Hydrilla is an aquatic plant that has earned the illustrious title “world’s worst invasive aquatic plant”. Listed as a federal noxious weed, hydrilla has made its home in just about every conceivable freshwater habitat including: rivers, streams, lakes, ponds, marshes, canals, ditches and reservoirs.

Hydrilla was first discovered in the United States in the 1960s in Florida. Since then, it has spread to many parts of the U.S.

Hydrilla can grow in a wide variety of water conditions (e.g., high/low nutrients, high/low turbidity, variable pH, up to 7% salinity) and water temperatures.  Unlike most native aquatic plants, hydrilla is capable of growing under extremely low light conditions. Hydrilla is able to begin photosynthesizing much earlier in the morning than native plants so it is able to capture most of the carbon dioxide in the water (which limits growth of other plants).  Hydrilla grows very rapidly (it can double its biomass every two weeks in summer) and has no natural predators or diseases to limit its population.

Dense infestations of hydrilla can shade or crowd out all other native aquatic plants, alter water chemistry, cause dramatic swings in dissolved oxygen levels, increase water temperatures, and affect the diversity and abundance of fish populations.

(Via the U.S. Fish and Wildlife Service)

Hydrilla is an invasive aquatic plant species that can negatively affect local ecosystems. Hydrilla forms dense stands underwater that can alter river flow, shade or crowd out all other native aquatic plants, replace habitat of sensitive species, alter water chemistry and pH, cause dramatic swings and reduction in dissolved oxygen levels, increase water temperatures, and negatively affect the diversity and abundance of fish populations. Hydrilla also has negative impacts on recreation, including making it more difficult or even potentially dangerous for both boating and swimming due to the denseness of its growth. Hydrilla grows in long easily fragmented strands, which readily spread and develop into new plants.

By treating the hydrilla to suppress its growth, the intent is to diminish these negative effects and in turn benefit the Connecticut River’s natural ecology and the local economy.

In partnership with the Connecticut Agricultural Experiment Station – Office of Aquatic Invasive Species (CAES-OAIS), and the Lower Connecticut River Valley Council of Governments (RiverCOG), the U.S. Army Corps of Engineers, and its Engineer Research and Development Center’s (ERDC) Aquatic Plant Control Research Program, are leading a demonstration project to determine the effectiveness of herbicides registered for aquatic use by the U.S. Environmental Protection Agency to safely reduce and control the spread of the Connecticut River hydrilla. The demonstration project and research began in 2023 to guide operational scale field demonstrations of herbicide efficacy in 2024. Additional field demonstrations are being planned for 2025.

How will hydrilla be treated?

The U.S. Army Corps of Engineers will be developed site-specific aquatic herbicide treatment plans for selected sites in the Connecticut River in the summer of 2024. The use of aquatic herbicides will be considered. There are several safe, well understood, and effective herbicides that are available for use. Any selected herbicides will be added to the website and shared with stakeholders once they have been chosen.

Post treatment monitoring surveys will be conducted throughout the fall months to assess the condition of the hydrilla as well as non-target impacts.

Will the hydrilla treatment impact recreation and fishing in the Connecticut River?

The hydrilla treatment should have minimal-to-no impact to recreation in the Connecticut River. Other than when contractors are on-site carrying out treatment, where some restricted public access may be needed, no long-term closures or restricted access is currently anticipated with this work. Regarding fishing, the treatment is expected to have minimal to no effects on fishing or to fishing access.

The five demonstration sites selected for aquatic herbicide demonstrations in 2024 were Keeney Cove, Portland Boat Works, Chapman Pond, Chester Boat Basin, Selden Cove. Phenology sites used for collecting hydrilla growth characteristics were located at sampling locations in Deep River, Mattabesset River, Selden Cove, and Selden Creek. Additional demonstration sites added in 2025 are under review, and final selection of these additional sites will be confirmed as funding and permitting will allow.