New Zealand Pigmyweed – the scourge of Derwent Water

woman in wetsuit walking into Derwent Water

You crunch along the shingle lakeshore the morning after a storm. Festooned along the bay are fat green sausages of washed up pondweed, so deep they squish down when you have to walk over them. You might think to yourself, ‘the plantlife in Derwent Water’s certainly doing well!’ And therein lies the problem – one particular plant is doing far too well.

It's easy to see New Zealand Pigmyweed washed up on the lake shore
new zealand pigmyweed in someone's hand
It's easy to see New Zealand Pigmyweed washed up on the lake shore

New Zealand Pigmyweed is an alien invasive species. In Derwent Water it’s thriving so well it’s formed thick mats of vegetation across the lake bed, out-competing the native species like Water Plantain which are on the Biodiversity Action Plan red list of species under threat. It's also covering up the spawning gravels for the UK’s rarest freshwater fish, the Vendace, which only survives in Derwent Water and one other lake in England.

Vendace are very rare freshwater herrings
A Vendace held in a person's hand
Vendace are very rare freshwater herrings

But that’s not the greatest danger

The greatest danger is the risk of infecting other, pristine, lakes in the Lake District.

New Zealand Pigmyweed excels at hitching a lift on people’s shoes, wetsuits, canoes, rubber dinghies, dog’s paws. It’s impossible to eradicate once it’s in a water body.

Have you ever tried to get rid of ground elder from your garden? It’s hard because the plant is so resilient it can regrow from just a 2cm scrap of root left in the soil.

New Zealand Pigmyweed is similar. The plant can survive in a damp fold of a wetsuit for weeks. Then the next time you go for a swim in a different lake or river, even a tiny scrap falling from your kit could spread the invasive species to a new water body – creating ever increasing pressure on our native wildlife species.

How did it get to Derwent Water?

New Zealand Pigmyweed is native to coastal regions in Tasmania and New Zealand. It was imported to the UK in 1911 for sale as an oxygenating plant in aquaria and garden ponds.

It may have been transferred to our lakes and rivers by hitching a lift on wild birds like heron, or it might even have been someone emptying out their aquarium into a natural water body.

It was first recorded in the wild in Essex in 1956 and has spread from there. The first recording in Derwent Water was in 1996, Bassenthwaite 1997, Grasmere 2002.

By 2003 nine native species of water plants had been displaced by this invasive species from Derwent Water alone.

" The scale of the problem is big, and getting worse – in recent years it's started to wrap itself around wild swimmers in the lake. The worst thing is we don't know how much there is – it's simply unmeasurable."
- Penny Webb, Countryside Manager North Lakes

We’re really concerned that it could spread to pristine lakes just a few miles over the fells – like Buttermere, Crummock Water and Loweswater – especially as wild swimming becomes more popular.

What can people do to help?

Whenever you swim, paddle, SUP or sail in Derwent Water, the really important thing is to make sure you don’t take the weed with you when you go.

  • CHECK – that you’re not carrying any weed on your kit before you load it into the car. Washing your wetsuit, stand up paddle board or canoe with lake water on the lakeshore before you leave will help the weed stay put.

  • CLEAN – clean your kit when you get home; but don’t do this in the bath or the washing machine where the weed could go down the drain and infect your nearby rivers! A bucket in the garden works well, where the weed will get washed onto the ground and eventually dry out and die.

  • DRY – make sure your wetsuit or canoe is thoroughly dry before you use it in a different water body. New Zealand Pigmyweed can survive for weeks in a damp crevice, but will die if it dries out.

Triathlons and aquathons

We’re working hard with companies organising mass swim events to make sure they’ve got good biosecurity measures to help stop the spread of invasive species like New Zealand Pigmyweed in the Lake District.

Next time you’re signing up for a swimming event, ask the organisers first how they plan to deal with biosecurity. The more people who ask, the more seriously they’ll take it.

We all have a part to play in a shared purpose for prioritising nature in our lives. The healthier our natural heritage is, the richer all our lives are.