Sap from maple tree

Why Is There Water Coming Out of My Tree?

We will do our best to manage the disease within the tree, so the tree can remain as healthy as possible. At Bushor’s Tree Surgeons, we know and love trees. We know what it takes to keep a tree alive. We’ve been in the tree business for more than 50 years, and we always strive to save and care for your trees and help you with all your tree needs.

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If you need tree help, Bushor’s Tree Surgeons are at your service. We are a tree service company based in Jacksonville, Florida, and service customers in Ponte Vedra Beach and St. Augustine, Florida. Call us at 904-731-9100 or click our Tree Service Jacksonville Fl contact page to learn more and to set up an appointment with one of our certified arborists. 

T- This is kind of a long answer, but the bottom line is – it won’t kill your tree! Wetwood is a condition caused by bacteria that enter wounds in a tree. This condition in trees is very noticeable because infected trees often have seepage coming from a major crotch or wound in the trunk. In some cases, the liquid emitted from the wounds has a foul odor because secondary microorganisms colonize it.

Wetwood causes a water-soaked condition of wood in the trunk, branches, and roots of many shade and ornamental trees, especially old street trees. Elms, poplars, cottonwoods, oaks, and maples seem most commonly affected in Illinois; but many other tree species are susceptible.

Wetwood is most visible externally as a bubbling seepage of bacteria and toxins from wounded tissue in V-shaped branch crotches, pruning wounds, injection holes, and trunk cracks. The liquid often runs down the trunk, leaving a white stain. You cannot always see the wound, but you can see the liquid. Bacteria in the inner sapwood and heartwood of the tree ferment, causing internal gas pressure. This pressure commonly reopens old wounds, and the sour liquid flows down the bark. As it dries, a light gray to white encrustation called slime flux remains. The liquid commonly causes localized death of the cambium. Fluxing occurs from April to December, but it is most conspicuous in the summer, especially now. This chronic but rarely serious disease of trees can contribute to general decline in tree vitality but is not known to cause tree death. There is no “cure”, but keeping the tree healthy never hurts! Brenda

Q: I have a ten year old river birch which has been steadily dripping a clear, odorless, tasteless (yes, I did) liquid from two branches for several weeks. Do you know what this is? What can I do about it?

A: River birch tree sap is known to cause babies to be born naked and to make your hair turn gray when you’re old. Other than that, the sap that is dripping from the cut branches is harmless.

Maple and birch trees are famous for the copious amounts of sap they bleed after being pruned in late winter. Once, when my father delayed pruning his grape vines, he measured a pint of liquid that trickled from a cut vine.

Sapsuckers (a type of woodpecker) take advantage of the slightly sweet nature of the tree sap and tap dozens of shallow holes in the bark of pecan, maple and magnolia trees each spring. Neither their pecking nor the bleeding does any harm to the tree. Your birch tree will gradually stop dripping as spring proceeds.

If you want to perform an interesting experiment next February, prune any maple and birch trees you own and collect the sap as it flows. Boil it down like New Englanders do to make syrup. Your efforts might yield a half-cup of sweetener from a gallon of sap.

Making Maple Syrup

Maple Tree Oozing Sap: Reasons For Sap Leaking From Maple Trees

Many people think of sap as a tree’s blood and the comparison is accurate to a point. Sap is the sugar produced in a tree’s leaves by the process of photosynthesis, mixed with water brought up through the tree’s roots. The sugars in sap provide fuel for the tree to grow and thrive. When the pressure changes inside a tree, usually due to changing temperatures, the sap is forced into the vascular transporting tissues.

Any time those tissues are punctured in a maple tree, you may see a maple tree oozing sap. Read on to find out what it means when your maple tree is dripping sap.

Why is My Maple Tree Leaking Sap?

Unless you are a maple sugar farmer, it is disconcerting to see your maple tree oozing sap. The cause of sap leaking from maple trees can be as benign as birds eating the sweet sap to potentially fatal diseases of the maple.

