Such a Sap

Q:  One of the sure signs of spring is when tree sap begins to run and the tradition of maple syrup-making begins. What determines when and how tree sap runs and what is the process that turns sap into syrup?

A:  Maple sap runs best when daytime temperatures are in the high 30s to mid-40s and overnight temperatures are below freezing. This cycle of above-freezing days and below-freezing nights needs to continue for several days, although nature has been known to occasionally provide a good run under less perfect conditions. Some sap may flow as early as January or as late as May, but the typical time for a "good" sap run in Minnesota is March 15 to April 20. Sap is converted to syrup by boiling off most of the water content of the sap, which leaves the sugar and flavor behind.  It usually takes 30-40 gallons of sap from a sugar maple to produce one gallon of pure maple syrup.
— Dave Crawford, Wild River State Park naturalist