The spring sap run in sugar maple (Acer saccharum) is driven by a mechanism that distinguishes this species from most other trees tapped for sap in North America. Unlike the root pressure that drives sap in birch or walnut, maple sap flow results from alternating freeze-thaw cycles acting on gas trapped within wood fibres. Understanding this mechanism helps producers predict when flow will start, how long it will last, and why some days produce high volumes while others yield almost nothing.
The Stem Pressure Mechanism
During winter dormancy, the sapwood of a sugar maple contains both liquid water and carbon dioxide gas held in the wood fibres. When temperatures drop below 0 °C overnight, the water in the outer wood layers freezes. Freezing expands the water volume by roughly 9%, compressing the gas in adjacent fibres. The compressed gas dissolves into the remaining liquid water, lowering the freezing point slightly and drawing water upward from lower in the trunk by osmotic and capillary forces.
When daytime temperatures rise above 0 °C, the ice melts. The dissolved gas comes out of solution and re-expands, generating a positive stem pressure that can reach several hundred kilopascals in actively flowing trees. This pressure drives sap outward through any opening — including a fresh tap hole. The flow continues until the stem pressure equalises with atmospheric pressure, typically within hours of the temperature crossing the freezing threshold.
Research published through Cornell University's maple programme and the University of Vermont's Proctor Maple Research Center has characterised this mechanism in detail. The specific involvement of gas storage in wood fibres (fibre cells) rather than vessels distinguishes A. saccharum flow from simple transpiration or root pressure.
Temperature Thresholds and Daily Patterns
The classic description of a good sap day — freezing nights followed by warm days — reflects the freeze-thaw cycle directly. Flow typically begins within one to two hours of temperatures rising above 0 °C and slows or stops as temperatures drop again in the evening. The volume and sugar content of sap on any given day depend on how cold the preceding night was, how warm the day becomes, and how quickly the temperature transitions.
Typical Sap Flow Conditions
Overnight low: −5 to −10 °CDaytime high: +4 to +8 °C
Flow starts: 1–2 hrs after temp rises above 0 °C
Flow stops: temperature drops below 0 °C or stays warm
Days where temperatures stay above freezing throughout the night — which occur as the season progresses into April — do not produce the necessary freeze that sets up the next day's pressure. The tree essentially needs to re-charge overnight. Extended warm periods with no overnight freeze will pause the run entirely, even if taps are open and the trees appear healthy.
Effect of Night Temperature Depth
Very cold nights (below −15 °C) can produce the opposite effect: the freezing penetrates too deeply into the wood, and the subsequent thaw is slow. Flow on the following day may be delayed or lower than expected. The highest-volume days tend to follow nights in the −5 to −10 °C range, when the freeze is thorough but not excessive.
Season Timing by Region
The maple season runs roughly six to eight weeks across Canadian producing regions, but the calendar dates shift significantly from south to north and from inland to coastal locations.
- Southern Ontario (Grey County, Bruce Peninsula): Season typically opens in late February to early March and may close by late March in warm years. Frost-free periods between runs can occur frequently.
- Ottawa Valley and Lanark County: Mid-March to early April is the typical window. The continental climate produces more reliable overnight freezes than southern regions.
- Quebec (Laurentides, Estrie, Chaudière-Appalaches): Generally runs from mid-March through mid-April, though production in the Laurentides region north of Montreal can extend into late April in cold years.
- New Brunswick and Nova Scotia: Coastal moderation means milder nights, which can reduce freeze-thaw frequency. The season tends to start later — often mid-March — and the run may be shorter than in continental Quebec.
Sugar Content Variation Through the Season
The sucrose concentration of maple sap typically ranges from 1.5% to 3% by mass, with averages around 2% used for production estimates. Higher sugar concentrations reduce the volume of sap required to produce a given volume of syrup; the commonly cited ratio of 40 litres of sap to 1 litre of syrup assumes approximately 2% sugar content.
Early-season sap from well-rested trees tends toward the higher end of the sugar range. As the season progresses and stored starch is metabolised, the sugar concentration may decline. Sap collected after bud break contains elevated amino acids and different sugars that affect both colour and off-flavour development during evaporation.
End of Season Indicators
The sap run ends when sustained daytime temperatures above freezing eliminate overnight frost. At this point, microbial populations in the sap increase rapidly, reducing sugar concentration and introducing off-flavours. A second indicator is bud development: once buds begin to swell and open, the sap chemistry changes and syrup produced from it carries a "buddy" flavour that fails grading standards. Most producers monitor both temperature forecasts and bud stage to determine when to pull taps and close the evaporator.