Sap Flow

/Home/Garden Basics/Trees/Trees: Problems & Solutions/Tree Sap

One of the great mysteries of maple syrup is what causes the sap to flow out of the trees

. Following is an explanation of how temperature fluctuations and pressure and suction in the tree cause sap to flow.

Early in the spring, when the maple trees are still dormant, temperatures rise above freezing during the day but drop back below freezing at night. This fluctuation in air temperature is vital to the flow of sap in sugar maple trees.

What causes the sap of maple trees to flow in the spring? During warm periods when temperatures rise above freezing, pressure (also called positive pressure) develops in the tree. This pressure causes the sap to flow out of the tree through a wound or tap hole. During cooler periods when temperatures fall below freezing, suction (also called negative pressure) develops, drawing water into the tree through the roots. This replenishes the sap in the tree, allowing it to flow again during the next warm period. Although sap generally flows during the day when temperatures are warm, it has been known to flow at night if temperatures remain above freezing.

Cornell Sugar Maple Research & Extension Program