Why Does Sap Flow from Maple Trees? Throughout the maple region, there are several weeks of alternating freeze and thaw temperatures each spring. This weather provides the right conditions for sap flow in maple. Unlike most trees, maples have tiny air-filled cells in their wood. Freezing causes ice to form inside these cells. As this frost thickens, a suction is created which pulls water from the soil into the tree. At the same time, the gas bubbles in the cells are compressed. When weather warms, the frost melts and the gas bubbles expand, causing pressure to form in the tree. This pressure pushes sap out of tap holes drilled by maple producers. In order for the sap to keep flowing, another freeze and thaw cycle must follow within in a day or two.