Not Up or Down But Sideways

By Larry Rettig (LarryR) on March 10, 2012

A sure harbinger of spring, and perhaps the earliest one, is the flowing of sap in sugar maples. Interestingly, the notion that sap rises up the tree from the roots in spring and goes back down to the roots in fall is completely erroneous.

Tapping vessels collect the maple sap

Rather, the sap travels sideways.  It moves outward to the bark during the spring and inward toward the center of the trunk and branches in the fall.

But, you say, the sap actually flows out into buckets when the tree is tapped in the spring, so it has to be rising in order for it to flow, right?  Here’s how it works.

When external temperatures rise above freezing, pressure develops in the tree.  This pressure causes the sap to migrate from the inner wood outward toward the bark.  The pressure is great enough to cause the sap to flow out from the bark and sapwood if the bark is ruptured, as it is when we pound a tap into it.

Everything hinges on pressure within the tree, or the lack thereof.  This pressure is created by a unique mechanism.  In the daytime, cell activity in the sapwood produces carbon dioxide.  This gas expands into the spaces between cells.  Already existing carbon dioxide dissolved in the sap is also released into these intercellular spaces.  A third source of pressure is something called osmotic pressure (a chemical/physical process) and is generated by the presence of dissolved sugar in the sap.

Sap about to drip off tap

At night, or at any other time when the temperature drops below freezing, the opposite happens.  That is, suction develops instead of pressure, through the cooling carbon dioxide gas as it shrinks.  Some of it also redissolves into the sap.  The suction created draws water up into the tree, which chemical processes then turn into sap.  The sap begins to move horizontally back out to the bark once more when the temperatures warm.

The duration of this whole process is about six weeks, depending on the ups and downs of the temperature.  When the temperature stops fluctuating between freezing and thawing, the sap stops flowing.

Once the sap is harvested and heated, a reduction process—mostly by the elimination of water via steam—turns the sap into syrup.  The flavor of this syrup is quite complex.  Our senses can’t tell us so, but through chemical analysis around 300 different natural flavors have been identified in maple syrup.  They include such common flavors as sugar, caramel, vanilla, nutty, buttery, floral honey, chocolate, and coffee.

Native Americans making maple syrup

The process for making syrup was already known to Native Americans in the 1600s.  The English chemist, Robert Boyle, documented this fact on a trip he made to the United States in 1663.  English colonists adopted the practice, and the maple syrup industry has flourished ever since.

Once you have the sap, making your own syrup is a relatively simple process.  Here are the steps:

1. Bring the sap to boiling in a large pan.
2. Take care not to let the syrup burn in the bottom of the pan.
3. As the sap is boiling, skim off any foam or foreign matter that might be floating on the surface.
4. Using a candy thermometer, boil the sap until the temperature rises to at least 219 degrees F.  Boil it a bit longer if you want the syrup thicker.
5. Once the syrup has reached the desired consistency, remove it from the heat and pour it into a glass container to let it cool.  During the cooling process, you will be able to see any remaining particulate matter that has settled to the bottom of the container.
6. Once the syrup has reached room temperature, pour off the 'good' syrup into a fresh storage container, leaving any impurities behind.
7. Enjoy!

Modern-day syrup making

Commercial maple syrup is produced in the northeastern United States and eastern Canada, but nowhere else in the world.  Although syrup isn’t produced, in some regions of Korea folks tap a species called Painted Maple (Acer pictum subsp. mono) and drink its sap raw.  (Birch trees are sometimes tapped in Alaska and Siberia but the sugar content in the sap and the general quality is much lower than in maple sap.)

So the next time you pour that maple syrup—whatever its source—over your pancakes or waffles, take a moment to appreciate this wonderful substance, brought to us complements of Mother Nature and human ingenuity.


Images are courtesy of and are used under the terms of the GNU Free Documentation License. 



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About Larry Rettig
"An enthusiastic gardener for over 50 years, my first plant was a potted Ponderosa Lemon tree ordered from a comic book ad at age 15. I still have it, and it’s still bearing lemons! My wife and I garden on 3/4 of an acre, both flowers and vegetables. Our garden, named Cottage-in-the-Meadow Gardens, is private and is listed with the Smithsonian Institution in its Archives of American Gardens. It is also on the National Register of Historic Places. We garden organically and no-till. Our vegetable garden contains a seed bank of vegetables brought to this country from Germany in the mid-1800s by my ancestors. My latest book, Gardening the Amana Way, is available at

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Great article CarolineScott Mar 14, 2012 8:01 PM 5
Export patgeorge Mar 12, 2012 12:03 AM 3

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