The Russian Thistle (tumbleweed) is an invasive species brought to North America a while back. You cannot convince anyone in El Paso, TX that the tumbleweed is anything but the scourge of the earth. It has no redeeming value to mankind and will crowd out any other species that gets in its way. Hydrilla is an invasive water plant that has invaded some Washington lakes and, if left unchecked, results in the extinction of many species and loss of wildlife habitats. More importantly, hydrilla can make kayaking really sucky.
Yeah. Hydrilla is bad news. I can see that dvg is probably right in some occasions, but not all.