Maple Tree Sap Dripping for Syrup

Those who harvest sap for maple sugar production reply on sap leaking from maple trees for their income. Essentially, maple sugar producers pierce the vascular transporting tissues of a maple tree by drilling a tap hole into those tissues.

When the maple tree is dripping sap, it is caught in buckets hung on the tree, then later boiled down for sugar and syrup. Each tap hole can yield from 2 to 20 gallons of sap. Although sugar maples yield the sweetest sap, other types of maples are tapped as well, including black, Norway, red and silver maple.

Other Reasons for Sap Leaking from Maple Trees

Not every maple tree oozing sap has been drilled for syrup.

Animals – Sometimes birds peck holes in the tree trunks in order to access the sweet sap. If you see a line of holes drilled in a maple trunk about 3 feet from the ground, you can assume that birds are looking for a meal. Other animals also deliberately take action to get the maple tree sap dripping. Squirrels, for example, might break off branch tips.

Pruning – Pruning maple trees in late winter/early spring is another cause of sap leaking from maple trees. As temperature rises, the sap begins to move and oozes out of the breaks in vascular tissue. Experts say that this is not dangerous for the tree.

Disease – On the other hand, sometimes it is a bad sign if your maple tree is dripping sap. If the sap comes from a long split in the trunk and kills the tree trunk wherever it touches the bark, your tree may have a potentially lethal disease called bacterial wetwood or slime flux. All you can do is to insert a copper tube in the trunk to allow the sap to get to the ground without touching the bark.

And if your tree is a silver maple, the prognosis could be just as bed. If the tree has cankers oozing sap and the sap leaking from the maple trees is dark brown or black, your tree may have bleeding canker disease. If you catch the disease early, you can save the tree by removing cankers and treating the trunk surface with an appropriate disinfectant.

Before sap starts flowing from the maple trees, you’ve got to tapThere’s a lot of work that goes into syrup!

By Tim Goodwin – Mar 1, 2016 |

If you look closely you can see the sap dripping out of the tap hole. (TIM GOODWIN / Insider staff)

Mapletree Farm owner Dean Wilber uses his trusty drill to notch out a spot for one of his 1,000 or so taps for this year’s sap collecting operation. (TIM GOODWIN / Insider staff)

Wilber inserts one of the taps into a sugar maple using his time trusted technique of tap, tap, boom. (TIM GOODWIN / Insider staff)

The point where the vertical lines meet the main lines. (TIM GOODWIN / Insider staff)

Look at all that tubing. (TIM GOODWIN / Insider staff)

It’s only a matter of time before the sap starts flowing through those taps. (TIM GOODWIN / Insider staff)

Tap lines waiting to be connected. (TIM GOODWIN / Insider staff)

Wilber connects some taps on his property. (TIM GOODWIN / Insider staff) Just look at Tim drilling that tap hole. (DEAN WILBER / For the Insider) Some of the early sap collection. (TIM GOODWIN / Insider staff) Wilber tests the sugar content. (TIM GOODWIN / Insider staff)

If you’re a fresh maple syrup connoisseur, you probably know that time of year is fast approaching.

It’s hard not to get excited about that first jug of the season, but be patient because the really sweet stuff isn’t quite ready.

You’ve probably started to pay a little closer attention to those plastic lines connecting maple trees on the side of the road these days and if you look real close, those large holding tanks will soon be filled to the brim with fresh sap.

But the process of getting that sap from the tree to on top of your pancakes is probably a bit more time consuming and labor intensive than you ever imagined. We found that out first hand when we spent an afternoon with Dean Wilber, owner of Mapletree Farm.

“You don’t just put a spout in a tree and get syrup,” he said.

If you didn’t know, the sap gathering season is a very short window.

“If you’re not ready, you’ll lose it,” Wilber said.

Wilber doesn’t like to tap the trees if the temperature is below 20 degrees in fear of splitting the bark. So he began the long and tedious process of tapping his 1,000 or so trees spread out over six locations in Concord and Loudon the third Saturday of February in order to be ready for what he calls the first good flow, which usually begins the first week of March.

