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How it works

Why does sap only run on some days?

Sap runs best when a freezing night is followed by a day that warms above freezing. Here is what is happening inside the tree.

The short version

Sap runs when nights drop below freezing and days climb above it. University of Maine Extension describes the classic pattern as daytime temperatures of about 4 °C (40 °F) or warmer after a freezing night. If it stays frozen all day, or stops freezing at night, the flow slows or stops.

Step through a freeze-thaw cycle

Illustration · simplified, not a forecast

Simplified drawing of a maple trunk with gas bubbles in the wood and a spout wood (sapwood) with gas bubbles spout

1. Freezing night

As the wood cools, gas in the wood shrinks and dissolves, and pressure falls. The tree draws water in. Ice then forms in the wood and traps the remaining gas, squeezing it.

2. Thawing day

As the wood warms above freezing, the ice melts and the squeezed gas expands. Pressure inside the trunk rises, and sap is pushed out wherever there is an opening, such as a taphole.

3. No more freezes

Without a freezing night to reset the cycle, pressure does not rebuild the same way and runs fade. Once buds break, the sap’s flavour changes too.

Try it: is it a sap day?

Rule of thumb · not a forecast

Enter tonight’s low and tomorrow’s high.

Real runs also depend on sun, snow cover, how many freezes came before, and the individual tree. That is the kind of local reading the MapleSAP app is being built to help with.

Animated chart from the MapleSAP app onboarding: a temperature line moving between freeze and thaw, with pressure bars rising after each thaw.
“Read the Rhythm” from the app’s onboarding (in development). An illustration of the idea, not a measurement.
Want to go deeper? Is this fully understood?

Not completely. The gas-and-ice explanation above is the leading one in textbooks. Researchers have also found that sap only flows if it contains enough sugar, so osmosis (water moving toward sugary sap) likely plays a part. Mathematical models combine ice melting, gas dissolving and osmosis. More on the deeper science.

Sources

  1. University of Maine Cooperative Extension, Bulletin #7036, How to Tap Maple Trees and Make Maple Syrup (2025 update). extension.umaine.edu
  2. S. G. Saupe, College of St. Benedict / St. John’s University, Biology of Maple Sap Flow (course notes citing Kozlowski & Pallardy, 1997). faculty.csbsju.edu
  3. University of Vermont Libraries, Sugar Maple: Sugaring — Physiology. libraryexhibits.uvm.edu
  4. M. Ceseri and J. M. Stockie, A mathematical model of sap exudation in maple trees governed by ice melting, gas dissolution and osmosis (2012). arxiv.org