Why Sap Runs: The Freeze–Thaw Cycle Explained Simply

Sap doesn’t run randomly, and it doesn’t run just because a tree is tapped. Maple sap flows only when very specific weather conditions occur. Understanding why sap runs explains why maple season is short, unpredictable, and so dependent on temperature.

This article explains the science behind sap flow in plain language— what happens inside the tree, why freezing nights and warm days matter, and why even a perfect-looking season can suddenly shut down.

For the full Michigan-focused overview, see: Michigan Maple Syrup: Season, Sap Flow, Production & Visiting Sugarhouses .

Short Answer: Why Does Sap Run?

Sap runs because alternating freeze and thaw temperatures create pressure changes inside the tree. These pressure changes move sap up and down the trunk, allowing it to flow out when a tap is present.

If nights don’t freeze, or if days don’t warm up, the pressure system fails—and sap stops running.

What Happens Inside a Maple Tree During Freezing Nights

When temperatures drop below freezing at night, gases inside the maple tree contract. This creates negative pressure (a partial vacuum) within the tree.

At the same time, water is drawn from the roots into the trunk and branches, replenishing sap inside the tree.

According to Michigan State University Extension , this freezing phase is essential for resetting the tree’s internal pressure system.

What Happens During Warm Daytime Temperatures

When daytime temperatures rise above freezing, the gases inside the tree expand. This expansion creates positive pressure, forcing sap to move outward.

If the tree has been tapped, this pressure pushes sap out through the tap hole. Without this warming phase, sap simply stays inside the tree.

This is why strong sap runs usually happen on days with temperatures in the 40s or low 50s°F following a freezing night.

Why Sap Needs Repeated Freeze–Thaw Cycles

One freeze and one thaw are not enough. The strongest sap runs occur when freeze–thaw cycles repeat over several days.

• Freezing nights reset pressure
• Warm days release pressure
• Repeating cycles maintain flow

Extended warm spells prevent pressure from rebuilding. Extended cold spells prevent sap from moving at all. Either condition can stop sap flow completely.

Why Sap Eventually Stops Running Each Season

Even with perfect weather, sap does not run forever. As spring progresses, maple trees begin to shift energy toward bud development.

When buds swell, the chemistry of the sap changes. This produces off-flavors in syrup and signals the natural end of the season.

This biological shift—not a calendar date—is what ultimately ends maple season.

Why This Matters for Maple Syrup Production

Because sap flow is weather-driven, maple syrup production varies from year to year. Some seasons produce large volumes in a short window. Others stretch out with fewer strong runs.

This variability is why maple syrup is inherently seasonal and why production planning requires experience, flexibility, and readiness.

To see how sap is collected and concentrated into syrup, read:  How Maple Syrup Is Made.

Seeing Sap Flow in Action

Visiting a sugarhouse during active sap flow allows you to see this process firsthand— sap moving through tubing, collecting in tanks, and being boiled into syrup.

You can explore maple events and tours here: Events and Farm Tours.