Q: Do you know how to control mosquito fern, Azolla caroliniana? It has completely covered the surface of my backyard ornamental frog pond. I’ve used a net to skim off much of it, but it reproduces so quickly that it is only a very short fix.

A: Mosquito fern reproduces rapidly and can cover the water in a pond or lake if not controlled. This is bad for fish because the shade reduces growth of oxygenating plants. Young plants are green but they change to red, and then brown, as they age.

Rampant mosquito fern growth indicates there are excess nutrients in the water. This could be caused by frog waste or excess fish feed. Koi are said to eat azolla, if you care to add another creature to your frog pond.

Continue skimming the plant out of the water. This permanently removes excess nitrogen and the green goo you take away can be added to flower beds as a green fertilizer.

Controlling Watermeal and Azolla