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 occasionally has been known to provide a good run under less perfect conditions.Sometimes sap flows as early as January or as late as May, but in Minnesota, sap usually runs from about March 15 to April 20.
