The much-loved sugar maple tree provides food and shelter for wild animals, leafy shade in the summer, spectacular colors in the fall, firewood in winter, and the finest syrup in early spring. Learning to know sugar maples better and understanding how they produce the sap for the syrup we love so well can only increase our appreciation of these delightful trees.
Maples are easy to identify if you take a closer look at their growth habit. They are one of the few trees in our forests with buds, twigs, and sometimes branches that grow directly opposite each other. This opposite arrangement occurs in only a few large trees. It’s easy to remember these with the phrase MAD Horse: M for Maple, A for Ash, D for Dogwood (mostly shrubs in New England), and Horse for Horse Chestnut. All of our other native trees have an alternate arrangement of buds, twigs, and branches.
There are eight different species of maple in New England, seven of which are native. The best known is the sugar maple. It is the maple species most commonly tapped to make syrup because it has the highest concentration of sugar in its sap. Black maple is similar to sugar maple and is also tapped for its sweet sap, but it is uncommon in New England, growing mostly in the Midwest and Great Lakes region. Red maple, named for its brilliant red foliage, is very common and occurs in a variety of habitats, from swamps to upland forests. It is sometimes tapped, even though its sap has half the sugar concentration of sugar maple. The other maples are rarely tapped since their sap contains little sugar. Striped maple and mountain maple are small trees in the forest understory that provide an important food source for wildlife. Silver maple and box elder (also called “ash-leaf maple”) are common along stream banks and floodplains. Norway maple, introduced from Europe, was extensively planted in urban areas and can now be found growing in the wild. At sugaring time, these eight species can be identified by differences in the color, shape, and size of their buds and twigs, as well as by their distinctive bark.
Wood is made up of inner heartwood, which gives strength to the tree, and sapwood, which transports water and nutrients from the roots to the branches. The sapwood (xylem) is surrounded by the inner bark (phloem), which carries the starches and sugars made in the leaves to the rest of the tree. Between the sapwood and the inner bark is the cambium, a microscopic growth layer where new sapwood and phloem are made each year. In maples, the sapwood is made up of vessel elements, cells like pipes that transport the sap, and fiber cells that surround them. In other hardwoods, these cells are filled with water, but in maples they are filled with carbon dioxide gas, which is produced during the daytime. Sapwood also contains ray cells that radiate out from the center of the tree. In the fall, sugars and starches are stored in the ray cells within the sapwood. In the spring, as the weather warms, they dissolve in the sap. The sweetness in maple sap comes not from the phloem but from the previous season’s sugars stored in the sapwood. Maples are nearly unique among hardwood trees in exuding sweet sap in the spring. A few other species have a sap flow (such as birch, grapevine, hickory, and butternut), but of these only birch produces sap that is sweet enough for commercial production of birch beer and birch syrup.
One of the great mysteries of maple sugaring is just what makes the sap flow in these trees. We do know that the combination of warm days and cold nights is vital to a sap run. As the temperature drops, the sap forms frost within the sapwood. When the temperature rises above freezing, the sap thaws, seeps into the vessel cells, and runs down through the tree. But there’s more to it than just freezing and thawing. Temperature fluctuations and the resulting pressure changes within the maple tree are key to the process as well.
When the temperature rises above freezing, the sap thaws and the carbon dioxide gas in the wood expands, creating positive pressure in the tree. This pressure is greater than the outside air pressure, so that when the tree is tapped, sap flows out of the opening like water from a leaky pipe. As the temperature drops below freezing in the evening, frost forms within the fiber cells and the gas within the wood is compressed, creating a suction or negative pressure (in relation to the atmospheric pressure) inside the tree. This draws water up from the roots, replenishing the sap inside the tree, so that it can flow again during the next warm spell. This recharge with water pumped up from the roots through the sapwood can occur as long as freezing and warming temperatures alternate. In most other hardwoods the internal structure of the trunk differs from maples and the temperature changes do not produce a sap flow.