“It hasn’t run much yet,” Wilber said early last week. “It runs when the weather conditions are conducive for sap to flow.”

What we learned is that in order for sap to run from the roots all the way to the leaf buds at the end of the limbs and back down, there needs to be a freeze/thaw cycle. That means it needs to be between 38 to 44 degrees during the day and then drop down to 26 to 28 degrees at night.

“The sap goes up during the day and drops down during the night,” Wilber said.

And as you can probably guess, the more times it goes from the roots to the limbs and back again, the sweeter it gets – at least to a point. It’s not like you’re going to get maple syrup flowing from the trees if you wait as long as possible.

“This is only the third time in 41 years I have boiled in February,” Wilber said over the weekend.

Putting the tap in an individual tree takes very little time. Since this is Wilber’s 41st year of operation, he’s got it down to a science.

“This was a hobby operation that’s gone to a retirement operation,” he said.

There’s some telltale signs that lets him know the sap is flowing. One is puddles in the roads from broken limbs or where tree crews have been working, as well as when the red squirrels nip the buds and drink the sap. Another sign is melting of snow around the base of a tree, but since there’s no snow that isn’t an option this year.

And with very little snow this winter, it’s been a little easier than a year ago.

“I was on snow shoes every day but one tapping last year,” Wilber said.

First, he looks over the tree, checking out the crown and limb sizes, as well as finding previous tap holes. He’ll see how far the tap stretches and determine where he wants to drill this year’s tap.

“I look for all fresh wood,” Wilber said. “You want time to look the tree over.”

He wants to make sure there’s no kinks in the tubing and that it’s above the line that will bring the sap to the filling tank.

Once he’s picked out the perfect spot, he uses his trusty drill and correct size bit to make a hole. Wilber wants a straight on hole – no wobbling with the drill – so the tap fits in perfectly. No one wants to see sap leaking out. Might as well just flush money down the drain.

And then with his trusty mallet, Wilber performs a tap, tap, boom as he calls it, where generally the first two hits tap it in and the last one tells you it’s as far as it will go.

The hole is anywhere from 1 1/8 to 1¼ inches deep and the tap goes in about ¾ of an inch. Got to make sure there’s space for the sap to flow in. He’ll tap trees once they’re about 9 to 10 inches in diameter and ones that are 18 inches can have two.

Now the tubing you see connecting all the trees in the woods off many roads in New Hampshire has a shelf life of about 15 years – and don’t worry, Wilber thoroughly cleans his each spring. But he does replace the taps each year and added a new check ball piece to all his existing line taps this year that prevents back flow of sap into the tree.

We say existing taps, because Wilber added about 220 new trees to his sap generating arsenal and is using the wooded area not too far from his Concord abode as a bit of an experiment. He used a smaller 3/16 inch tubing (normally it’s 5/16) and is looking to see what kind of natural vacuum the downhill slope can create.

“It’s an ideal situation to use the natural vacuum,” Wilber said.

Over the weekend, he saw some favorable results.

And sure, it will give him extra sap this year to make delicious tasting syrup (and we know because we’ve had it), but it also added quite a bit of labor to get ready for the season.

“It’s hard work. These things don’t just happen,” he said. “You don’t just drill a couple holes and make syrup.”

It took a little over a week’s worth of work to lay out the 4,500 feet of 3/16 tubing, along with about 700 feet on main line connected to a 225-gallon tank. There’s an average of 22 taps leading into each lateral line off the hill.

“The whole point is to have it straight, tight and down hill,” Wilber said.

And during the season, Wilber walks all his lines about once a week.

“We look for animal damage, typically squirrel or deer, and for any blow downs,” he said.

When sap really starts flowing, he’ll keep a close eye on his seven tanks, which range from 65 to 335 gallons.

“If it runs exceptionally well, I might have to gather it night and morning,” Wilber said.

And soon, all that sap, which is only about 5 percent of what the tree produces, will be turned into maple syrup you can buy. That is also a very involved process, but a story for another day.

