[Pond with Azolla at Under tallarna in July 2019]
Various species of Azolla are indigenous to areas with temperate and tropical climates around the world, including large regions in Asia, The Americas, Africa and Australia. In many of these regions Azolla is considered as a weed and invasive species. Since the 1800’s Azolla has also been spreading increasingly in Europe and it is now considered as a weed and invasive species in for example the UK. In colder regions, such as in Scandinavia, Azolla has not been considered as an invasive species or potential weed because of its inability to survive winters with prolonged freezing.
In Sweden f.ex. Azolla has not yet been classified in terms of risk as an invasive species by ArtDatabanken (The Swedish Species Information Centre) who is the national authority that classifies invasive species. However, researchers at ArtDatabanken are aware of the species and it is considered for classification in the future. Their initial impression is that Azolla would not be considered as a high risk invasive species because of its inability to survive freezing but with climate change, as the prognosis is now, it could become a high risk invasive species in around 10 years time. Read more about the classification process here (in Swedish).
In 2019 Under tallarna who have been growing Azolla in ponds and evaluated it for different uses have discovered that in fact Azolla can survive winters with freezing temperature and reestablish itself when it gets warmer. Because of this, its extremely rapid growth rate under the right conditions, and the risk of warmer winters due to climate change it is probably best advised to not introduce Azolla in areas where it is not already established, including in Scandinavia. In general care should always be taken to not introduce new species in ecosystems where they can spread and outcompete other species.