Birches are high maintenance trees with characteristic peeling bark. Though they can live up to 50 years, most will not survive more than 20 years as a landscape tree. They are highly susceptible to insects and sensitive to environmental changes. When pruning the top of a birch tree, timing is the most important factor. Improperly timed pruning will lead to birch borer or beetle invasions. The process of trimming the top of a tree is known as heading back.
Prune the birch tree in late fall, after the leaves have fallen and the tree is dormant. Never prune a birch between May and August. This is when the birch borer is most active, and the insects are naturally drawn to sap-producing pruning wounds.
Prune back select branches from the top of the birch tree. Identify less than 25 per cent of the branches to be pruned. Leave the rest for next year. If the pruning isnt' spread out over several years the tree could become overly stressed and die.
Cut back the selected branches to a lateral branch or one that grows out sideways from the main branch. The lateral branch should sit at an angle less than 45 degrees to the main branch being cut. The lateral branch also should be at least half the diameter of the branch being cut. Do not cut the top branches back more than 25 per cent.
Make the cuts at 45-degree angles, using pruning shears. This will prevent water from gathering in the wounds of the upward-facing branches.
Do not dress the pruning wounds with tree wound dressing. It is ineffective at closing wounds and will not keep out insects.