Tree Warts, as my 7-year old calls them, are actually burls. And what exactly is a burl, you ask? A burl is a gnarly-looking, extraneous growth found on a tree.

Scientists are still not sure what causes these bulges to occur, but have theorized that a burl may be made when a tree is experiencing stress, injury, virus, or fungal infection. Other scientists believe that certain trees may have a genetic predisposition to forming burls as a result of certain environmental factors, like pollution or the mineral content in the soil.

Although they seem ugly on the outside, burls are highly prized by woodworkers who know what magnificent designs are often found on the inside. Kevin Smith, a plant physiologist at the U.S. Forest Service’s Northern Research Station, published a piece in 2012 on burl biology in the newsletter of the Massachusetts Tree Wardens’ and Foresters’ Association. Next to his article was a report detailing several Massachusetts burl thefts, including one where arboreal bandits climbed 30 feet into a “very old sugar maple” to steal two burls from either side of the trunk. 

I’m no burl-poacher, but I will steal a word from Berkshire cave-explorer Clay Perry. Instead of going spelunking, why not try burl-lunking! Skip the cavernous outcroppings, and instead hunt for the burliest growths. Grab a ruler, dive through the winter forest and see who can find the biggest, baddest, burl. (I’m making this up as I go folks, as we do). 

Some great places to go burl-lunking are Golden Hill Town Forest in Lee, Old Mill Trail in Hinsdale/Dalton, Dorothy Frances Rice Wildlife Sanctuary in Peru, Greylock Glen in Adams, Kennedy Park in Lenox and Bullard Woods in Stockbridge. 


Want to recreate the swirlicious beauty found on the inside of the burl? Try out one of the painting techniques below! Do a Google image search “inside burls” for inspiration. 

Burl Fork Painting


  • Heavier paper or cardstock
  • Paints
  • Paintbrushes
  • Forks

Cut out a loose cross-section of a tree. Do it freehand, trees are all shapes & sizes! (I saved the scraps for another project.)

We taped our paper wood cookie down on the table and started to paint a small section. Then while the paint was still wet, the kids grabbed a fork and created their designs!

Milk Swirl Painting


  • Shallow dish
  • Milk (Fuller fat milk works best)
  • Food Coloring
  • Dish Soap
  • Q-Tips
  • Paper

Fill a shallow dish with milk just so the bottom is covered. Choose 3 or 4 colors of food coloring and add a couple of drops of each in the center of your dish. 

Carefully pour one drop of dish soap into the center of the food coloring and observe what happens!

The kids then dipped their q-tips in soap and swirled the food coloring into different designs before it all mixed together.

We dipped a few of our paper scraps into the swirls before they were thoroughly mixed.

 Sources: The Biology of Burls

What is a Tree Burl

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