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Theo Wargo // Getty

9 Ways Christmas Trees Are Reused After the Holidays

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Theo Wargo // Getty

You don’t need a calendar to tell you when the holidays have ended—just take a look outside to see if rows of skimpy, dried-out Christmas trees are lining the curb. Each year, roughly 33 million live Christmas trees are purchased in North America, many of which end up rotting in landfills once the new year arrives. But making our days merry and bright isn’t the only thing a felled evergreen is good for. Here are some ways Christmas trees continue to serve a purpose long after their decorations have been packed away.


The tree that’s erected in Manhattan’s Rockefeller Center each November is arguably the most iconic Christmas display there is. It’s also one of the largest, reaching up to 100 feet tall and often weighing more than 10 tons. That’s a lot of lumber, and luckily, Habitat for Humanity makes sure it’s put to good use. Every year since 2007, Rockefeller Center has donated its tree to Habit for Humanity International after taking it down on January 9. From there, the festive behemoth (usually a Norway Spruce) is divided into sections in the plaza before it's shipped to a mill in New Jersey for additional sawing. It’s eventually made into 2-by-4 and 2-by-6 beams used in construction projects around the country. Homes in Bridgeport, Connecticut, Morris, New Jersey, and Philadelphia all contain pieces of what was once the world’s most famous Christmas tree in their walls.


If you were to walk along the bottom of Lake Havasu between Arizona and California long enough, you’d eventually come across the site of a Christmas tree graveyard. What may be a creepy scene to holiday lovers is a lush utopia for fish—the branches of the spruces, firs, and pines provide a hiding place from predators and attract food for the fish to nibble on.

The 875-acre artificial reef resting on the lakebed consists of PVC pipe, cinder blocks, concrete sewer pipe, brush, and thousands of Christmas trees weighed down with sandbags. Decades of decomposed plant matter have built up a healthy layer of moss and algae around the non-degradable structures. This green coating attracts insects, which in turn attract fish looking for a snack. The end of the holiday season marks the introduction of 500 new trees to the reef, each of which will take about five or six years to break down completely.


Spend a day on the beach in summertime and Christmas trees will likely be far from your mind—but on at least one beach along the East Coast, there are thousands of abandoned conifers buried in the sand. That’s because Bradley Beach, New Jersey depends on recycled Christmas trees to build its sand dunes. Discarded trees are laid out on the beach and held in place between two parallel fences. Sand that blows in from shore gets caught in the branches, eventually packing into a full sand dune over the course of several seasons. Unlike piles that have been pushed together with bulldozers, sand dunes that are allowed to build naturally over time provide a more stable barrier against storm surges. When the time is right, the town plants dune grass to give the structures even more stability, with the vegetation's hairy roots anchoring trees in the sand.


An elephant plays with a Christmas tree at a zoo in Germany. Image credit: Odd Andersen // Getty Images

In the wild, many animals encounter plant life that changes with the seasons. The Oakland Zoo in California hopes to simulate this seasonal variety in captivity with annual Christmas tree donations. Each year, a local Christmas tree lot hands over whatever’s left of their inventory at the end of the season. The zoo’s residents are more than happy to take the trees that others didn’t want—zebras munch on the needles, squirrel monkeys swing from branch to branch, and otters play games of “smell and seek” with treats hidden in the trees by zookeepers. Oakland’s zoo isn’t the only one to take advantage of the surplus of trees at the end of year. The Staten Island Zoo, the North Georgia Zoo, the Linton Zoological Gardens in the UK all accept tree donations.


Christmas trees are a key tool in the fight to save Louisiana’s marshland. The state loses 25 to 35 miles of coastal wetlands a year to advancing ocean tides, and one thrifty way to prevent further damage is by building fences around the marsh’s perimeter. Since the Santa Saves the Marsh project began in 1986, over 1.5 million Christmas trees have been used for this purpose. Following the holiday season, bundles of timber collected from around the country are flown in via a helicopter on loan from the Army National Guard and dropped into the wetlands below. These trees are used to stuff pre-built wooden pens surrounding the bayou. Today, more than eight miles of Christmas tree fencing lines the vulnerable habitat, and it’s already proven valuable: When Hurricane Katrina hit Louisiana’s southern coast in 2005, the shoreline behind the barricade was better protected.


Christmas trees likely aren’t the alternative fuel source of the future, but that hasn’t stopped Burlington, Vermont from ringing every watt of energy they can get from their seasonal haul. The Joseph C. McNeil Wood and Yard Waste Depot collects hundreds of unwanted trees from households and Christmas tree lots at the end of each holiday season. That organic waste gets fed to a wood chipper, and part of the mulch that comes out is sent to the local power plant where it’s tossed into a boiler. The heat generated from the boiler evaporates water into steam that's used to power the turbine in the plant's generator. Each tree that's incinerated amounts to about 36 cents worth of energy for the town.

