India, Oct. 10 -- There's a lake in East Africa that's so big it feels like a sea. Now imagine a thick green carpet spreading across its edges. That really happened on Lake Victoria. According to a U.S. State Department case study on water hyacinth, this plant has infested 53 countries across five continents. The same case study says it covered some 90 percent of Lake Victoria's shoreline. It all began with a plant that people grew because it was pretty.

NASA's Earth Observatory describes how such invasions usually start. Someone plants a water hyacinth in a garden pond because they like its lavender flowers. Then the plant begins to double its area every 6 to 18 days. The real trouble begins when the gardener throws it away. The plant i...