Noon Edition

If you take a spring walk through the woods in many parts of the Northeast, you’ll find buckets hanging from the sides of maple trees. Drop by drop, the buckets are filling up with the slightly sweet sap from the trees. When that sweet liquid is boiled down, it becomes maple syrup, all ready for a plate of pancakes.

Sweet Syrup

Colonists arriving from Europe learned from the American Indians how to tap maple trees and boil the sap into syrup. For a long time, maple syrup was the only concentrated sweetener available on this continent.


Today, little is known for certain about why a sugar maple produces such sweet sap. The sugar in the sap is made in the leaves by photosynthesis the same process that all green plants use to convert water, sunlight, and carbon dioxide into food.

Getting The Best Sap

So the best year for maple syrup is one with the best conditions for photosynthesis a sunny summer and fall with a late frost. A cold winter and a heavy snow fall to keep the roots cool in the spring also improve the syrup harvest.

And any year with extreme variations in temperature from night to day tends to be a good year for maple syrup as well.

Do Other Trees Produce Sugars?

All trees produce some natural sugars, which they use for energy in their own growth processes, but not all trees, or even all maples produce enough or the right kind of sugar to make syrup.

Out of about 100 species of maple trees, only four are good for syrup and of those, one species the rock, or hard, maple produces most of the syrup we use.

If you’re wondering why something that flows from trees should be so expensive, consider how little syrup you get when the sap is boiled down. The pint jug of syrup you buy in the store came originally from about five gallons of sap.

Related Wonders for You to Explore

You’re sleeping peacefully when suddenly the sound of pots and pans gets you stirring. You’re able to drift back to sleep for a few minutes, but it’s not long before you’re pulled into full consciousness by your nose.

As you sit up in bed, you sniff the air and recognize the unmistakable smell of pancakes! Your mouth begins to water as you jump out of bed and head to the kitchen.

It won’t be long until you’re seated at the table before a stack of flapjacks, ready to soak them in that sweet and sticky substance that turns ordinary pancakes into a breakfast treat: syrup!

As you wait for your pancakes to be served, you take some time to WONDER about where that syrup comes from. Like the paper you write on at school, syrup comes from those tall, woody perennials you often take for granted. What are we talking about? Trees, of course!

You can’t drill a hole in just any old tree and have syrup pour out. You can, however, tap certain kinds of trees, such as sugar maple trees, and collect gallons of sap at the right time of year.

Syrup makers use tree sap to make syrup. On average, it takes about 40 gallons of sap to make just one gallon of delicious syrup.

Sap inside a tree is a bit like blood inside the human body. Sap flows through a part of the outer tree trunk known as sapwood, delivering water, sugars, and nutrients throughout the tree.

Sap production begins in warm summer months when the process of photosynthesis creates carbohydrates that get stored in the tree as starch. The starch then gets converted to sugar in the form of sucrose that dissolves into the sap, which is stored for the winter.

When spring begins to arrive (often around the month of March), the weather provides just the right conditions to produce sap that can be collected to make syrup. Those special conditions consist of alternating freeze/thaw cycles in which temperatures drop below freezing at night and rise above freezing during the day.

The rising temperatures during the day create positive pressure within the tree that will force sap out of any holes in the tree. In addition to taps purposefully placed in the tree to collect sap, sap will flow out of broken branches or any other cracks or holes in the tree.

At night when temperatures fall back below freezing, negative pressure develops inside the tree. This creates a suction that stops the sap from flowing out of the tree. This suction force also pulls water through the tree’s roots to replenish the sap. Experts believe these differences in pressure as a result of temperature fluctuations occur because of the expansion and contraction of carbon dioxide gas within the sapwood.

This cycle continues until temperatures stay above freezing, at which point sap will stop flowing and begin the sap production cycle anew. Scientists believe that the sap’s main purpose is to provide nutrients to new leaves as they grow. In turn, those leaves will fuel the process of photosynthesis that starts the sap production process.

