This week Canberra received 70mm of rainfall in three days; for context, the entire month of March normally only gets 55mm. Luckily this caused little damage, but it did result in an unusual sight around the city. In what appeared to be a bizarre act of vandalism, trees were covered in foam.
It looked like someone had used soap to try and wash the trees, without rinsing it off afterwards. However, the froth is actually the result of a completely natural process.
Plants use a range of chemicals to protect themselves from all their predators. One such type of chemical is called a saponin. These stop animals from eating the plant because they’re mildly poisonous and have a bitter taste.
When heavy rain hits the saponins on the outside of a tree, it froths up just like soap in water. The heavier the rain, the frothier it will get, ultimately resulting in these bizarre-looking, but completely natural, piles of foam.
Thanks to Ellie van Veen for providing the photos and inspiration for this article.
Lachlan Writes Science 