The Merry Mulch Project isn’t able to produce enough fuel to keep the plant running on 100-percent Christmas tree power (for that, the boiler would need to be fed the equivalent of 100 trees per second), but luckily, Burlington uses other renewable resources like wind and water to keep the city running throughout the year.


It’s hard to go for a hike through Dunbar Cave State Park in Tennessee without trampling on ghosts of Christmas past—all of the mulch used to cushion their trails is made from old Christmas trees. A thousand trees are mulched by the park as part of their annual Trees to Trails program and laid along pathways by volunteers. According to the National Christmas Tree Association, Friends of Dunbar Cave board member David Boen said they stick to Christmas trees exclusively because “by definition they don't have any invasive species or seeds.” In addition to making them easier to walk on, mulch also protects trails from damaging water run-off.


Since 2012, artist Michael Neff has installed a seasonal art project in New York City. “The Suspended Forest” started with a handful of forgotten Christmas trees hung illegally beneath an overpass in Williamsburg. The most recent iteration included 40 floating trees harvested from sidewalks and tree lots after Christmas. They were on display in a warehouse in Queens through the month of January (this time around, Neff had actually received permission to put them there). He hopes to keep bringing the exhibit back to New York and potentially re-imagine it for different cities in the future.


If a Christmas tree doesn’t end up hanging in a warehouse, decomposing on a lakebed, or providing festive scenery for a landfill, it’s most likely turned into mulch. Plenty of towns pulverize their discarded Christmas trees for use in parks and public spaces, but San Diego does something a little different with theirs. For decades the Miramar Greenery has invited city residents to pick up free mulch and compost for use on private property. After dropping off unwanted trees at locations around town or dumping them on the curb, families can visit the Greenery later in the year to collect the mulch they helped contribute to. In a single year, the recycling program can make mulch out of nearly 1000 trees, making the city’s Christmas trees the gifts that keep on giving.

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iStock // Ekaterina Minaeva
Man Buys Two Metric Tons of LEGO Bricks; Sorts Them Via Machine Learning
May 21, 2017
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iStock // Ekaterina Minaeva

Jacques Mattheij made a small, but awesome, mistake. He went on eBay one evening and bid on a bunch of bulk LEGO brick auctions, then went to sleep. Upon waking, he discovered that he was the high bidder on many, and was now the proud owner of two tons of LEGO bricks. (This is about 4400 pounds.) He wrote, "[L]esson 1: if you win almost all bids you are bidding too high."

Mattheij had noticed that bulk, unsorted bricks sell for something like €10/kilogram, whereas sets are roughly €40/kg and rare parts go for up to €100/kg. Much of the value of the bricks is in their sorting. If he could reduce the entropy of these bins of unsorted bricks, he could make a tidy profit. While many people do this work by hand, the problem is enormous—just the kind of challenge for a computer. Mattheij writes:

There are 38000+ shapes and there are 100+ possible shades of color (you can roughly tell how old someone is by asking them what lego colors they remember from their youth).

In the following months, Mattheij built a proof-of-concept sorting system using, of course, LEGO. He broke the problem down into a series of sub-problems (including "feeding LEGO reliably from a hopper is surprisingly hard," one of those facts of nature that will stymie even the best system design). After tinkering with the prototype at length, he expanded the system to a surprisingly complex system of conveyer belts (powered by a home treadmill), various pieces of cabinetry, and "copious quantities of crazy glue."

Here's a video showing the current system running at low speed:

The key part of the system was running the bricks past a camera paired with a computer running a neural net-based image classifier. That allows the computer (when sufficiently trained on brick images) to recognize bricks and thus categorize them by color, shape, or other parameters. Remember that as bricks pass by, they can be in any orientation, can be dirty, can even be stuck to other pieces. So having a flexible software system is key to recognizing—in a fraction of a second—what a given brick is, in order to sort it out. When a match is found, a jet of compressed air pops the piece off the conveyer belt and into a waiting bin.

After much experimentation, Mattheij rewrote the software (several times in fact) to accomplish a variety of basic tasks. At its core, the system takes images from a webcam and feeds them to a neural network to do the classification. Of course, the neural net needs to be "trained" by showing it lots of images, and telling it what those images represent. Mattheij's breakthrough was allowing the machine to effectively train itself, with guidance: Running pieces through allows the system to take its own photos, make a guess, and build on that guess. As long as Mattheij corrects the incorrect guesses, he ends up with a decent (and self-reinforcing) corpus of training data. As the machine continues running, it can rack up more training, allowing it to recognize a broad variety of pieces on the fly.

Here's another video, focusing on how the pieces move on conveyer belts (running at slow speed so puny humans can follow). You can also see the air jets in action:

In an email interview, Mattheij told Mental Floss that the system currently sorts LEGO bricks into more than 50 categories. It can also be run in a color-sorting mode to bin the parts across 12 color groups. (Thus at present you'd likely do a two-pass sort on the bricks: once for shape, then a separate pass for color.) He continues to refine the system, with a focus on making its recognition abilities faster. At some point down the line, he plans to make the software portion open source. You're on your own as far as building conveyer belts, bins, and so forth.

