DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
New Zealand pygmyweed (Crassula helmsii) growing at the shore of Derwentwater, Borrowdale, Cumbria, UK, in June. C. helmsii is an invasive alien plant species, introduced into the UK in 1927, when it was sold under the name Tillaea recurva by Perry's Hardy Plant Farm at Enfield in Middlesex. Its first appearance as a naturalised escape was in 1956 at Greenstead in Essex. It is now widely spread throughout Britain, appearing in ponds, lakes and gravel pits. It is easily spread from small fragments, and very difficult to eliminate once established. Methods of control that have been tried include herbicides, light exclusion by opaque plastic sheeting, and freezing with liquid nitrogen. None is especially effective, and over large areas, all are impractical. The picture shows C. helmsii (bright green), growing both on shore mud (bottom left), and as an emergent underwater plant.
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