We want people in garden centres to please use only native species for their ponds

Phil Taylor, Lake District National Park Authority

Phil Taylor, senior ecologist with the Lake District National Park Authority said: "The weed can colonise down to three metres deep, and block out the light.

"It may take over totally.

"There is no realistic way to get rid of it, and we are asking boat owners and fishermen to make sure they do not have these plants entangled in their propellers or nets so they infect other sites."

Just one centimetre of the weed is capable of rooting and growing, meaning there is no realistic way of mechanically removing it from the water.

Pond weed

Lake District rangers do not want to use herbicides, because that would harm plants like the rare floating water plantain, which is already dying out.

Crassula helmsii is freely sold in garden centres as a pond weed, and is sometimes mislabelled as Tillaea helmsii or Tillaea recurva.

Warning leaflets have been distributed

It was introduced to Europe from Tasmania in the 1920s, and first recorded in Derwent Water in 1992.

But by 2000 it's presence in the Lake District was extensive.

British Waterways, English Nature and the Environment Agency are all trying to control it.

The Department for Environment, Food and Rural Affairs is now trying to make it an offence under the Countryside and Wildlife Act to release the weed into the wild.

Mr Taylor said: "It is the classic example of an introduced non-native species.

"We want people in garden centres to please use only native species for their ponds, as they escape into the wild."