By Rachel Foebel, Master Gardener in Training

Recently, I have grown to love maple syrup for more than just its delicious taste. The harvesting and boiling of the sap brings me outside in the fresh air during a time of year when I need it most. But I’ve wondered what makes this harvest season so short, so specific and so unique.

First, the Sap

During the growing season, sugars needed for tree growth are created via photosynthesis and stored in the tree as starches in the sapwood. When the weather cools, starches are converted back to sugars and passed into water flowing through the xylem of the tree, creating sap.

How Water Moves Through the Tree

Typically, water moves up the tree from the roots through cell channels called xylem. Since the xylem is made up of dead elongated cells, these cells don’t contribute to the movement of the water, they rely on pressure gradients. In maples, a unique pressure is present in the springtime, but only when it freezes at night and thaws during daytime.

Maple Magic in Spring Time

Just outside of the xylem vessels are dead wood fibre cells with air inside. When the temperature drops in the evening, the smaller branches at the top of the tree freeze first, and sap frost collects in the air-filled wood fibre cells surrounding the sap-filled xylem. This creates suction, pulling sap up the tree. When the temperature rises in the morning, the frost melts and falls down the tree through the xylem by both gravity and pressure created by the warming air in the wood fibre cells. This is when you get to collect!

What about Fall?

But wait! We don’t just get freezing at night and above freezing daytimes in the spring. That happens in the fall too. Can you tap maple trees and make syrup in the fall too? The answer is yes, but researchers have found you won’t get as much as springtime, and the sap won’t be as sweet. Additionally, it will reduce the sweeter springtime sap yields as well. Worth it? Probably not.   

The Pros Know What They’re Doing

Tapping only once a year also helps to reduce injury and therefore risks to the tree caused by severing its protective layers and exposing it to external factors. Hobbyists and large-scale producers alike want to foster a healthy tree that provides delicious syrup for years to come.

References

Giesting, K. (2020). Maple Syrup. Climate Change Resource Centre. www.fs.usda.gov/ccrc/topics/maple-syrup

Merhaut, D. J. (Feb, 1999). How do large trees, such as redwoods, get water from their roots to the leaves? Scientific American. https://www.scientificamerican.com/article/how-do-large-trees-such-a/

Tyree, M. (Jan, 1984). Maple Syrup Exudation: How it Happens. Maple Syrup Journal. 4(1). 10-11.https://www.uvm.edu/~uvmaple/maplesapexudation.pdf