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The Basics of Tree Trimming

January 27, 2020

Written by Dave G.

In our previous blog, Should I Trim My Trees? we talked about how to look at a mature tree and decide if the time has come for some trimming. On a larger tree this can be a considerable undertaking, needing an arborist or tree surgeon, and we will look in a future blog about hiring one. If the tree is smaller it may be possible to do the work yourself, so some basic guidance is needed. Even if you hire an expert, it helps to have an idea of the basics yourself, so you can discuss what needs doing, and understand what is going on when the project gets underway.

Safety First

Trimming large trees is dangerous work. The insurance premiums for trained specialists are very high, which proves the risk. It is dangerous work for them, so you can imagine how dangerous it is for someone without training and experience. Think carefully before deciding to do tree work for yourself. Hand pruning a young tree to improve its form is a reasonable goal, if you have good ladders and a hard-hat. Tackling a large tree with a chain saw is a completely other matter. Serious accidents and even death are possible, so don’t take chances and take on more than you can handle. That said, knowledge the basics is something all gardeners need, so let’s see what they are.

Some Terms You need to Know

Like all professionals, arborists have their own language. If you want to talk to one, you need to know what some basic words mean. Here is a quick run-down:

Trunk – the main, central part of the tree, beginning at the ground.

Leader – The uppermost slender branch, most obvious in many evergreens (conifers), that creates the trunk.

Scaffold Branches – major structural limbs that give the tree its basic outline.

Lateral Branches – smaller branches growing from the scaffold, carrying the leaves

Water Sprout – a vigorous, fast-growing long stem, growing from scaffold branches. Usually undesirable, and these should be removed unless they are needed to develop a new branch.

Sucker – one or more branches growing from the base or roots of a tree. Often these are part of the root-system used for grafting, and not part of the tree you are growing. They should always be removed as soon as they are noticed. Depending on where the graft was made, suckers could be only from ground level, or higher up, on the trunk itself.

Tree Health and Removing Branches

Trees live a long time, and most trees end up dying because the wood inside them rots. It is amazing to think that only the outer skin of tree is alive – the leaves and buds, and the green inner bark – that’s it. The rest is a dead framework of lumber, holding up the growing parts. Nature has equipped trees with natural methods to protect that wooden interior from the attack of microbes, mostly fungi that live by consuming dead wood. Besides natural preservatives that are injected into that wood by the living parts, the most important protection is the integrity of the outer ‘skin’ – the bark – of the tree. This acts as a barrier to infection, just as our own skin keeps microbes out of our bloodstream.

When we cut ourselves, bacteria can enter our blood. It’s the same for a tree – once the inner wood is exposed, a whole ecosystem of different fungi sees it as an opportunity for lunch. Once they have finished, the strength of that wooden ‘skeleton’ is destroyed, and collapse of parts of all the tree is inevitable.

Tree trimming exposes wood, so we need to help, not hinder, the natural defenses of the tree. In time a tree will cover a wound with new bark, so the first step is to make that easier for the tree to do quickly. At the base of every branch, where it meets the trunk or larger branch it grows from, there is a swollen area, called the ‘collar’, which can quickly grow new bark to cover a removed branch.

Don’t Cut into the Collar

The first good trimming practice is to leave that collar intact. Decades ago we thought it was best to trim a branch flush with the trunk, leaving a large, flat surface. Today we know better. The collar, and the thick bark ridge that often lies at the fork, should be left intact. This means cutting at an angle of between 45o and 60o .

The Sequence of Cuts

When removing all but the smallest branches, two main cuts are made, and three cuts in all. The first gaol is to remove most of the branch, and so remove most of the weight. Then you can make the final cut in exactly the right place, without having to handle a heavy branch. The worst thing that can happen when removing a branch is tearing the collar and bark of the trunk, if the weight of the limb falling tears away bark.

Even if you remove most of the limb at a higher point, tearing can still ruin the job, so the first cut you make should be a shallow cut underneath the branch, through the bark, to protect against bark tearing (see diagram). Then make a 2nd cut off the main part of the branch. The 3rd and final cut is just above the collar, leaving it intact, with as short as stump as possible. Avoid long stumps, as these are ugly, but more importantly they cannot be covered by new bark from the collar, so they act as permanent ‘doors’ for disease into the heart of the tree.

Don’t Use Wound Sealers

Another abandoned practise is using ‘pruning paint’. Those black paints were once widely used to ‘protect’ exposed wood, but we now know that they don’t. Instead they trap moisture, making a perfect environment for fungi to develop, and increasing, not decreasing, the risk of decay. Leaving the wood smooth and clean is the best approach.

Heading or Thinning?

Trimming is used to change the natural form of the tree in three different ways. Several basic positions for cuts are used, depending on your goal. Heading means shortening back a long branch to a side branch, closer in to the main trunk. Thinning is removing a branch from the center, to make a more open crown. Sometimes arborists talk of crown lifting, which means removing branches lower down the trunk, or crown dropping, which means heading back upper branches to reduce the overall height of the tree.

What is Your Goal?

Having a clear idea of your overall goal, and the kind of tree you want, is important in guiding how you approach trimming your trees.

A Taller Tree

By heading back lower branches, and lifting the crown, a taller, slimmer tree can be developed.

A More Open Tree

Thinning out some branches will open the crown, letting more light through, and improving air circulation, which in turn can reduce disease.

A Shorter Tree

Heading back upper branches will reduce the overall height of a tree, so it throws less of a shadow, for example. There is a limit as to how much you can shorten the tree without destroying its beauty. This means that it’s important, when choosing trees, to pick ones that fit the space, including overhead clearance from wires and cables.

If you can’t safely and adequately trim your own trees, consider hiring an arborist. In our next blog we will look at how to make sure you get a good one.