Sap starts to flow from a metal spile on a sugar maple tree at County Farm Park. The hump midway along the spile provides a place to hook your bucket and collect the sap.

A brutally cold wind buffets the group huddled around a sugar maple at the Washtenaw County Farm Park. They’re looking at a small metal device that’s been gently hammered into a hole drilled in the tree. Faye Stoner, a park naturalist for the county, sounds doubtful. “It’s probably too cold,” she says.

But wait – a kid in the group cries out: “It dripped … it dripped!” And sure enough, a globule slowly rolls off the end of the spout.

The maple sap is rising, and two dozen very cold people are learning about what Stoner calls “a gift from nature.”

We decided to sit in on Stoner’s Saturday class, called “Sap to Syrup,” after one of our readers filed a Stopped.Watched item about the “sapcicles” forming on maple trees across the city. How exactly does one tap the sap, we wondered? And then what happens?

Stoner describes the process as very simple: “It’s not easy, but it’s simple.”

Trees should be at least 10 inches in diameter before tapping. These kids are using wooden calipers to measure the trunk. It's plenty big.

Step 1: Know your trees

Someone asked whether you could make syrup from pine sap. Sure, Stoner replied, but “do we want turpentine on our pancakes or ice cream?” That would be “No” –  so knowing which trees to tap is important.

The reason maple sap makes great syrup is that it contains a relatively high percentage of sugar, compared to other trees. “Relatively,” in that the maple sap is still about 97% water. Sugar maple is, as its name implies, the best for making syrup, though red and silver maples “will do in a pinch,” Stoner said.

Leaves are the easiest way to identify maples, but since tapping is done in the late winter, you’ll need other ways to locate the right tree. There are two key ways: 1) Sugar maple buds are pointy, not rounded; and 2) maples have “opposite growth pattern,” meaning that twigs shoot out from each side of the branch directly opposite each other.

The size of the tree is also important. Stoner uses the analogy of a dog and a puppy: Which one would be least harmed if you cut off its food supply? Like a puppy, young trees need more nourishment because they’re growing. So look for maple trees at least 10-12 inches in diameter. And the bigger the trunk, the more taps you can put in – up to three per tree – without harming the tree. Stoner warns: “Don’t overstress your tree by taking too much.”

Faye Stoner watches as one of the children in her "Sap to Syrup" class uses a hand drill to make a small hole in the silver maple tree. If you understand gravity, you'll know why they're drilling a hole angled this way.

Step 2: The two Ts (temp and tools)

Sap flow depends on temperature – the perfect time to tap sap is when nights are still cold but the days are getting warm, at least above freezing. Late February into March are typically when this occurs, and you’ll have about a six-week window to tap. As soon as the sap reaches the buds, it starts turning milky and bitter, and your season is over.

You need three things before you start: A tool to drill the hole in the tree, a tap or “spile,” and some kind of container to collect the sap. Stoner pointed people to two local sources for supplies: Downtown Home & Garden in Ann Arbor, and Dexter Mill.

Hand drills are old school, but if you’re tapping several trees you’ll likely want to use a cordless power drill, Stoner said. In either case, use a 7/16-inch drill bit.

This metal bucket with a sloped lid is a traditional container for collecting sap. The lid prevents rain (and deer) from getting into the sap.

Metal buckets that hook over the tap are the traditional way to collect sap – they come with sloped covers to prevent deer from snacking, and rain from diluting the sap. You can buy plastic sap bags designed specifically for that purpose, or an empty plastic milk jug works fine, too, Stoner said.

Step 3: Make your tap, collect your sap

You should drill at a slight upward angle, going only 2 to 2.5 inches deep. Clear out all the small wood chips from the hole, and insert your tap with a gentle, er, tap of a hammer. The metal tap is essentially a tube, and assuming conditions are right, you should soon see clear liquid start dripping out the end.

Stoner had tapped a nearby tree on Thursday, and collected her sap in a plastic milk jug. Whatever you use, make sure the container is securely affixed to the tree so that it won’t pull out the tap as the container fills and grows heavier.

When the days get warmer, you should collect your sap from the container every day to prevent bacteria from growing, Stoner said. Even when temperature isn’t a concern, it’s good to check the container frequently to make sure it doesn’t overflow. Sometimes you can get two gallons of sap within a 24-hour period, depending on how much sap the tree is producing. A single hole will typically yield 10 gallons of sap per season.

Faye Stoner pours sap into a small pan to boil it down into syrup. The sap she's pouring is amber, not clear, because it's already been partially reduced. The large pot on the left is sap that's in an early phase of being boiled down.

Step 4: Boil, boil, boil

Ten gallons sounds like a lot until you’re told that it takes 40 gallons of sap to make 1 gallon of syrup. Of course, you might also want to drink it straight: On Saturday, Stoner passed around small Dixie cups filled with sap she’d collected on Friday for people to sample. It tastes refreshing, like water that’s been spiked with sugar.

To get from this clear liquid to a dark amber syrup mostly requires time. Stoner suggests using a large pot for the first phase, and you can boil it 30-40 minutes without paying much attention. Beyond that, however, you need to keep a watchful eye. The closer sap gets to becoming syrup, the more likely it is to boil over or burn.

