When Does Birch Sap Flow?

Dormancy

The start and end of the annual upward flow of sap from the roots into the trunks and branches of birch trees varies by latitude and elevation. Little or no sap flows from the time all leaves drop from the tree and daytime temperatures no longer get above 35 degrees F. Even though no leaves are present in mid- to late winter, repeated daytime temperatures in the range of 40 to 60 degrees can end the winter dormancy. However, a premature up-flow of sap when winter temperatures drop down well below freezing can cause branches and twigs to crack from the expansion of water molecules if sap freezes.

In the southern and central United States, where winters are cool to cold, birch trees may start a sap flow as early as February, with flowers occurring in March. Farther north and in the highest mountain elevations, sap flow may not start until April or early May, depending on when brutal winter temperatures finally subside. Conversely, leaf drop, onset of subfreezing temperatures and tree dormancy begins in October in northern states, but delays until November or early December farther south.

Sap bleeding is pronounced on birch trees if pruned or wounded, especially from late winter to late spring. While the sap bleeding is not harmful to the birch, the liquid can look unsightly or stain the attractive bark. Pruning birches in late winter and early spring before leaves appear is horticulturally acceptable, but usually pruning is delayed until late summer to avoid copious sap bleeding issues altogether.

Another reason horticulturists in the United States avoid pruning birches in spring to summer is the threat of the bronze birch borer insect. When female insects are mating and laying eggs, they seek out wounds in birch trees to lay eggs -- pruning wounds and the smell of sap attracts the pests. As the eggs hatch, the larvae bore into the pruning wounds, causing damage to vascular tissues and leading to branch dieback. Do not prune birch tree species susceptible to bronze birch borers between May 1 and August 1.