The tropical water hyacinth, Eichhornia crassipes (Mart.) Solms, occurs in some of the sloughs and rivers of northern California's Central Valley, a possible northern limit for its weedy distribution. The growth rates and reproductive rates of these water hyacinths, at least for brief periods each year, were comparable to those of water hyacinths growing in the tropics. Their rapid reproduction and high productivity in the summer months enable the populations to be maintained from year to year in northern California despite a high death rate in winter.