About Hydrilla

The highly invasive aquatic plant, Hydrilla verticillata, commonly known as 'hydrilla' or 'water thyme' was found in the Cayuga Inlet in 2011. The biotype that was found in the inlet is native to southeast Asia and was brought to the United States through the aquarium trade. Hydrilla was first found in the wild in Florida in the 1950's. It has since spread to many parts of the US. We will never know how hydrilla arrived in the Cayuga Inlet, but it was probably spread by a boater or someone dumping an aquarium irresponsibly.

Hydrilla was often used in aquariums for many of the same reasons it makes for such an invasive pest. It is extremely hardy and can grow in many conditions (including low light levels and poor nutrient areas). Hydrilla can spread quickly since fragments of the plant can sprout roots and establish new populations. Fragments are easily caught and transported by boats and boat trailers, and can be dispersed by wind and water currents. Hydrilla also reproduces and spreads by buds produced along the stems (called turions), or overwintering tubers. Rarely does hydrilla flower and set seed.

Click on image for larger view

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 pondweeds 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 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.

Hydrilla-Elodea image: University of Florida, Center for Aquatic and Invasive Plants

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. View image of hydrilla and look-alikes, that is other water plants that look similar to hydrilla.

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)

1. Glomski, L.A. and M.D. Netherland. 2011. Does Hydrilla Grow an Inch Per Day? Journal of Aquatic Plant Management. (click to go to full report)

Last updated July 13, 2020