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Tree Foam

Welcome back to Ask a Naturalist: your own personal ‘Google’ for all things wild on PEI! I received these photos of foamy trees from Rish McGlynn who wondered what was going on. I’m happy to have a chance to explain this cool natural phenomenon!


Photo by Rish McGlynn, used with permission.

There are two common causes of foam on trees. The bubbles shown here were associated with rainfall, so are most likely the result of a natural soap. Dust that’s always present in the air (even outdoors and in forests!) includes acidic and alkaline particles as well as tiny bits of salts; tree bark has oils. Together, those are the building blocks for soap. When rain mixes dust on the surface of a tree with oil in the bark, a simple soap is created. As gravity pulls the soapy mixture down the rough bark, it gets agitated – just like detergent in your washer – and foams. Voila: tree foam! That’s essentially the same process that causes foam on pavement when we get a heavy rain after a dry spell.


Photo by Rish McGlynn, used with permission.

A second cause is a bacterial infection called flux. If a tree has been injured or otherwise stressed, bacteria can invade the wood and start to breed. This causes a natural fermentation of the sap, creating carbon dioxide and alcohol; eventually pressure builds and the sap bubbles out of the tree. Like the simple soap described above, this mixture gets aerated as gravity pulls it down the tree’s stem resulting in a foamy appearance. Flux has a fermented, beer-like smell, whereas the natural soap is mostly odourless. While flux does indicate that your tree is stressed (often by drought or injury), it is usually temporary and doesn’t kill the tree. If you see it on your landscape trees, you may want to check to see if they need to be watered.

If you have a question about PEI’s wild side, it’s likely others do too! So, follow me on Facebook, join the conversation, and Ask a Naturalist about PEI untamed!

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