New Zealand pigmyweed Crassula helmsii

New Zealand pigmyweed (Crassula helmsii) hanging over a fence, after being removed from the shoreline of Derwentwater in Cumbria, England. This is an aquatic or semi terrestrial perennial with a floating or creeping growth habit. It is an invasive alien species introduced to the UK as an oxygenating plant for aquaria. It has become naturalised in the UK where it colonises wetlands and slow flowing or standing freshwater. It reproduces vegetatively from nodes along its stem, taking only a small piece of stem with one node to produce a new plant. Fragments of the plant are easily transported, leading to local dispersal. Its vegetative growth leads to dense mats which outcompete native flora and impoverishes the ecosystem for invertebrates and fish. The cost of control of New Zealand pigmyweed in the UK has been estimated at 3 million British sterling.