Have you seen a foaming tree? Stemflow is the term used to describe the basic act of water flowing down the exterior of a tree or plant. “Plants and trees produce all kinds of different chemicals they use in the biological process of being alive,” said Jay Wasanat the University of Maine. “Certain ones have waste products that are distributed to the [outer] bark of the tree.” In dry conditions, those chemical deposits build up in the nooks and crannies of the tree trunk’s bark. The chemicals include acids, salts and other organic compounds — ingredients used in soap making. If the dry conditions last long enough, a good rain will mix those chemicals together and create a sort of natural tree soap.