You may be WONDERing if drilling holes into trees is damaging. Using proper tapping procedures, trees sustain only minor wounds that are not damaging. Trees repair drill holes and heal in a matter of time.

Maple syrup mechanics: xylem, sap flow, and sugar content

It’s maple syrup making time in the Northeast. Jeanne explains the mechanics of sap flow, collecting sap for syrup making, and why maples are special in this regard.

Proctor maple research field station, Underhill, VT

I had the great pleasure last weekend to visit the University of Vermont’s Proctor Maple Research Center in Underhill, VT, where the sugar maple (Acer saccharum, Sapindaceae) sap is flowing. Sugar maple trees all around the northern hardwood forests in the Midwestern and Northeastern United States and southeastern Canada can now be “tapped,” fitted with a hollow tube in the sapwood, out of which sap flows and is collected and boiled down to maple syrup.

Tapping sugar maple to collect sap, Proctor research station, Underhill, VT.

Maple syrup might be the oldest agricultural product in North America. Early 17th-century written records from Europeans exploring North America describe Native American use of sugar maple sap. We of course can’t know how Native Americans discovered sugar maple sap, but it may have been by sampling a “sapsicle,” icicles made from frozen maple sap that forms at the end of a broken twig. The evaporation of water during ice crystallization partially concentrates the sugar in the sap, making the sapsicle particularly sweet. To understand how that sap got to the end of the twig in the dead of winter and why it’s so sweet, we need some basics about plant vasculature and carbohydrate storage and must figure out what makes maples so special.

Sugar maple canopy leaf sampling

On a hot day in summer, a big sugar maple tree with its crown at the top of the forest canopy might move around 200 liters of water from the roots to the evaporating surfaces of leaves, 30 meters above the forest floor. Mineral nutrients from the soil, along with sugar and hormones and other physiological constituents manufactured or stored in the roots or stems sometimes also rise dissolved in the water and collectively form the sap. The sap ascends from root to leaf in the xylem.

Xylem out of xylem: vessel structure sculpture, Kew Botanic Gardens, UK

Xylem consists of tracheary elements through which sap flows and the living and structural tissues (parenchyma and fibers) that surround and support them. Tracheary elements—made of vessels in angiosperms and tracheids in gymnosperms and more evolutionarily basal vascular plants—form a network of interwoven pipes, made of dead, hollow cells, joined end-to-end. Phloem is another set of pipes made of living cells that moves solutions within the plant with the aid of metabolized energy. The xylem and the phloem together comprise the vascular system in most land plants and evolved not long after plants colonized land.

Tracheary elements sculptures, Kew Botanic Garden, UK

Sap in the xylem moves passively, in response to physical forces, pressure and osmotic gradients. Three primary processes can cause xylem sap to flow: transpiration, root pressure and stem pressure. In order to turn carbon dioxide into sugar through photosynthesis, the leaf must open its stomata (small pores on the leaf surface) so that carbon dioxide can diffuse into the leaf. In doing so, however, the leaf must inevitably lose water by evaporation out of the stomata. This water loss is called transpiration and creates tension, or negative pressure, in the water in the xylem. Water moves up through the vessels to replace water lost through transpiration, in response to this pressure gradient. Transpiration during photosynthesis therefore pulls water up through the plant from the soil. When photosynthesis is done for the day, tension relaxes on the column of water in the xylem, and vessel pressure approximately equilibriates with that of soil water at the root surface.

Positive root pressure causes early spring sap flow in paper birch (Betula papyrifera)

Sap might also rise in the absence of photosynthesis because of root pressure. Sometimes a pressure gradient develops between the soil water and roots, usually because of high concentrations of minerals and organic compounds in the roots when photosynthesis is inactive. Water then flows into the roots and produces positive root pressure, pushing sap up through the xylem. Root pressure-generated sap flow in trees increases as the soil warms in the spring and ceases only when leaves develop enough to begin transpiring, thus eliminating day-time root pressure. Positive root pressure may still develop at night during the growing season and is partially responsible for dew production.

Pressure gauge attached to tap in sugar maple used in vacuum-assisted sap collection, Proctor research station, Underhill, VT.

