A burl is a strange-looking collection of tree cells, which are called callus tissue. It is a tree growth in which the grain has grown in a deformed manner. Burls are commonly found on tree trunks or branches and are filled with small knots from dormant buds. They are made up of a tree’s bud tissue that has not sprouted into foliage or grown into a twig. Burls form as a result of stress that the tree has undergone, such as an injury, virus, or fungus. They are a tree’s wound response used to compartmentalize or seal off injured areas. Burls are not harmful to the tree and are prized by woodworkers for their beautiful patterns.

Burls can grow on many types of trees but are more commonly found on some than others, including:

• Maple
• Oak
• Redwood
• Walnut
• Cherry
• Birch
• Elm
• Redcedar
• Black locust
• Apple
• Pear

It is important to note that cutting off a burl is like making a wound that the tree can’t seal off. Burls don’t have the same response chemicals as normal branch wood does, so a tree can’t close over a cut-off burl the way it can when a branch is pruned off. Therefore, it is recommended not to cut off a burl as it can leave a burl-sized wound with wood tissue that’s exposed to disease.