Of all our plants, trees take the longest to develop and so it is not only heart-breaking, but a significant set-back to a landscape when a tree that is 10-20 years old is destroyed in a storm. Many of these disasters could be prevented with proper pruning early in a tree’s life. Besides preventing disasters, pruning trees properly when young will help them to develop more beautifully, make them stronger, less expensive to maintain as they get older and keep them healthier.
A young tree, like any young being, is vulnerable and needs some extra care. And trees are often a costly investment, both for the plant and for the planting. So since few arborists will come out for the fifteen minute job of pruning a young tree, and since few lawn crews are trained in proper pruning, it is good for home-owners to understand the basics of pruning in order to get their trees off to a good start.
For a basic understanding, it is important to learn three fundamentals:
- The common hazards and how to avoid them.
- How to make a correct pruning cut.
- How to train a young tree so it will have a strong structure.
The common hazards to young trees are many, but these are the main ones to avoid. Don’t let lawn mowers or weed trimmers touch the bark. This tearing of the bark, called “lawn-moweritis” is often a cause of disease and decline in young trees. Put a loose protection, like a plastic or hardware cloth cylinder, around the trunk or mulch 2—4′ around the tree so mowers won’t have to come close. A protector will also prevent cats from using a young tree for a scratching post, which is very harmful. Naturally, deer, especially bucks with their antlers in the velvet stage, can destroy a young tree with munching and rubbing. So keep a circle of fencing around a young tree for three or four years if deer visit. If a tree is planted too close to a street or sidewalk, people will start breaking off branches. If you plant under an electrical line, Public Dis-Service will carve a huge hole in the tree’s canopy. If you forget to remove stabilizing ropes or wires, the tree will grow over them, become girdled, and will break off or be seriously weakened. And lastly, beware of human over-reactions. Don’t over-water and don’t expect the tree to live on Colorado rainfall. Don’t prune aggressively and don’t leave the pruning to nature. Don’t pile mulch against the trunk and don’t let the soil bake with no mulch. Don’t plant too deep and don’t plant too high; plant right where the trunk begins to flare into the root. The best pruning cannot make up for these hazards.
Learning how to make a correct pruning cut is of the utmost importance. Since pruning is surgery on a living being, an improper cut will have far more serious consequences than cutting a 2×4 off at the wrong angle. About 20 years ago, Dr. Alex Shigo’s research for the Forestry Service revealed new information about how trees should be pruned. Dr. Shigo identified the branch collar, which is often a swollen area at the base of a branch. He discovered that the common practice of making a flush cut (see Figure 1. A-C) slices through a protection zone at which a tree can wall-off decay. So by making a pruning just outside the branch collar, trees’ natural defenses are left intact. (Fig. 1) What is not simple about this advice is that trees are variable, so there is no simple formula for judging the distance from the trunk or the exact angle for a proper cut. See Figure 2 for variations. Unfortunately, even some university teachers advise their students to leave a short stub (Fig. 1. D), but this will lead to decay. In general, look for the swelling of the branch collar and cut just outside it. If you can’t see a swelling, find the bark ridge (Fig. 1. A), and begin your cut just outside that ridge, sloping the cut out, usually less than 90 degrees from the branch. (Fig. 1. A-B) In most cases it is better to remove the branch in two steps; first take of most of the branch and second, remove the remaining stub. This will prevent splitting and tearing of the bark. Armed with this knowledge, you can remove dead, broken and diseased branches, vertical-growing sucker shoots and rubbing branches.
Whereas learning to make a proper cut is science, learning to create a strong structure is part science and part art. The science is learning what makes a strong crotch, the union of a branch to the trunk or to a larger branch. Basically the strongest branches are at a 60 to 90 degree angle from the trunk. This sounds counter-intuitive since we would normally think that a branch that stands out perpendicular to the trunk would be more likely to break. However the greatest possibility for weakness occurs in branches that are at a 30-degree angle or less, because with these, the wood fibers run parallel rather than interlocking. You can easily tell if a crotch is weak by looking closely where the branch is connected. If the bark is pushed up into the bark ridge (Fig. 1. A), the union is strong. If the bark is folded in, forming a crack, the union is weak, and the branch is likely to fail sooner or later. The most dramatic example is called a co-dominant leader. (Fig. 3) In this case, two branches arise from the same place on the trunk and grow up nearly parallel. You will almost always find the bark folded in between the trunks. As the two trunks grow, they reach for light, leaning away from each other. This makes them vulnerable to heavy wet snows and strong winds, which can cause the tree to split down the middle. (Fig. 4) This usually means the death of the tree.
There are two approaches to dealing with branches with weak crotches and co-dominant leaders. One is to remove the weak branch or less important trunk. This is easiest and least harmful to do when a tree is young. If removing the entire branch or trunk would be too severe, the weak branch or leader can be dwarfed by shortening the branch significantly. This is called a training cut and can also be used to dwarf the height of a tree while it is still young. See figure 6 for the proper method.
The art of creating a tree with a strong structure is learning how to recognize balance and proper proportion. Young trees that are fertilized and over watered often shoot up and become gangly and vulnerable to breakage. Whether a branch is strong or weak is relative to the proportion of length to diameter. This varies with the type of tree, but roughly, a 1″ diameter branch 4′ long can be strong, whereas a 1″ branch 8′ long will be weak. In terms of the overall structure, remember that the trunk is the pillar holding up the entire tree, so the better the top is balanced over the trunk, the stronger it is. If the highest point of the tree is far from being directly above the trunk, the tree is not balanced and will be weaker. The time to correct this is when the tree is young, by pruning the wayward leader back to a branch that will direct the growth more over the trunk. In general, round and conical are the most stable forms in terms of strength.
Storm damage can also be prevented and health supported by removing crowded and rubbing branches. This thinning is best done when the branches are small, and never remove more than a third of the branches.
Especially in very young trees, every leaf adds to their photosynthesis. But also remember that we live in Colorado with high winds and wet snows that sometimes catch our trees in leaf, so it is good to prune to more compact forms than would be necessary in California or even Iowa.
In general, if the proportion of a tree, height to width is pleasing or beautiful, it is stronger. If it is awkward or ugly, it is weaker. And the same is true for individual branches. Be patient with young trees because they often have an adolescent phase before they develop symmetry and real beauty. Be gentle and not too aggressive; don’t top them or chop them. And since it takes little time to prune a young tree, and since they can change so quickly, plan on doing some corrective pruning every year or every other year as needed. Watch the structure as it develops and responds to your pruning. Use your imagination to visualize how each branch will grow. Pruning can be artful, creative and fun. Before long, your care and insight will take form in a massive being that will tower above you and your children and your house, providing shade, protection, character and beauty, and putting that greenhouse gas, carbon dioxide, to a constructive use.