The pale bark and graceful growth of birch trees make them an attractive addition to any landscape. Unfortunately, birch trees often fail to thrive in cultivation. While a healthy birch tree can live 40 to 50 years, many birches planted in gardens die before reaching 20 years. Choosing the proper site is essential for the health of birch trees, as is a sound knowledge of pruning practices.
Gardeners prune trees to repair damage, to avoid damage and to improve aesthetics. A tree is damaged any time the bark is broken, since that leaves the tree vulnerable to both infection and infestation by insects. Check trees after severe wind or hail, when breakage is common. Damaged limbs should be removed immediately because the tree will heal more quickly with a clean wound.
Always cut the limb flush with the trunk of the tree. Stumps left behind will die back, and the rotten wood will attract insects. Small branches should be pruned flush with the larger limb. If the branch is too large to be cut with hand loppers, use a pruning saw. Make a small cut on the underside of the branch to prevent the weight of the branch from stripping away bark when it falls, then cut from the top of the branch. Large, heavy branches, or any branches near utility wires, should only be pruned by professionals. Never paint the wound or apply wound dressings, as this old-fashioned practice only delays healing.
Never prune a birch tree between May 1 and Aug. 1 (in the Northern Hemisphere). During this period, the bronze birch borer is in flight, and female borers are attracted to the fresh wounds left by pruning. If the tree is damaged, making pruning necessary, paint the wound with an insecticide. Repeat the application about every 30 days until after Aug. 1. The birch borer is a real tree killer, so avoiding infestation is vital to the health of the tree. Late winter, trees are dormant, is the best time to prune most trees, and birches are no exception. The tree suffers less disturbance of its growth cycle if pruning is done during this time.
Trees are more resistant to damage and need less pruning if they get good care. Choosing the proper site for birch trees is particularly important. They require half to full-day sun, but they must have cool, moist soil. This tree cannot tolerate soil that is very wet or very dry. Birch trees have shallow roots, and they need a light soil. Compacted soil will limit root growth. To avoid root damage, plant where there will be little or no traffic over the roots.
Mulching keeps the soil cooler and will conserve moisture. Spread 2 to 5 inches of mulch over the soil around the tree. Keep the mulch slightly away from the trunk so it does not touch the bark. Extend the mulch circle as far away from the trunk as the branches extend overhead; this is about as far as the roots extend horizontally below the soil. When conditions are very dry, water thoroughly once a week. Fertilize lightly in late spring if desired.