How to Identify Hydrilla

  • Hydrilla has pointed, bright green leaves about 5/8 inches long.
  • Leaves grow in whorls of 3-10 along the stem; 5 is most common. Leaves have small teeth or serrations on the edges and at the tips. The reddish center spine shown in some images is often lacking.
  • The most characteristic identifying features are the small, white to yellowish, potato-like tubers attached to the roots, however, they form up to 6 inches deep in the soil and can be difficult to dislodge.
  • Hydrilla grows aggressively, up to a foot a day. 1  Early in the season, it grows horizontally along the bottom of the waterbody. Side shoots and new tubers can develop at the nodes as the plant grows. As the water temperature increases, the stems elongate, sending the shoot tips toward the water surface, creating a thick mat of vegetation. It quickly shades out other aquatic plants, displacing beneficial native species like pond weeds and wild celery.
  • Hydrilla has long slender stems that can grow underwater to lengths of up to 25 feet. Its identifying characteristics are displayed to the right.

Hydrilla Look-Alikes 

  • Hydrilla is easily confused with a native water weed, Elodea Canadensis, whose leaves typically occur in whorls of three and appear smooth‐edged. Hydrilla in the Cayuga Inlet typically has leaves in whorls of five with toothed (serrated) edges
  • It also resembles the invasive Brazillian waterweed  (Egeria densa) which is found downstate in New York and has finely serrated leaves (3/4 ‐ 1.5 inches) in whorls of 3 to 6.
  • Visit New York Invasive Species Clearinghouse for more information about hydrilla and other invasive species. (Adapted from a press release by New York Invasive Species Research Institute at Cornell University) 

Monoecious & Dioecious Biotypes

  • Not much is know about the type, or biotype of hydrilla that grows in NYS. The monoecious biotype grow in the north and the dioecious biotype is more common in the south. The 2 are different enough that some scientist say they are like 2 different species.
  • Hydrilla has both dioecious and monoecious biotypes. Dioecious plants have separate reproductive units; individuals are either male or female and cannot self-fertilize. Whereas, monoecious is classified by each indiviual having both reproductive units that are merely female and reproductive units that are merely male.
  • The dioecious has female flowering plants and originated in Southern India. It was often used in aquariums and probably dumped in Miami and Tampa in the 1950s. From there it has spread and is found in much of the Southern US.
  • Monecious hydrilla is thought to have originated in Korea and was introducted in the US decades later then the dioecious strand in the Potomac Basin and now spans north from South Carolina.

1. Glomski, L.A. and M.D. Netherland. 2011. Does Hydrilla Grow an Inch Per Day? Journal of Aquatic Plant Management, 50:54-57.Click citation to read article online.

Last updated September 4, 2020