How to Prune & Care for Young Trees
by Mikl Brawner:
Trees have it hard in Colorado. If it’s not the shallow and lean topsoil, it’s the low rainfall and low humidity, or the heavy, wet, late spring or early fall snows, or the dramatic temperature changes. Of all our plants, trees take the longest to develop, 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 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.
Learn How to Make a Correct Pruning Cut
Correct pruning is of 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. He 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 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. Since trees are variable, there is no simple formula for judging the distance from the trunk or the exact angle for a proper cut.
Unfortunately, even some university teachers advise students to leave a short stub, 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 and begin your cut just outside that ridge, sloping the cut out, usually less than 90 degrees from the branch. In most cases it is better to remove the branch in two steps to prevent splitting and tearing of the bark. First take of most of the branch and second, remove the remaining stub.
Train Young Trees for Strong Structure
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 the wood fibers run parallel rather than interlocking. The most dramatic example is called a co-dominant leader, where 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, making them vulnerable to heavy wet snows and strong winds, which can cause the tree to split down the middle. This usually means the death of the tree.
One approach to dealing with weak crotches and co-dominant leaders is to remove the weak branch or less important trunk. This is easiest and least harmful to do when a tree is young. If this would be too severe, the weak branch or leader can be dwarfed by shortening the branch significantly. This “training cut” can also be used to dwarf the height of a tree while still young.
The art of creating a tree with a strong structure is learning to recognize balance and proper proportion. Whether a branch is strong or weak is relative to the proportion of length to diameter. Roughly, a 1" diameter branch 4' long can be strong; 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; the better the top is balanced over the trunk, the stronger it is.
Especially in very young trees, every leaf adds to photosynthesis. But since we live with high winds and wet snows that sometimes catch our trees in leaf, it is good to prune to more compact forms than in California or even Iowa. But never remove more than a third of the branches.
How to Avoid Common Hazards
Don’t let lawn mowers or weed trimmers touch the bark. This tearing of the bark, called “lawn-moweritis” often causes 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. This will also prevent cats from using a young tree for a scratching post, which is very harmful. If deer visit keep a circle of fencing around a young tree for three or four years. If planted too close to a street or sidewalk, people will start breaking off branches. If you plant under an electrical line, Public 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 or expect the tree to live on Colorado rainfall. Don’t prune aggressively or leave the pruning to nature. Don’t pile mulch against the trunk or let the soil bake with no mulch. Don’t plant too deep or too high; plant right where the trunk begins to flare into the root. The best pruning cannot make up for these hazards.
Mikl Brawner was an arborist for 35 years. He and his wife, Eve, are co-owners of Harlequin’s Gardens, specializing in organic veggie starts and herbs, natives, sustainable roses, xeriscape, unusual perennials, and products to build healthy soils.