What makes maple sap run a bit of a mystery
March is typically the month when local maple syrup producers have their best sap runs. Maple sap “runs” when environmental conditions are suitable for this phenomena to occur, usually. Those conditions are cold nights, with temperatures well below freezing, followed by warmer, sunny days, with temperatures above freezing.
The mid-March blizzard we just had probably will have little to no effect on the sap run, but it might make it tough to collect and process it for those who still use buckets or pails to collect it by hand. Some places in Greene, Ulster and Schoharie counties have almost three feet of snow on the ground to trudge through, but with the longer days of spring upon us and the stronger
sunlight we get every day, the snow will soon recede.
I asked an “old-time” maple syrup producer when he tapped his trees each spring, and he told me he waited until the snow at the base of the sugar maples had receded to where there was a “doughnut” of bare ground at the base of the tree. Of course, this observation is only valid when we have snow cover for most of the winter, but it is a good indicator that the sap is going to run.
Tapping too soon or too late is a common mistake. If the trees are tapped too soon, the tap holes may clog before the main run, and, if tapped too late, the sap may become “buddy” as the buds begin to swell. Syrup made from “buddy” sap will have a distinctively sour taste. I recall judging maple syrup at the Greene County Youth Fair years some ago and occasionally a batch was entered that tasted buddy. The only thing worse tasting than “buddy” syrup was syrup that also had been burned! I quit judging maple syrup shortly thereafter.
Although maple syrup has been produced for hundreds of years in our region and the process of making it has been highly refined over the past few decades, until recently no one really understood just what made the sap run. We knew the conditions, but not the chemistry or the physics. It is a unique process that defies much of what we know about plant physiology. For most of the year, the xylem vessels conduct only water from the roots upward, while the phloem vessels conduct sugars downward and sideways. In the spring, however, dissolved sugars are moved upward in the xylem water, but why do they squirt out of the holes that we drill in the trunks of maples? If you drill holes in ash, oak or hickory trees or most other hardwoods, you get sawdust, but no sap.
A few other tree species — such as birch, butternut and black walnut — will also “leak” sap if tapped in the spring, but, in these species, it has been determined that it is root pressure that
causes the sap to flow. That is not the case in maples.
As it turns out, it is dissolved carbon dioxide in the cells surrounding the xylem vessels that pushes out the sap. Carbon dioxide gas can create quite a bit of pressure, as anyone who has dropped a warm can of carbonated soda and then opened it up, is well aware! Carbon dioxide gas is far more soluble when the temperature is cold, so when we have
freezing cold nights, it goes into ion.
When it warms up the next day, the dissolved gas volatilizes (becomes gas again) under pressure. The sap that we collect is not being pushed upward
from the roots, but it is actually falling downward from the tree’s limbs during the day, under pressure caused by carbon dioxide gas.
This current theory does not answer all the questions though. Sometimes, sap will run all night, even on freezing nights and sometimes it will also run well on
cloudy, or rainy cold days. Sometimes it does not run at all, even when conditions are ideal. Despite all we now know, the mystery of maple sap run is still somewhat of a mystery.