Maintaining healthy trees
Diseased wood can be a source of infection that can move through the tree and dead wood attracts insects and other diseases.
“Pruning shapes, controls the size and spread of a tree, removes broken branches and prevents weak crotches that are subject to breaking under fruit or snow loads,” said Shelley Barkley, horticulture information officer with Alberta Agriculture. “Pruning also promotes flowering and rejuvenates old shrubs.”
When wounds are made, trees produce a callus to cover the wound. If the wound isn’t covered before disease sets in, the tree forms compartment walls. These walls stop the spread of disease into the tree. Trees vary in their ability to compartmentalize, so make the smallest wound possible to encourage speedy callus formation.
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“It’s important to understand how the tree reacts when pruned,” said Barkley. “Tree branches grow longer from the bud at the end of the branch. This bud is dominant over all the other buds on the branch. When the terminal bud is removed, a nearby bud is then forced to take the lead. New branches grow in the direction the terminal bud points, so cutting to an outward facing bud forces plant growth outward.”
When a plant or tree is cut back severely, the nutrients that sustained the length of that branch exceed the needs. The plant directs the nutrients into new vigorous growth like water sprouts and suckers. This also results in the plant setting fewer but healthier flower buds that produce large flowers. This can be an advantage when growing prize winning flowers.
Prune trees at early age
“Prune annually, leaving the branch collar intact on the tree,” adds Barkley. “The branch collar is the swollen area at the base of the branch where it comes off the trunk or another branch. Begin pruning trees while they are young, since the wounds callous faster and there is less stress to the tree.”
Pruning is usually done in the dormant season (end of March to end of April) or during early summer to invigorate tree growth. Spring or summer pruning dwarfs growth. Pruning in late summer (mid-July to August) forces the tree to grow rather than harden properly for winter, which can increase the chance of winterkill. Fall pruning can lead to winter damage to the cambium tissue around the pruning wound. Some trees, such as birch and maple, are bleeders. They have a high sap pressure in the spring and need to be pruned once the tree has leafed out, in June.
Make clean cuts and leave no stubs. Make cuts above a bud that is growing in the direction you want the branch to take. To choose the correct angle, place the pruning tool so the top of the cut is slightly above the top of the bud and the bottom of the cut is even with the bud’s bottom.
– Alberta Agriculture
