1. Thinning Cut:
First, let's talk about a thinning cut. A “thinning cut” is one that cuts a side branch from the main branch. The term “thinning” is a bit of a misnomer. Thinning trees is not something that is done any more. Researchers found that trees that had been thinned were more likely to sustain storm damage in high winds. This is because branches actually bump into each other and create a buffer effect. Furthermore, as mentioned in a previous post, if the branch wasn’t actively contributing photosynthesis, the tree would have eliminated it on its own. So, every green leaf counts. Finally, thinning your tree is opening up your tree to infection unnecessarily. The fewer wounds the better.
However, there is a time and place for a thinning cut. To make a thinning cut we want to cut outside the branch collar, starting at the branch bark ridge and following a 45 degree (ish) angle down and out from the trunk. You want to make sure you are not cutting into the branch collar. In my opinion it is better to leave a little nub than to cut into the branch collar. What we don’t want are “flush cuts”, for obvious reasons at this point.
As an astute observer will witness, sometimes the bark will rip on the bottom as you cut down the branch. This is because the fiber cells on the bottom of the branch are wanting to stay intact and are therefore ripping until they finally sever. To avoid this, sever the fibers on the underside of the branch first. I suggest you use the three cut method shown below.
The three cut method is simple and easy (and what arborists use 90% of the time). Start by making an undercut several inches to a foot (depending on how big the branch is) out from the branch collar or intended final cut. This undercut should be about ⅓ of the way through the branch. Next, cut down from the top just outside this cut. Cut all the way until the branch falls to the ground. Lastly, go to where your final intended cut is to be and cut away. You won’t have to worry about ripping at this point because there is no weight to drag the branch down.
You can see here that they encourage a three cut approach to prevent any tearing of wood fibers down the branch and into the branch collar (which is the bulging part of the branch/trunk union). Your final cut (cut 3) should be outside of this branch collar.
2. Crown Reduction Cut:
A “reduction cut” means you are taking a large branch back to a smaller “lateral” branch. It is still debated on whether you should actually cut where the dotted line is in the diagram or on the straight line perpendicular to the stem. I cut on the straight line. The thought here is there is less surface area for the tree to enclose in the wound healing process and less surface area that could be exposed to fungal and bacterial spores.
3. Heading Cut:
Heading cuts are not very commonly used. They are internodal cuts that are designed to release one or more dormant buds to create branching where there isn’t any. Think of a hedge. When you shear a hedge you are arbitrarily cutting branches. These cuts mostly fall somewhere between two existing lateral branches from the parent stem. Along the parent stem are buds that are dormant, but will be released if the right hormone cocktail (specifically elevated levels of cytokinin) tells them to. This is why hedges are so thick. Like Medusa’s head, you cut one off and two more take its place.
Heading cuts can be used deliberately in the nursery setting to promote branches on young stems. But you don’t see them used a lot in arboriculture because they are not “clean” cuts, meaning trees usually cannot heal quickly from these cuts. Therefore there is a greater chance of infection. Also, usually the branching from dormant buds (called epicormic shoots) isn’t very desirable. They are usually vertical sprouts that are unsightly, are a suck on the trees resources, and are weakly attached leading to possible branch failure in the future.
Long story short, steer clear from heading cuts.
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