This weed is found mostly in waterways in the Piedmont region of North Carolina. It reproduces itself usually by stem and root fragments which disperse through the water. It grows vigorously and forms mats along the surface. It degrades water and blocks waterways.
It is an aquatic plant, found both submersed (as a vine) and on the surface (as a mat). It roots to the bottom and is free floating. Stems can reach as long as 25'. Serrated leaves (0.2 - 0.8" long) grow in whorls of 3 to 8, along the stem. Along the reddish midrib of the leaf, on the underside, there are small spines.
Fact Sheet: Hydrilla
Research: Sarah Meadows, Rob Richardson, and Steve Hoyle - Phenology of Monoecious Hydrilla Over Three Years
Photo credit: Chris Evans, River to River CWMA, Bugwood.org