New Zealand pigmyweed (Crassula helmsii) on the shore of Derwentwater, Cumbria, England. This invasive, alien plant forms dense mats that out- compete native British species, choking ponds and drainage ditches. It first became naturalised in the UK in the 1950s and is now a notifiable weed. Control methods include herbicides, freezing with liquid nitrogen and covering with opaque materials. None of these methods are effective or practicable on large bodies of water such as Derwentwater.