During the four to six weeks of sugaring season, ten gallons of sap may be gathered from each tap, if using buckets, and twenty gallons per tap if tubing and a vacuum system is used. This is only a very small portion of the sap produced in a season, and its removal will not hurt a healthy tree. However, it is important to carefully evaluate maples in summer and fall and choose only the healthiest to tap. To support a single tap, a tree should have a diameter of at least ten inches (circumference of thirty-one inches). A tree that is eighteen inches or more in diameter (circumference of fifty-six inches) can support two taps, the maximum number currently recommended. The tree should be healthy, with no large wounds and with a dense crown of leaves. If the tree suffered from insect damage or other stresses during the year, it should not be tapped. A sound tree can be tapped for 100 years or more without endangering its health. A healthy sugar bush should contain a mix of maples of different ages and a variety of other tree species, standing snags, coarse woody debris on the forest floor, and layers of vegetation from seedlings to saplings to understory. A diverse forest is good for maple sugaring, good for the birds that help control insect outbreaks, and better able to respond to stresses from disease or climate change.
When the conditions are right for the sap to flow, it is time to tap the trees. This involves drilling a hole about one to two inches into the sapwood. The hole slopes gently upwards to aid in the flow of sap down and out of the tree. A tap or “spile” is then gently hammered into the hole, and a bucket with a lid to keep out rain and snow is placed on the tap hook. Any hollow tube will work; sumac stems with the pith removed, turned and drilled wooden taps, and traditional metal 7/16 inch taps have all been used. Modern taps are 5/16 inch in diameter or smaller, leaving a smaller wound in the tree. If pipeline is used, a plastic tap with a fifteen-inch section of tubing connects the tree to the pipeline, which leads to holding tanks. The pipeline needs to have at least a two-percent slope to keep the sap flowing downhill. The size of the pipeline varies depending on the number of trees that are connected to it. To aid in sap collection, some sugar makers use a vacuum pump to draw additional sap from the trees.
The sap is then brought to the sugarhouse to be boiled down. It is poured into the evaporator pan, which is divided into partitions. Sap constantly flows into the back of the pan and then moves towards the front as water boils off, with the most concentrated sap at the front of the pan where the syrup forms. Using a metal scoop, the sugar maker can check if the sap is approaching the consistency of syrup. If the liquid slides off the edge of the scoop in a sheet, it is almost done. A hydrometer or thermometer is used to determine when it is syrup. In larger operations, the sap is first run through a reverse-osmosis machine to concentrate it further, removing some of the water before it is piped into the evaporator. This reduces the boiling time and the amount of fuel needed to make syrup.
An average sugar maple tree produces sap with a sugar concentration of two to three percent, though this will vary from tree to tree and from day to day. Some trees are known to produce seven- to eight-percent sap or even higher. A lot of water boils off the sap in order to make maple syrup, which has a sixty-seven percent sugar content. On average, around forty gallons of sap are needed to make one gallon of syrup. When the syrup is ready, it is drawn off, filtered, and packaged into containers. The best syrup is made from the freshest sap, so sugar makers often boil late into the night. The syrup is graded by color and flavor, with new standards adopted in Vermont in 2014. Grade A syrup ranges from Golden Color with Delicate Taste, to Amber Color with Rich Taste, to Dark Color with Robust Taste, to Very Dark Color with Strong Taste.
Boiling sap to make syrup is a long-standing and much-loved tradition in New England. Maple sugar makers look forward eagerly to the wonderful smell of boiling sap, the taste of the syrup, and the pleasure of working outdoors among the maple trees during this important part of their life cycle.
Heiligmann, Randall B., Melvin R. Koelling and Timothy D. Perkins. North American Maple Syrup Producers Manual, 2nd ed., Columbus, OH: Ohio State U, 2006.
Mann, Rink and Daniel Wolf. Backyard Sugarin’: A Complete How-To Guide, 3rd ed. Woodstock, VT: Countryman Press, 2006.
Cornell Sugar Maple Research; Extension Program: http://maple.dnr.cornell.edu/index.html.
Mass Maple Producers Association: http://www.massmaple.org/flow.html.
St. Johns University: http://employees.csbsju.edu/ssaupe/biol327/Lab/maple/maple-sap.htm.
University of Vermont, Proctor Maple Research Center: http://www.uvm.edu/~pmrc/
Vermont Maple Sugar Makers Association: www.vermontmaple.org
Vermont Audubon Bird Friendly Sugarbush Management: http://vt.audubon.org/sites/default/files/documents/bird_friendly_sugarbush_management_guidelines_final.pdf