Maple sap flow during the leafless season is physiologically unique in that it is largely independent of root pressure and only occurs on occasions between October and April when warm days follow freezing nights. Maple winter sap flow is caused by pressure in the stem generated by alternating daily cycles of night freezes and warm days. Cool evening temperatures generate negative pressure from the dissolution of gases in the xylem, which were seeded in from adjacent parenchyma and intercellular spaces. The negative pressure replicates the effect of transpiration, which draws still-liquid water from the soil into the roots. As the night freeze deepens, water freezes along the inner walls of the hollow fiber cells adjacent to the xylem and in intercellular spaces.

Cross section of xyelm in a sugar maple twig (2-years-old), 10x magnification. The large holes are vessels; the dark radial bands are rays

Eventually vaporized water on the surfaces of all cells freezes. The ice formation compresses and traps gases in the stem. The heat of the day melts the ice and causes expansion of the compressed gases, which generates positive pressure in the stem that pushes the sap up the stem and out the nearest exit, if one exists, such as a maple producer’s spile. Applying vacuum pressure to the tap allows a maple producer to collect up to three times the normal amount of sap, and doing so has been the industry standard since the late sixties.

Tubes run from the vacuum device to the tapped sugar maples and carry sap to the collection location, Proctor research station, Underhill, VT.

Because it depends on weather (and water availability) instead of spring leaf flush, maple winter sap flow is intermittent and highly variable from year to year. In any given season, a maple producer may have the opportunity to collect sap on one or ten or more daily “runs” of sap. The process occurs in all maples and sycamore (genera Acer and Platanus) and some other species, including butternut and walnut (Juglans spp., Juglandaceae) that spend the winter with water-filled vessels with air-filled fiber cells adjacent to the vessels. Most other species in the northern woods (including willow (Salix), aspen (Populus), elm (Ulmus), ash (Fraxinus), and oak (Quercus)) do not exude sap and spend the winter with gas-filled vessels and water-filled adjacent fibers. Some tree species (see below), most notably the birches (Betula), will exude sap from a spile in the later spring in response to positive root pressure.

Walnut (Juglans) has maple-like late winter sap flow

The sugar in the sap stream is from carbohydrates stored in the living ray parenchyma and fiber cells adjacent to the vessels in the xylem and is mobilized in late winter and exuded into the xylem sapstream to fuel flowering and leaf expansion (maples flower before they leaf out). Most of the carbohydrates in the rays are actually stored as starch grains, and an enzyme released into the xylem throughout late winter converts the starch to sucrose and mobilizes it in the xylem. Sap sugar content also varies within a sap flow season, peaking in the middle. Therefore, there is quite an art to the timing of the tree tap. Producers should tap right before the peak sugar content sap flows. If they tap too early, the tap site might dry out, and they will collect mostly lower sugar content sap.

Maple syrup is mostly sucrose, but the maple xylem sap stream also contains glucose, inorganic salts, protein precursors (peptides and amino acids), some enzymes, and a few mystery organic compounds. The distinctive maple flavor is still something of a mystery but is variously attributable to the organic constituents of the sap and is amplified by heating. That is why maple syrup tastes differently from syrup from other trees, such as birch, or from syrup made from other plants, like sugarcane or sorghum (molasses is made from the combined xylem and phloem sap from both species).

Riverside yellow birch (Betula alleghaniensis) in the sun are good candidates for sap collection

Sugar maple is not the only tree species from which xylem sap is collected for consumption. Many species of birch trees (Betula spp., Fagaceae) are tapped for sap throughout the northern hemisphere. Sometimes the sap is boiled down for syrup, and there’s a growing birch syrup industry in Alaska, Russia and Scandinavia. Often, however, the birch sap is drunk or bottled as is, as a refreshing tonic. In North America sap is also sometimes collected from species in the Juglandaceae family (hickories and walnuts) and basswoods (Tilia spp: Tiliaceae). I suppose one might be able to extract sap from conifers during periods of root pressure sap flow, but this might be practically unfeasible because the collected sap might be inevitably contaminated with resin, the sticky pitch that visibly oozes from conifer wounds (which is not sap, even though it is sometimes called that). Throughout the tropics sap is collected from the crowns of palms for miel de palma (“palm honey”) and palm sugar. Maple species other than sugar maple also apparently make delicious maple syrup but have lower sap sugar contents, so one needs more sap to make syrup.

