Molting season

I recently got a call from a client who was concerned about her tree because a large section of bark was flaking off. My first instinct was, "Uh-oh." When plates of bark come off a tree trunk, it usually means someone ran into it with a car, or hit it with something, or maybe that the underlying wood was rotting. I was quite relieved to find it was a Sycamore (Platanus occidentalis).

All trees shed bark. Unlike animals, trees do not grow in a scalar fashion. Instead, they get longer from the tips of their branches, while secondary growth over time increases the diameter of the wood. This means that we can put a nail in the tree trunk four feet above the ground and come back years later to find it is still four feet high (assuming the grade has not changed). It might be buried deep inside the wood, but a determined search would reveal that it has not climbed any higher. So, the sheet of bark around the stem must crack apart and crumble away over time. It's like a strip of paper wrapped around a balloon. If you blow up the balloon, it will get larger and tear open the paper.

Most people do not notice this phenomenon. It usually happens gradually, with small chunks of bark breaking free here and there, leaving little or no visible sign they were ever there. The underlying bark looks the same as the piece that came off. But Sycamores are unusual in two ways. First, large plates of bark come loose all at once. Second, the inner bark looks quite different than the outer bark. Over time, a mature Sycamore changes from dark and chunky, like most trees, to thin, white sheets that are later shed in flakes.

It's a large part of what give a Sycamore its unique character, but can be rather shocking to the people who live with the tree every day. When caught midstream, the bark loss looks quite bad and leads some people to call their arborist for a look.


But, happily, there is no cause for concern. The tree is doing what comes naturally, and later will be almost entirely white. If you see your Sycamore showing signs of this molting, enjoy the process and don't worry about calling for help.