Also called Esthwaite waterweed and waterthyme.

Native to 🇪🇺 Europe, Asia, Africa and 🇦🇺 Australia.  Invasive > learn+quiz Invasive > Global Invasive > in USA+Canada Invasive > report it! Invasive > in USA Invasive > in Michigan

For kids!  This plant described in invasive species placemat (.pdf) series

Learn more about hydrilla Hydrilla verticillata

Biological Control in Eastern USA (.pdf page 99)
Discover Life Encyclopedia of Life Google Google images USDA PLANTS db USGS Wikipedia