It's time for another tree care lesson! If you need a refresher on why we value trees so much, check out our post on the top 10 benefits of urban trees. Today we'll talk about proper pruning technique and why it's so important. The tree you see above has completely closed off some cuts and is working on others with even, donut-shaped callus growing over the wound. That's what you want to see, but getting that result is harder than you might think.
If you've ever needed to prune a tree branch and you haven't been trained on the best methods, chances are you've done one of two things: cut the branch somewhere in the middle to meet whatever clearance needs you're trying to achieve, or cut the branch off flush with the trunk. Both of these are bad for the tree. Trees can't heal wounds in the way people heal cuts on their skin. The tree has to grow new bark from the areas around the cut to seal off the damage, and some parts of the tree are better at this than others. When you cut a branch in the middle, the bark just below that wound can't wrap around and seal off the cut, so instead the branch just dies back to a spot lower down the branch where it can seal itself off, and you're left with a stub that will die. If you cut the branch flush with the trunk, you're creating a much larger wound that provides a bigger area for insects and disease to invade, and it takes longer for the tree to seal it off. The tree pictured above has been pruned properly, and you can see that new bark has totally covered the cuts over the years. Learn how to get these results below.
So where should you make a cut? There's a sweet spot at the base of a branch just before where it connects to another limb or to the trunk. There is a ridge of bark on the top of the branch and a little bump called the branch collar on the bottom. Your goal is to cut just on the outside of these two structure. See the diagram on the right for clarification (click to see larger). It won't always be easy to spot the branch collar on the bottom, so if you aren't sure make your best guess. The angle of the cut should follow the angle of an imaginary line connecting the bark ridge and branch collar. If you're having trouble finding the branch collar, make the angle match the angle of the bark ridge as it extends into the trunk.
If you look at a branch that has broken off in a storm, you will sometimes notice that the weight of the falling branch caused a strip of bark to rip off of the trunk as it fell. This can also happen when a branch breaks as you work on sawing through it, and ripping bark like that leaves a wound that can be difficult for a tree to heal over. If you are removing a branch larger than 1" in diameter, you need to use the three cut technique to avoid tearing the bark. First you make a cut on the bottom of the branch further up than where your ideal cutting zone is. This cut should go about a third of the way into the branch. Next, you cut off the branch on the outside of this pre-cut. If the bark starts to rip, it will stop when it gets to your first cut. Your third cut should be at the base of the branch in the sweet spot we discussed above.
Congratulations! You now know how to prune your trees to keep them happy and healthy for years to come! Please spread your knowledge, and never hesitate to ask a professional for help if you have questions. If you’re more of a visual learner, here’s short video that shows how to cut medium-sized branches and shows where you want the finishing cut for any size of branch. For more details written instructions, you can also check out this Purdue Extension bulletin about pruning trees and shrubs.