The Crassulaceae is a family of mostly succulent plants. British native plants in the family include stonecrops (Sedum species), Wall Pennywort (Umbilicus rupestris) and Roseroot (Rhodiola rosea), which are adapted to well-drained soil, such as cliffs and walls.

New Zealand Pigmyweed removal, Sweetbriar Marshes, 2012

The New Zealand Pigmyweed, Crassula helmsii, differs from the above members of the family in two ways. Firstly, it grows in ponds and secondly, it is an introduced species from Australia and New Zealand, first introduced into the UK in 1911 as an oxygenating plant for garden ponds. It was first encountered in the wild in the UK in 1956 at Greensted Pond in Essex. It is very invasive and competes with native plants, often forming dense patches of growth where other plants can’t grow. There are pictures, a distribution map and more information on the GB Non-Native Species Secretariat’s website here.

We accidentally bought the plant twenty years ago from a garden centre, as an oxygenator for our back garden’s pond, but we identified it a few years later and managed to remove it all by hand and destroy it. This is difficult to do on a larger scale, as small, broken off pieces of the plant can root and grow.

We walk at Sweetbriar Marshes in Norwich every few weeks and, since our last visit, two ponds have been cleared of the plant and filled in to stop it from regrowing and spreading to other parts of the nature reserve. Five new ponds have been dug on another part of the marshes to provide alternative accomodation for wildlife. The former ponds are now fenced off and bear notices to inform visitors what is going on.

Frog and New Zealand Pigmyweed at Sweetbriar Marshes in 2010

Hopefully the new ponds will provide spawning grounds for frogs and toads. These used the old ponds, though with a couple of dry springs most of the amphibians seem to have decamped to nearby dykes, which remained full of water when the ponds dried out.

Filling in ponds to remove an invasive non-native species is drastic but sometimes necessary. New Zealand Pigmyweed is just one species that has been introduced by humans and done rather too well in its new home. Others are also covered by the GB Non-Native Species Secretariat’s website.