Water hyacinth (Eichornia crassipes):

Water hyacinth grows and reproduces rapidly and is considered one of the world's worst invasive plants. It is a free-floating plant with thick glossy leaves, inflated leafstalk and lavender flowers. Dense floating mats (up to 200 tons per acre) interfere with navigation and other lake uses, outcompete native plants and provide good breeding ground for mosquitoes. A native of Brazil, water hyacinth is now established in over 150 drainage basins in eight southern states, California and Hawaii. Herbicides, mechanical removal and biological agents (e.g., water hyacinth weevils) are used for control. Water hyacinth will die in freezing weather.