During the last week of February, I was out pruning some trees and got to experience an amazing sap flow on one of the river birch trees we have planted in our front yard. Usually, I prune my trees earlier in the winter, but got behind with some other chores this year. When I removed a couple of low-hanging branches on the river birch, I was immediately drenched in a heavy sap flow. It literally appeared as though someone had turned on a water faucet. This heavy sap flow continued for several minutes while I was working in the area.
Eventually, the sap flow on my river birch trickled down to a slow, steady drip after a few days. My return visit a week later revealed some busy insects feasting on the sugary sap. The sap had attracted a variety of ants, flies, bees, and wasps—all harmless to trees.
This kind of sap flow reminded me of how maples are tapped for syrup. My grandpa, Jesse Currier, was a maple syrup farmer in Orford, New Hampshire back in the 1940’s. I can only imagine the amount of time and effort that went into tapping maple trees by hand, collecting buckets of sap, and boiling it down with a wood burning stove.
The ability of trees to hold this much water and move it from roots to shoots each spring is physiological marvel. According to the University of Maine Extension, the yield of sap varies greatly with the method of tapping, the size of the tree, and seasonal differences. The average yield for a taphole is from five to 15 gallons. However, under favorable conditions, a single taphole can produce as much as 40 to 80 gallons of sap in a single year. It takes about 10 gallons of sap to produce one quart of syrup!
The sugar content of sap produced by different trees in a grove can vary a great deal. The average maple tree produces sap with a sugar content of two or three percent. Sugar maple trees, also known as rock or hard maple, are usually the best producers. Red maples and river birch also produce sweet sap, but have a lower sugar content and take a long time to boil down into syrup. Prolonged exposure to heat from boiling causes the glucose and fructose in birch sap to develop a very strong molasses-like flavor, which is less appealing to most consumers.
Understanding how maple sap is formed requires some knowledge about tree physiology. In the later summer and fall, maple trees virtually stop growing and begin storing excess starches throughout the sapwood, especially in cells called ray cells. This excess starch remains in storage as long as the wood remains colder than about 40 degrees F. Whenever wood temperatures reach around 40 degrees F, enzymes in the ray cells change the starches to sugars, largely sucrose. This sugar then passes into the tree sap.
As the temperature increases to about 45 degrees F, the enzymes stop functioning and sugar is no longer produced. In March and April, the sugar changes back to starch—except during periods of sap flow. Rising temperature creates pressure inside trees, causing sap to flow. When a hole is bored into a tree (or a branch is pruned out), wood fibers that are water-carrying vessels are severed, so sap drips out of the tree.
Every year our office receives phone calls about pruning maples, birches, or grape vines and the alarming amount of sap that comes from these plants. However, rest assured that this type of sap flow is perfectly normal and will not harm the tree. Eventually, the tree will stop dripping as spring approaches and those busy little insects will move on to the next source of sap or nectar that flows in your landscape. If you’re really ambitious, you catch some of that sap and boil it down to make a quart of syrup the old-fashioned way.
Paul Pugliese is the Extension Coordinator and Agricultural & Natural Resources Agent for Bartow County Cooperative Extension, a partnership of The University of Georgia, The U.S. Department of Agriculture, and Bartow County. For more information and free farm, lawn, or garden publications, call (770) 387-5142 or visit our local website at extension.uga.edu/bartow.