Invasive Azolla Fern in a recirculating landscape pond

Asked August 16, 2016, 3:49 PM EDT

I do not have any idea where this invasive plant came from, nor have I found a way to control it locally.

This from the UK: "At CABI we mass-rear the Azolla weevil Stenopelmus rufinasus for the biological control of Azolla and would recommend its use as the best practice control method where the weevil is present. Unfortunately we do not ship outside of the UK and do not have US distributor. I am not aware of any Azolla weevil suppliers in the US, so it may be best to contact local environmental authorities to see if they have any records of the weevil's presence nearby as it is a US native."

I appreciate any help you may offer.

Ethan Shapiro

Wake County North Carolina pond and water gardens horticulture

1 Response

Azolla caroliniana is native to North Carolina and is found in Wake County. It could have come to your system from spores blown in or from birds, or maybe frogs. The first link below is a description of the native mosquito fern for you to compare and see if that is what you have. There are other species, and as you have found it is a big problem in the UK where they are using mainly biological controls, such as the weevil. In this country there are several herbicides approved for use in aquatic systems and those are given in the second link below. If you do not want to use chemicals to control the fern, there are few options. Dipping it out with a net doesn’t work because it fragments so easily and grows so rapidly. It likes acidic water, so maybe raising the pH might help. Again, I don’t know if that would cause problems for your pond. The extreme solution might be to drain the system, let it dry out completely so all the fern is killed and start up again. That is what I had to do with a water garden in my yard that was infested with duckweed, another tiny aquatic plant that grows quickly. We took the plants out, dipped out the fish and frogs and put them all back in a day later. I know of no sources for weevils.

https://plants.ces.ncsu.edu/plants/all/azolla-caroliniana/

http://aquaplant.tamu.edu/management-options/mosquito-fern/