This is the third part of an extended blog about the pruning of young trees. Many people just plant a new tree and expect it to develop perfectly all by itself. Sometimes that can happen, and a tree is just fine. Much more often, though, the best trees grow when there has been some intervention while they were young, to set them on the right path to be healthy and long-lived. If you have just stumbled on this blog, we suggest you go to Part 1, and look at the ‘why’ of pruning your new tree. Then move on to Part 2 to see what equipment you need, and the basic techniques of pruning. Then come here, to Part 3, where we look at the ‘how’ of the process, and guide you through the pruning steps that will give you the best trees possible.
The Goals of Formative Tree Pruning
It always helps when you are doing something to know the outcomes you are trying to achieve. There are a few basic goals we strive for, and these are all things that give your tree the best chance of longevity. Aesthetics are secondary, but a healthy tree is of course always going to be more attractive than a damage, oddly-shaped one.
A Single, Strong, Central Leader – particularly with evergreens like spruce, fir, cedars and many pines, the most important thing to pay attention to is keeping one single growing stem at the top of your tree. This creates a tree with one tall, straight trunk and balanced branches radiating out around it. In time, with age, other branches may become strong too and grow out, but by then the lower section will be a good strong trunk better able to deal with the weight of that.
Of course not all trees grow that way, and most mature deciduous trees and some pines have more complex, open branching structures. Nevertheless, early development around one central trunk will develop into a stronger crown than a free-for-all from an early age.
Develop a Clean Trunk with Appropriate Clearance – nothing makes a tree more ugly than a big scar on the trunk from removing a large limb. So its important to steadily raise the crown as the tree develops so you have the necessary clearance beneath the lowest branches, for vehicles, mowing, views, or whatever other reasons you may have.
Avoid ‘Split Crotches’ – the place between two branches is called the ‘crotch’. If that space is narrow, leaving the two branches growing almost parallel to each other, as the trunks expand bark becomes trapped in this space. The result is a weakness there, which in strong winds can lead to one of the two branches tearing away, leaving a large, gaping wound and lop-sided tree. Not only is that unsightly, it can lead to infections that in time can kill the tree.
Avoid Heavy Side Branches – Reducing the length of branches makes them develop more smaller side shoots, and keeps them closer to the trunk. That in turn reduces the risk that branches will, in the future, collapse under the weight of a snowstorm, or break in a high wind.
Maintain an Open Crown – tight crotches and branches growing against each other is always going to lead to problems, so the ideal tree has well-spaced branches. To avoid creating scars, which are not only unsightly but lead to disease, this should be developed early, and maintained as the tree grows.
Now let’s take each of these one by one, and look at how to achieve that outcome in different circumstances, with different types of trees.
Creating a Single, Strong, Central Leader
Especially with taller evergreens, having a single straight central trunk gives the strongest and most durable tree, and the best looking one too. In deciduous trees too, keeping a central leader for a while at least gives a nice straight lower trunk and helps develop height rather than spread.
This is a simple and straight-forward process, and one that can be done at almost any time of year. Step back and look at your tree. Is there one straight stem at the top standing well above the surrounding branches? At least 6 to 8 inches taller? If there is you have nothing to do except keep an eye on it and check every year or two that it still looks like that.
If instead there are two or even more stems, all about the same length, then you have a ‘double leader’. Take your pruners, pick the strongest and most central stem, and remove the others around it. You don’t always need to remove them completely, and it is often better to just shorten them, so the one you selected stands at least 6 inches above all the others. When pruning evergreens remember that almost all of them will never sprout from a bare branch, so always leave some healthy green needles or growth on any stem you cut. In a while new buds will develop on the cut piece and it will re-sprout just fine.
With a deciduous tree shorten back surrounding branches, or remove some entirely. If the tallest stem isn’t vertical, you could attach a stake and straighten it a bit, but usually it will get the idea and straighten itself if the bend isn’t too big.
Developing a Clean Trunk
As trees grow they all focus more and more energy in the upper parts, so that the lowest branches weaken and eventually die. This can takes years or decades, depending on the type of tree. There are a few obvious reasons for speeding up the process. Clearance for walking and driving are obvious ones, or you might just want to be able to sit in the shade. Consider when planting the likelihood that over time lower branches will grow enough to reach paths and drives, and decide if you want that – before planting.
Rather than wait for the tree to grow large and then start cutting off big low limbs, start early. That way you will have a smooth, unblemished trunk to admire. Remove a few of the lowest branches each year, obviously starting with the one’s closest to the ground. Those lower branches encourage the trunk to thicken, and taking them all in one go leaves a small crown on a thin stem, which is not a good look and harms the tree.
Avoiding ‘Split Crotches’
Sometimes a branch develops that has a narrow angle with the main trunk, especially if you have branches of similar sizes. Called a split, or narrow crotch, if the angle between the branches is less than 30 degrees, it represents a future problem – as you can see from this picture – and is one of the most common causes of sections of trees breaking away. When pruning, if it’s possible, always choose to remove completely the less important branch when you see this situation.
Avoiding Heavy Side Branches
As mentioned in the first part of this blog, trees grown as isolated specimens have no reason not to keep spreading sideways. This is very picturesque, but from the point of safety and durability, it should be kept within limits. Regularly taking the last foot or two of the end of branches tending towards the horizontal will reduce the risk of them later collapsing.
Maintaining an Open Crown
Again as mentioned earlier, nurseries tend to trim young trees so that they have dense, bushy crowns – it’s the way customer’s like them. As branches thicken, though, the center of the tree can become very dense and overcrowded, leading into narrow crotches and crowns more likely to collapse or snap off.
So as your tree is developing, keep taking some of the branches away from the inside of the crown – if it ends up looking a bit ‘thin’, that’s probably about right – it won’t look that way for long.
One Final Word. . .
So that’s about it. With this knowledge you are all set to grow the perfect tree – it doesn’t take much effort, it’s a lot of fun, and the end result is a tree that has a much better chance of reaching a venerable old-age – let’s hope it will still be around long after you and I are just memories.