) way triggers the CODIT process and is called natural target pruning. If you’ve ever pruned a tree, the way you performed the prune would have broadly fallen into one of these three methods. It’s ok if you’ve ever walked up to a tree with a panel saw and “lopped a bit off”. We’ve all done it, but don’t do it again.
Natural Target Pruning
Natural target pruning is the correct way to prune a tree. Below is one of many trees at treeschool. Every one of these circles is an example of natural target pruning. In the yellow circles you can make out what look like donut shapes. They cover and seal the exposed wounds where a correctly executed pruning cut has taken place. In one or two yellow circles you can make out eye shapes. These are where the tree has completed the recovery process and has totally sealed. There is also one coral coloured circle. This is where the right technique was perhaps just off. There may have been a reason for this. You can make out that there has been no attempt on behalf of the tree to seal the wound up in the coral circle. There is a crack next to the wound, so there could be something else going on here.
In the photo below, just under each yellow line you should be able to make out a distinctive line where the bark of the main limb fuses into the branch. This is called the branch bark ridge. Above each of the coral lines is the region called a branch collar. I’ll go into the depth of why natural target pruning works in the future, but all you need to know is that if a pruning cut is made so that it starts just in front of the branch bark ridge and ends just in front of the branch collar then the CODIT response should initiate and the donut will form.
Next time you’re walking about in the world and you pass some trees, especially in urban settings, have a look for donuts (tree ani). Once you see them you’ll never stop seeing them.
So, what about the incorrect ways to prune a tree?
Flush cuts were once considered the standard way to prune trees back in the day, but now we know that was wrong. Below you can see a flush cut. Notice that the branch bark ridge has been cut into and the branch collar has been completely removed? This wound will never seal and it will always be a point of ingress for pathogens. You can already see a dark clover shaped area which would indicate this tree is already in the process of decay. This cut was likely the death penalty for the tree.
With a flush cut the mindset was that you’d cut the branch off completely. You’d be aiming to get the cut flush (translates into “level with” in trade speak) with the trunk. The branch bark ridge comes off, the branch collar comes off. All of it. “Just lop it all off and make it flush mate” was the general idea.
Also, it isn’t in the above example, but in some extreme cases the removal of any wood that was remotely part of the branch occurred. Just as if a carpenter took a giant spokeshave (but was actually a chainsaw) to the trunk to gain back that circular tree form. Whilst this left a nice shape, it also left a massive wound in the trees cambium that would have never healed.
A stump cut is made when you cut a limb nowhere near the branch bark ridge or branch collar leaving a long or short “stump” or a “peg”. It is a cut will never seal and it will no doubt become infected. Below, highlighted in coral is a stump cut, and I was the person who made this cut. Before you judge me, the job I was performing on this tree was a removal. In the vein of redemption, the cuts in the small yellow ovals are just about passable. Not ideal, but passable. However the larger branch in the coral oval is a textbook example a stump cut.
Again, the job I was doing here was a tree removal. The customer wanted the light back into the garden for summer so the tree had to go. However, they use this tree as an anchor for their hammock, so I left this limb as a stump cut. When I go back to finish the job in October, I’ll remove the limb, grind out the stump and replace the tree with another Acer palmatum, but this time a weeping dwarf variety.
Here’s another example of bad pruning using the stump cut technique. This tree belongs to a friend. He emailed this picture to me as "I had correctly prophesied" in September that the tree would blossom in April3. I should point out that he inherited this tree as part of a property purchase. He didn’t butcher this tree, the previous owner did. Every one of these yellow circles4 is a stump cut. This tree will die soon in any case, as there is a lot wrong with it, but this army of stump cuts isn't helping matters at all. My friend calls it the “haunted-ass frankenstein tree” which is pretty spot on.
Just to wrap up, this was the second of the trees I pruned for my exam. As you can tell, it is not a big exam but it was bit nerve wracking as these trees are part of the Houghall Arboretum and a bad prune could kill a prize specimen tree. I didn’t want to be the guy who killed the tree.
And here’s a close up of the first tree I correctly wounded.
If you’re going to prune a tree yourself, then brush up on natural target pruning and the series of three cuts you would use. As I am a student, I am not offering this information as a tutorial as I’m not quite at the point I feel qualified or comfortable to start handing out free advice.
When pruning, understand that a tree of any age could be easily wounded by an ill executed pruning cut. You’ll know if your cuts are correct a few years down the line as they’ll start to seal. As a rule, never flush cut something when pruning and never use a stump cut. The tree won’t heal and it’ll be open to infection by any one of the 1000000’s of fungal spores, bacteria or viral pathogens that hang around in the all of the air on Earth.
Whilst arboriculture is the correct term, it has five syllables. So we say “arb”. Not only is “arb” one syllable but it also sounds like a grunt when pronounced correctly. This allows us arb workers to continue the myth that arb workers are just slightly above drummers in the intellect department.
The logic is this: A tree has ten branches and has stored enough sugars to power ten branches. You correctly prune two branches off and even though it now has to seal and ground around two wounds, ultimately it is left with more sugars that it needs for minimal existence, so that extra energy goes into the existing branches. Hence more vital.
Prophesied was not my term or choice of word. It is a cherry tree and they tend to do that in April.
I know some are ovals and not circles.