Now that you're armed with an understanding of how to cut, let’s talk about where and when to cut.
I would start with the easy stuff. The three Ds…
Dead
Diseased
Damaged
You can take these guys out any time of year.
After that you can think about the tree’s structure.
Central leader
The “central leader” is the main parent stem of your tree. From it all branches start… ideally. Most of the time this isn’t true. But evidence and studies show that this is the strongest structure a tree can have. Therefore if you have two large competing trunks it is best to subordinate one by pruning it down to size. Simply pruning off the apical meristem will have an impact on that branch's growth because it changes the hormonal interactions in the stem. Or you can make a more severe pruning cut by taking it down to a lateral branch. This will slow the growth of the subordinated stem and allow space for the competing stem to grow into, thereby further outcompeting the subordinated stem.
Bad branch unions
Some branches connect to the parent stem in a not-so-good way. Through genetics or environmental triggers the branches get tricked into thinking they are rival stems instead of daughter branches. Normal branches connect in such a way that layers of the parent stem’s tissues get deposited over layers of the daughter branch tissue. Each year a new layer is added and the union is strengthened. For rival branches this isn’t true and what can happen is bark can become “included” in the union. For several reasons this branch union is prone to failure. A keen arborist will detect these unions and eliminate them sooner than later.
Lifting and Lowering
This is where your needs as the tree owner come in. Does your tree need to be lifted? Does your tree need to be pruned to provide space for a structure? Don’t be afraid to make these cuts if they are needed. Making the cuts when the tree is younger is better than having to make big cuts when the tree is bigger. Remember if you make the pruning cut in the right way and in the right place you are just simulating a natural branch die off.
Advanced cuts (optional)
I will call them advanced because this is where the nuances come in. These are cuts that will help set up the trees structure for the future. Arborists think of branching structure like scaffolding. Sometimes you want to encourage some branches and sometimes you want to subordinate others. It is also helpful to think of where you want your first permanent branch to be. Is this going to be a street tree where the first branch needs to be 12 high or is it going to be a specimen tree in a yard where the branches can droop down to the grass? What is the tree’s natural look? What are some natural characteristics that can be accentuated with a couple of cuts? I call these cuts optional because I think a tree will naturally set itself up. The first 3 steps to my pruning lesson are fairly important for safety and health. I too often see this 4th step abused, and in my experience a tree will establish its own branching in its own unique way. So leave this 4th step to the experts.
Common myths
"My tree needs to be thinned." WRONG! As I mentioned earlier trees naturally thin themselves. So you don’t need to remove all the little branches in the interior. Those guys are important too!
"My tree needs to be topped." VERY WRONG! Topping trees is about the worst thing you can do besides cutting them down. Here are the reasons:
It is opening up the tree to infection.
It requires the tree to use up massive amounts of stored energy to overcome this set back, thereby weakening the tree.
The new branches (epicormic shoots) are and will be weakly attached for a number of years because they don’t have that layered branching like I talked about earlier.
The tree will respond to the drastic cuts with very thick and very vertically straight branches, creating more maintenance and care down the road.
It’s ugly.
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