When the temperature falls to 20-25 degrees F the pressure inside the tree may become negative in relation to atmospheric pressure.
As the maple tree begins to freeze, the sap is drawn up into the tree through the sapwood, which acts as a series of straws, that connect with the tree’s roots.
During the day the rises above 40 degrees F pressure is created inside the tree that pushes the sap back down through the sapwood to the bottom and through its sprouts and roots. During this process, there is an excess of sap and that is what sugar makers gather to boil into delicious maple syrup.
This sap flow continues while the cycle of warm and below-freezing temperatures repeats until the nights are no longer freezing.
Sugar makers generally look for other signs as a signal that the season is coming to an end.
One indication is that the sap may become discolored and have a bitter taste. Another is when the buds on the trees become the size of a mouse’s ear.