Australian Swamp STONECROP / New Zealand Pigmyweed - invading New Forest pond, Alien Mass of pygmy weed (Crassula helmsii). Introducted to the United Kingdom from Tasmania in 1911, sold into the garden trade as an oxygenating pond plant. It was first detected in the wild in 1970 and has become invasive, choking and outcompeting our native plant species. Once established, it is very difficult to eradicate.