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Outside/Inbox: What is that white foam that forms on trees when it rains?

 Foam at the base of a broad-leaf maple
Jo Zimny on Flickr (CC BY-NC-ND 2.0)
How does this all happen at the base of a tree? These soapy components are readily available, simply drifting around in the air.

Every other Friday on Morning Edition, the Outside/In team answers a question from a listener about the natural world.

This week, we tackle a question from Mihaela LaRoche, who lives in northern New Hampshire:

Every time it rains, there is a white foam forming at the base of the tall big white oak and pine trees on our property… and it disappears when the rain stops. What is that? 

The bubbles at the bottom of LaRoche’s trees are evidence of a “crude soap.”

Soap molecules require four basic elements:

So, how does this all happen at the base of a tree? These soapy components are readily available, simply drifting around in the air.
“Salts and acids and alkali substances [float] in the air, in dust,” explains Rebecca Roy, the manager of educational programming for Vermont State Parks. “When it’s dry, these particles are floating around and sticking to surfaces all over the place.”

Tree bark produces oils, which are drawn to the surface of the bark in warm, dry weather.

When it rains, these ingredients dissolve in the water and flow down the tree trunks. They start to bond chemically and create soap molecules. The rough surface of the bark helps to agitate the mixture — just like the friction created when you scrub a soapy dish in the sink — and by the time it gets to the bottom, the process has resulted in a little patch of soapy foam.

While LaRoche observed suds at the foot of white oaks and pines, the tree soap phenomenon is not exclusive to those trees. But the rougher bark on pines and oaks, compared to other species like birches, might contribute to the amount of foam generated.

“It’s a combination of surface area — they’re really big trees — with that really furrowed rough bark, working together to make it something that we observe,” Roy explained.

This phenomenon is also observable in another outdoor setting with a lot of surface area: paved roads. The oils in the pavement combine with the ambient dust, and passing car wheels provide plenty of churn.

But here’s the real question: Could you actually use the tree soap?

“You probably could,” Roy said. “It’s very crude. That would be a wonderful experiment... I would love to see if your listener is up for trying that out and reporting back to us what they discover.”

If you’ve got a question about the natural world, send it as a voice memo to, leave a voicemail on our hotline at 1-844-GO-OTTER, or share it with us on Twitter or Instagram.

Outside/In is a podcast! Listen and subscribe on the streaming platform of your choice.

Justine Paradis is a producer and reporter for NHPR's Creative Production Unit, most oftenOutside/In. Before NHPR, she produced Millennial podcast from Radiotopia, contributed to podcasts including Love + Radio, and reported for WCAI & WGBH from her hometown of Nantucket island.
Outside/In is a show where curiosity and the natural world collide. Click here for podcast episodes and more.

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