As the water evaporates, the liquid turns darker and thickens. The process can take quite a while, depending on how much you’re cooking. You’ll also see foam form on the top of the liquid while it’s boiling – it’s not harmful, Stoner says, but most people like to skim it off with a wooden spoon or strainer. Sometimes there’ll be other sediment, like bits of wood – you might want to strain the liquid through cheesecloth or some other kind of filter to get rid of that, she said.

Faye Stoner uses an eyedropper to put a sample of maple syrup she made on the hand of each person who attended her Saturday class. Why this method of distribution? For a large group, it's sanitary, fairly quick, and shows you how thick the syrup is, Stoner said.

Step 5: Eat and enjoy

Stoner had examples of several types of maple syrup on hand, including some she’d made. But she also held up bottles of Aunt Jemima and Log Cabin, saying, “These are not maple syrup!” To make her point, she read the ingredients: corn syrup, high fructose corn syrup, water, natural flavor (“But what kind of flavor?” she asked) sorbic acid and so on.

And what’s in real maple syrup? Do you really have to ask?

Everyone who attended Saturday’s class got to sample two kinds: Syrup made by Stoner, contained in an old mustard jar, and syrup from Snow’s Sugarbush in Mason, Mich. She dispensed samples onto each person’s hand, so you could see its consistency before licking it to taste. Her homemade syrup wasn’t as thick as the sample from Snow’s Sugarbush, but both were pretty darn rich and tasty.

At the end of the 1.5-hour class, Stoner asked how many of the people there planned to tap their trees. A few people, but not all, raised their hands. “If you don’t have a tree on your property, you can come take the sap from these trees,” she said. “Just call me.”

And by way of ending the class, Stoner had a final piece of advice: “Be kind to your trees. Don’t be greedy.”

Bonus: Maple Cocktail Recipe

3/4 oz. pure maple syrup

3/4 oz. dry gin

1 oz. lemon juice

1 oz. bourbon

(Serves one)

That stick is no ordinary ruler. It's called a Biltmore stick, and has a scale that allows the user to sight the outside limits of a tree's diameter from a single point of view.

On Thursday near 7th and Madison streets, The Chronicle noticed a guy wearing a bright yellow vest with electronic gear and some sort of measuring stick. We had a pretty good idea what it was about, having recently reported on city council’s approval of a $243,500 contract with Davey Resource Group for a GIS-based inventory of trees in the public right-of-way as well as in parks.

Marcia Higgins, one of two councilmembers for Ward 4, had cast the lone vote against the contract, and had explained at Sunday night caucus two weeks later that she would prefer to see the money for the project, which is coming out of the storm water fund, spent directly on storm water.  She also wondered if the work could be completed more cost-effectively as a Boy Scout service project.

It’s not Boy Scouts who are doing the work, but rather four guys from Davey Resource Group.  One of them is Wes, the guy in the yellow vest, who chatted with us as he took down a couple of trees’ vital statistics: height, trunk diameter, type (genus and species), condition, and location.

Early in the conversation, we asked him what kind of tree he was recording at the moment, and he said he figured it was a sugar maple, “but it looks a little different from the sugar maples I’m used to seeing back east.” In this case, “back east” means New Hampshire, where he lives.

Wes uses a clinometer to get a bead on the height of the trees. The device measures angles, and the rest is the magic of geometry.

He was flown in on Tuesday, just two days earlier, and he figures the four-man crew, which is working Monday-Saturday from around 7:30 a.m. to 5:30 p.m., will need a couple months to complete the inventory of around 60,000 trees. The pace of around 260 trees a day that Wes reported would take around 60 working days.

Not all of the four tree guys who are working on the project came from outside the region.  For example, the project manager drove up from Ft. Wayne, Indiana, which means the crew has a vehicle to drive people from their hotel out by Briarwood Mall to their job site for the day. On Thursday Wes was working 7th Street from north to south and needed to get to Pauline.

In Wes’ tool kit, along with the GPS unit (for precise location information) and tablet-style computer, were two old-school analog devices: a Biltmore stick for estimating tree diameter and a clinometer for measuring height. Through the power of applied geometry, they’re not just guessing those stats. Wes wasn’t just guessing about that sugar maple, either. He’s an ISA-certified arborist, and has been working in the field for around 10 years.

Why is the city of Ann Arbor conducting a tree inventory, and why is it being paid for out of the storm water fund? The 1994 inventory was based on street addresses, and a pilot attempt to geocode that inventory to make it compatible with the city’s GIS system proved not to be cost-effective. In addition, the arrival of the emerald ash borer in 2002 meant that city staff focused on removal of dead and dying ash trees, which diverted resources from the ongoing update of the inventory. The 1994 inventory did not include park trees like the current project does.

As for the rationale for using storm water monies to pay for the tree inventory, there’s a connection between trees and storm water runoff. A mature deciduous tree uses up an average of 625 gallons of water per year, preventing that amount of water from  entering the storm water system.

So over the next couple of months, if readers spot guys in yellow vests and GPS units sticking out of their backpacks who are taking a really close look at a tree, one of them is Wes. Here’s a question that Wes, the tree guy from New Hampshire, asked The Chronicle: “What’s a can’t-miss thing to see in Ann Arbor?” Readers might want to convey their own answers directly to Wes, or perhaps in a comment below.