Tree care #5: Why and how to prune trees
I've talked about all the selfish-people reasons for tree pruning, but there are a few situations when it's okay for us to interfere and meddle with our trees. First one is, if our tree has a lot of dead wood on it. Meaning, it has branches that are dried up or infected with rot, maybe as a result of some pest or accident; they're falling down and creating debris, possibly dangerous for falling on your head. That is something that is always acceptable to prune off, since the tree has already written those parts off, and she won't miss them when they're gone.
Second scenario in which you can cut something off, is when there's disease showing on a particular branch. If it's happening on the main trunk, there's not much to do, but if it's starting from a branch that can be taken off, it's better to take this potential problem away, rather than risking your entire tree getting sick. Trees can often deal with sickness on their own, so in a lot of cases, you can just wait and see how she does before deciding to do anything.
Another scenario is when you're growing a large tree, and it has multiple leaders; meaning multiple branches are trying to grow into trunks and are competing to be the new main trunk. In smaller trees this is often normal, decorative, and done on purpose, a small tree can sustain multiple trunks. However, a large tree will develop balance problems, and is very dangerous for you, and herself, to have multiple leads.
Only the original trunk is connected to the ground and the roots; other branches are not a part of the trunk, and are merely laminated in place. If they grow into a leader, there's not much holding them onto the tree; as soon as they grow too large, they will develop bark, possibly rot between themselves and the original trunk, and break off. This is a game of balance and chance, a tree can grow very large before one of the leaders breaks off, and the larger it gets, the more dangerous it is.
Here's a picture illustrating just how a branch is connected to the trunk:
You can see they're not one continuous body; there's a part that belongs to the branch, and a part that belongs to the trunk. While the trunk is bigger and stronger than the branch, it will grow without a problem. If the branch competes with the trunk and grows vertically, it's clear this structure will not be able to support it, and it will eventually break off. After it starts breaking off like this, the tree is likely to lose her balance completely, and rot from the inside. One-leader, healthy trees, can live forever.
This picture will also help you understand where to cut off a branch in order for the tree to be able to heal correctly. Here's three ways in which people cut off branches.
Now why would one way be correct, and other two not? Because every time you cut a tree, you're introducing rot. In the first case, you're leaving the tree with an entire rotting stub to deal with. In the second, you're digging into the collar ring that connects the tree and the branch; this collar belongs to the trunk, not the branch you're cutting off. If you cut into it, you're damaging the trunk. Ideal way to cut is to go exactly between that (sometimes invisible) ring and the rest of the branch. This way, the rot will only go as far as the branch was being held inside of the tree, and the tree will be able to heal the damage safely. Here's a diagram to explain it better:
Second set of trees illustrates just how far we've allowed rot to enter the tree. If you do it right, the tree will be able to heal properly from it, and she will not be terribly bothered by it.
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Here for Types of Pruning Cuts!