I’ve read that some people have made early spring syrup from basswood (Tilia americana).

This list begs the question of whether any sufficiently large tree can be tapped for sap. The short answer is no. Whether a tree is worth the tap depends on geography and how much work you’re willing to invest and what flavor of syrup you’re willing to settle for. Sap can be collected from any tree in which sap flows from positive root pressure in the spring or from any maple (including sycamores) or walnut/butternut (Juglans) growing in a place with abundant soil water that experiences daily freeze-thaw cycles in late winter. The list of candidate trees is therefore winnowed down by first by location. The remaining questions to ask of your tree are: (1) will enough sap volume with (2) a high enough sugar content be collected to make it worthwhile to boil it down for syrup; and (3) what will the flavor of the syrup be when it’s done?

Measuring volume production by sugar maple trees, Proctor research station, Underhill, VT.

The volume question depends on the size of the tree, the diameter of the xylem storing water in the trunk (minimum 12” in diameter for a sugar maple, around a 40-year old tree), and more critically, on the water availability to the tree and the weather. For sugar maples, the larger the temperature difference between the night freeze and the daily thaw and greater the number of days in a season with big freeze-thaw cycles, the greater will be the volume of sap collected.

The sugar content question is economically critical. To make syrup, sap is boiled down until it is about 2/3 sugar. This means that if the sap has 2% sugar, as is typical for sugar maples, it takes 40-43 gallons of sap to make one gallon of syrup. Birch sap usually hovers around 1% sugar, so it takes over 80 gallons of sap to make one gallon of syrup. Some syrup producers run their sap through a reverse osmosis machine to remove some of the water from the sap to concentrate the solution before the boiling begins, so that less time and fuel will be necessary to reach the syrup stage. The higher the sugar content at the beginning of boiling, the lighter and more delicate will be the resulting syrup when it hits 66 or 67%, because the sugar will be less caramelized.

Dark Grade B maple syrup

That is the difference between Grade A maple syrup and Grade B. More boiling, more heat, was necessary to produce Grade B because the sugar in the starting solution was more dilute. All the other stuff in sap, though, that contributes to the maple flavor, may or may not be more dilute in the sap destined for Grade B syrup, so Grade B might have a more intense maple flavor than the lighter Grade A. I actually always buy Grade B for this reason.

Northern hardwood forest in Wisconsin with shaded sugar maple saplings in understory

The sugar content depends on the species of tree and how much sugar it was able to store in its trunk over the winter. Shade tolerant species, such as syrup star sugar maple, tend to store more sugar in their trunks than do shade-intolerant species, such as aspen. If a sugar maple canopy sees enough sun and has enough water and nutrients in a summer to photosynthesize more sugar than it needs for its immediate, perhaps pre-programmed determinate, amount of growth and maintenance, it stores the extra sugar in the ray parenchyma cells in its sapwood, adjacent to the xylem. A shade-tolerant tree seems to always expect to have all or part of its future canopy shaded, either by itself or a neighbor, so it saves sugar to survive the lean times.

Quaking aspen (Populus tremuloides) incursion into a sunny abandoned field

Shade-intolerant, species, however, have more of a “can’t take it with you when you go” philosophy, and quickly burn through much of their photosynthate by growing as much as they possibly can during the season. They’ll store enough sugar in their roots and stems to support respiration over the dormant season, provide frost hardiness (sugar is antifreeze), and to fuel budburst and leaf expansion or maybe even reproduction in the following year, and they might even hedge their bets by storing enough to fuel healing after injury or pest attack. The end result, though, is less stored sugar, so less sweet sap. So, even if sap can be collected from relatively shade-intolerant trees, which is sometimes the case, as with paper birch (Betula papyrifera), the sap will have a low sugar concentration relative to sugar maple sap, so it will take much longer to boil it down to make any kind of a syrup out of it. The sap of even a shade tolerant tree, however, might be too dilute to merit syrup making if the stored sugar doesn’t make it into the xylem sap when the sap could be profitably collected before budbreak. Such apparently is the case with American beech (Fagus).