Check out Mattheij's writeup in two parts for more information. It starts with an overview of the story, followed up with a deep dive on the software. He's also tweeting about the project (among other things). And if you look around a bit, you'll find bulk LEGO brick auctions online—it's definitely a thing!

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Sponsor Content: BarkBox
8 Common Dog Behaviors, Decoded
May 25, 2017
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Dogs are a lot more complicated than we give them credit for. As a result, sometimes things get lost in translation. We’ve yet to invent a dog-to-English translator, but there are certain behaviors you can learn to read in order to better understand what your dog is trying to tell you. The more tuned-in you are to your dog’s emotions, the better you’ll be able to respond—whether that means giving her some space or welcoming a wet, slobbery kiss. 

1. What you’ll see: Your dog is standing with his legs and body relaxed and tail low. His ears are up, but not pointed forward. His mouth is slightly open, he’s panting lightly, and his tongue is loose. His eyes? Soft or maybe slightly squinty from getting his smile on.

What it means: “Hey there, friend!” Your pup is in a calm, relaxed state. He’s open to mingling, which means you can feel comfortable letting friends say hi.

2. What you’ll see: Your dog is standing with her body leaning forward. Her ears are erect and angled forward—or have at least perked up if they’re floppy—and her mouth is closed. Her tail might be sticking out horizontally or sticking straight up and wagging slightly.

What it means: “Hark! Who goes there?!” Something caught your pup’s attention and now she’s on high alert, trying to discern whether or not the person, animal, or situation is a threat. She’ll likely stay on guard until she feels safe or becomes distracted.

3. What you’ll see: Your dog is standing, leaning slightly forward. His body and legs are tense, and his hackles—those hairs along his back and neck—are raised. His tail is stiff and twitching, not swooping playfully. His mouth is open, teeth are exposed, and he may be snarling, snapping, or barking excessively.

What it means: “Don’t mess with me!” This dog is asserting his social dominance and letting others know that he might attack if they don’t defer accordingly. A dog in this stance could be either offensively aggressive or defensively aggressive. If you encounter a dog in this state, play it safe and back away slowly without making eye contact.

4. What you’ll see: As another dog approaches, your dog lies down on his back with his tail tucked in between his legs. His paws are tucked in too, his ears are flat, and he isn’t making direct eye contact with the other dog standing over him.

What it means: “I come in peace!” Your pooch is displaying signs of submission to a more dominant dog, conveying total surrender to avoid physical confrontation. Other, less obvious, signs of submission include ears that are flattened back against the head, an avoidance of eye contact, a tongue flick, and bared teeth. Yup—a dog might bare his teeth while still being submissive, but they’ll likely be clenched together, the lips opened horizontally rather than curled up to show the front canines. A submissive dog will also slink backward or inward rather than forward, which would indicate more aggressive behavior.

5. What you’ll see: Your dog is crouching with her back hunched, tail tucked, and the corner of her mouth pulled back with lips slightly curled. Her shoulders, or hackles, are raised and her ears are flattened. She’s avoiding eye contact.

What it means: “I’m scared, but will fight you if I have to.” This dog’s fight or flight instincts have been activated. It’s best to keep your distance from a dog in this emotional state because she could attack if she feels cornered.

6. What you’ll see: You’re staring at your dog, holding eye contact. Your dog looks away from you, tentatively looks back, then looks away again. After some time, he licks his chops and yawns.

What it means: “I don’t know what’s going on and it’s weirding me out.” Your dog doesn’t know what to make of the situation, but rather than nipping or barking, he’ll stick to behaviors he knows are OK, like yawning, licking his chops, or shaking as if he’s wet. You’ll want to intervene by removing whatever it is causing him discomfort—such as an overly grabby child—and giving him some space to relax.

7. What you’ll see: Your dog has her front paws bent and lowered onto the ground with her rear in the air. Her body is relaxed, loose, and wiggly, and her tail is up and wagging from side to side. She might also let out a high-pitched or impatient bark.

What it means: “What’s the hold up? Let’s play!” This classic stance, known to dog trainers and behaviorists as “the play bow,” is a sign she’s ready to let the good times roll. Get ready for a round of fetch or tug of war, or for a good long outing at the dog park.

8. What you’ll see: You’ve just gotten home from work and your dog rushes over. He can’t stop wiggling his backside, and he may even lower himself into a giant stretch, like he’s doing yoga.

What it means: “OhmygoshImsohappytoseeyou I love you so much you’re my best friend foreverandeverandever!!!!” This one’s easy: Your pup is overjoyed his BFF is back. That big stretch is something dogs don’t pull out for just anyone; they save that for the people they truly love. Show him you feel the same way with a good belly rub and a handful of his favorite treats.

The best way to say “I love you” in dog? A monthly subscription to BarkBox. Your favorite pup will get a package filled with treats, toys, and other good stuff (and in return, you’ll probably get lots of sloppy kisses). Visit BarkBox to learn more.