What Month to Prune a Birch in Wisconsin?

Birch Sap

• Depending on species and variety, birch trees are hardy in U.S. Department of Agriculture plant hardiness zones 3 through 9. Despite their differences, they respond to Wisconsin's cold winter temperatures by becoming fully dormant. As spring approaches and temperatures rise, birch sap begins to flow within the tree. This happens when temperatures rise above freezing, but before leaf buds begin to break on the trees. While sap can be collected and boiled down into tasty syrup, it can make pruning at this time messy. The sap will run freely, and dried sap will stay on your tree.

Birch Pests

• Bronze birch borer is a serious pest that birch trees battle. This insect can severely injure and kill a birch tree in a matter of years. Environmental stresses such as improper site conditions or care make birch trees susceptible to borers. While proper pruning should not draw insects to a healthy tree, it can stress an already unhealthy tree even more. Bronze birch borer larvae overwinter beneath birch bark. In Wisconsin, the adults emerge in May, June and July. Pruning during these times can inadvertently stress your tree, making it susceptible to borers and attract other insects to the wounds.

When to Prune

• With young or newly planted birch in Wisconsin, always allow at least one to two years for the tree to become established. Waiting helps your tree develop healthy, extensive roots. Mature trees that were shaped in their formative years don't require pruning unless damaged or diseased. Pruning when sap is running in late winter or early spring -- the exact time depends on weather -- won't harm your birch tree, but the dried sap can be unsightly. To avoid it, prune in late spring, after leaves are fully expanded but before insects become active in May. Never remove more than one-fourth of the tree's foliage at any time. Once May arrives, insects and pathogens are active. Delay further pruning until fall.

How to Prune

• For young branches up to 3/4 inch in diameter, handheld pruning shears do the task well. Wear protective gloves and long sleeves, and sterilize your blades before and after each cut to prevent the spread of disease. Keep pruning to a minimum; don't alter the tree's natural form. Remove damaged, broken and crossing branches that could interfere with future growth. Place your pruners against the branch you want to keep, with the branch you want removed firmly between the base of the blades, never at the tips. This makes a clean cut that heals quickly. For larger branches, a small pruning saw works well. In Wisconsin and elsewhere, leave large branches on established birch trees to qualified tree care professionals with appropriate safety equipment and tools.