Water hyacinth Eichhornia crassipes is often rated as one of the world's top ten worst weeds, thanks to its prodigious capacity to spread over the surface of lakes and rivers. It's said that just one plant can multiply to cover an acre of lake surface in eight months, thanks to its ability to produce stolons that sprout new plants from their tips. While vegetative spread explains its local abundance, its short-lived but beautiful flowers have also played a part in its current worldwide distribution in the tropics. It comes from South America but is now a problem in Africa, India and the Far East where it has been introduced as an ornamental species that has quickly rampaged out of control. It carpets parts of Lake Victoria in Africa, impeding navigation, and within a year of being introduced onto the Sudanese Nile in 1957 it had spread along 620 miles of river.