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13th October 2020

Ebsford Environmental are working in partnership with London Borough of Lewisham and The Friends of Beckenham Place Park to manage an Azolla issue at Stumps Hill Pond, part of Beckenham Place Park.

Azolla is a floating water fern, one of the UK’s most invasive non-native aquatic species; proscribed under Schedule 9 of the Wildlife and Countryside Act 1981 (as amended) and it is an offence to allow the plant to spread into the wild.

The plant forms a thick and dense mat over the surface of a pond preventing sunlight reaching the water and depleting oxygen resulting in a catastrophic loss of native diversity. Spreading extremely easily through the transference of organic material, the fragmented nature of the plant means that minute parts can easily regenerate and be transported between water courses, inadvertently on boots, on fishing or pond dipping equipment, dogs and perhaps most hard to control – wildfowl.

Like all ferns, in autumn the plant releases thousands of spores, meaning transmission of the species is almost impossible within the immediate vicinity. 

Mechanical control of the plant is often equally damaging to the biodiversity of the pond and it is almost impossible to avoid damage to banks. Stumps Hill Pond was regenerated last year by Ebsford with banks recreated, and planting carried out, so we were very keen to limit any damage to these. The use of Glyphosate based chemicals can be employed for control, however there is increasing demand to limit the use of chemicals within sensitive and public locations.

Ebsford were approached by Lewisham with the floating fern issue, which we quickly ascertained was extremely serious. A dense mat of azolla covered the entire surface of the pond and had started to terrestrialise the marginal damp areas of the newly created pond edges. 

Within the wider Beckenham Place Park, there is an extensive new lake complex which it was extremely important to try and keep azolla free.

Ebsford opted to manage the weed naturally using a Bio Control agent – a north American weevil which predates the plant in its native range. A seed population of the weevil Stenopelmus rufinasus is released onto the mat and as the population explodes the weed is depleted. The weevil relies exclusively on Azolla and are host specific, meaning they are completely safe and bio secure – posing no risk to native ecosystems. Because of its long occupancy, the weevil is now considered by DEFRA to be ordinarily resident and therefore no licensing is required.

With the help of local volunteers, a staggered seed weevil population was released at Stumps Hill in July 2020, over a period of 4 weeks. By week 2 a significant reduction in both density and extent of the mat, with clear water starting to show and within 8 weeks the Azolla was almost completely gone from the surface of the pond with clear open water visible.

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