Birch trees make elegant landscape decorations, with their thin, papery bark. Like all trees, birches benefit from pruning to control their size and shape, and to stay healthy. A major insect pest and a spring sap flow greatly limit the pruning window for birches.
Pruning in the early spring can cause your tree to weep, or ooze sap. To bypass the sap flow, avoid trimming birches when the tree is dormant, which is before the leaves develop. However, summer is also a poor choice for birch pruning due to the bronze birch borer, a prime pest of birch trees. This leaves mid spring or autumn as good times to prune birch trees.
The United States Department of Agriculture recommends gardeners not prune birches from May 1 to August 1 because of the bronze birch borer. Fresh wounds on a birch tree attract this pest, which bores inside the wood. Borers are lethal to birch trees, and can kill them within one season. If you have to prune during the May-to-August window -- for example, if a limb gets damaged in a storm and you need to cut it off -- treat the cut with insecticide to kill any borers that try to invade.
When pruning a birch, do not remove more than one-third of the growth in a given season. Check your tree for dead or damaged branches, and remove these first. Next, cut off limbs that rub against other limbs, or vertical growth that can interfere with tree branches. Thin out the canopy by removing wood from crowded areas to promote air circulation. Clip back long branches to a Y-intersection to control the tree's size.
While you may not want to perform a heavy pruning each year, you should check your birch tree annually. Dead or diseased wood that remains on the tree can rot or spread disease to healthy branches. At the bare minimum, remove unhealthy wood every year to protect your birch tree. When you remove dead or diseased wood, clean your pruning tools after each cut with a disinfectant spray or 10-percent bleach solution, so you don't spread the disease.