Water fern Azolla filiculoides
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Banned From Sale after April 2014. An invasive, non-native plant.
Water fern is a small free-floating water plant that forms dense mats. It was introduced for ornamental use in ponds and aquaria but its introduction into the wild has meant it has spread rapidly throughout England in the last 50 years.
What's the problem?
It spreads mainly vegetatively, which makes it difficult to eradicate as it only takes a small section to float downstream to start a new colony, but it can also spread by producing minute spores. It out-competes native species by forming a dense covering on the surface of the water. This blocks out light and can also deoxygenate the water. The dense mats can also appear to be a solid surface so animals and humans can fall into the water.This species is listed on Schedule 9 of the Wildlife and Countryside Act in England and Wales therefore, it is also an offence to plant or otherwise cause to grow these species in the wild and after April 2014 this species will no longer legally be for sale in England and Wales.
Removing water fern
The easiest way to ensure you get all of the plant matter out of your pond is to scoop it up with a net. You can use a hose to force Water Fern to side of your pond, which should make it easier to scoop up. The RHS suggest that fountains may help to reduce infestations by disturbing the water surface. In larger ponds and lakes, a floating boom can be used to sweep the water surface. Complete control is very difficult and so repeated clearance will be necessary. It is important to attempt control before the spores are released (at the beginning of winter, or once dense mats have formed). If spores have already been released then be extra vigilant the following year to catch reinfestation early.