Red maple (Acer rubrum) also makes good syrup but flowers earlier than sugar maple

Budbreak, the opening of the flower or leaf buds on the aerial shoots in the spring, requires the addition to the xylem sap of additional hormones, amino acids, defense compounds, and other physiological constituents that are either stored or largely manufactured in the roots and sometimes stems. According to most palates, these budbreak precursors in sap negatively affect the flavor of syrup produced from all tree species. Consequently, even if sap is still flowing, sap collection abruptly ends at budbreak for anyone wishing to avoid “buddy” syrup. Also, if you’ve got your eye on roadside maples for tapping and are particular about your syrup flavor, beware de-icing salt. Some studies have demonstrated that road salt in winter not only hurts the trees, because salt is toxic to most plants, but also imparts a bitter, salty flavor to the syrup made from sap of roadside trees subjected to salt.

Sugar content of sugar maple sap is highly variable across trees, as a consequence of genetic variation among individuals, soil fertility and water availability of the growing site, and previous year’s photosynthetic success. Sugar maple sap sugar content varies from less than 1 to over 10% but usually averages around 2 or 3%. More water and soil fertility and sun exposure is better for sugar yield.

Sugar maple branch growing into a sunny forest gap

Interestingly, then, if you wanted to plant a plantation of sugar maples, you’d have to space your trees far more widely for optimal syrup making than for optimal harvest of the highly-prized sugar maple timber. Wider spacing allows the branches to sprawl out and take advantage of the most sun per leaf, which maximizes seasonal sugar storage, whereas close spacing creates competition for light and causes the trees to invest in tall, straight trunks in their race to the top of the canopy, at the expense of stored sugar.

Herbivore damage on sugar maple leaves

Drought stress or pests or diseases, especially defoliating ones, can also decrease seasonal sugar storage. A bad summer for the trees will lead to a bad spring for the sap collector. As long as there is water in the soil, there may be enough sap volume to collect, but the sugar reserves in it will be low. Research is ongoing into the genetic variation underlying sap sugar content and involves an effort to identify particularly “sweet” trees and propagate their offspring. Particularly high-sugar-yielding trees might have more and larger xylem rays than low yielding trees.

The hole in the sugar maple trunk from the tap heals in about 6 weeks. While the tap is in place and after it is removed, diseases could enter the tree through the hole. While the amount of sap removed by the syrup producer, accounting for an average 10% of stored sugar, will not mortally harm the tree, it does reduce its vigor a little bit and may make the tree slightly more susceptible to diseases during the healing period. Therefore, the conscientious syrup producer should seal the tap wound with an appropriate compound.

Young sugar maple stand, Proctor research field station, Underhill, VT

Sugar maple trees like it cold, and repeated freezing temperatures are necessary for a good spring sap flow. The current range of sugar maples is centered in the northern part of the Midwestern and Northeastern United States, with fingers of the tree’s range extending south to high elevation (coldish) sites. The range of sugar maple in North America is projected to shrink and move north, almost entirely out of the United States, over the next century in response to global climate change. The last few warm winters in the Mid-Atlantic and Northeastern U.S. have not been kind to maple sugar producers. So, if you love your maple syrup—not to mention the northern hardwood forests where sugar maples live—be sure to educate yourself about climate change and climate policy.

Pallardy, S. G. 2008. Physiology of Woody Plants. Third Edition. Academic Press.

Valladares, F., and U. Niinemets. 2008. Shade tolerance, a key plant feature of complex nature and consequences. Annual Reviews of Ecology, Evolution and Systematics 39: 237-57.

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