Connecticut River Hydrilla graphic

Connecticut River Hydrilla

Hydrilla was first identified in the Connecticut River near Glastonbury, CT in 2016 and has since spread to Essex, Connecticut. It has also spread into the river’s many coves, tributaries, and boat basins. The Connecticut River hydrilla is unique from other known hydrilla strains, as it is genetically distinct. The plant’s biology is largely unknown at this time. The highly invasive aquatic plant, Hydrilla verticillata, known commonly as 'hydrilla' or 'water thyme' was first detected in the Connecticut River in 2016 around Glastonbury, CT. In 2019 and 2020 a survey was conducted from Agawam, MA south to the Long Island Sound. Hydrilla was found as far north as Agawam, MA. The 2019 and 2020 surveys of hydrilla in Connecticut/Massachusetts waters, confirm the plant is spreading and the risk of it spreading further to other regional waterbodies is significant. Fragments of the plant, which are easily caught and transported by boats and boat trailers, can sprout roots and establish new populations. Fragments also float and are capable of dispersing via wind and water currents. Due to the importance of the Connecticut River as an environmental resource and driver of the local economy, stakeholders are seeking an aggressive eradication and management program.

Invading the Connecticut River - The Spread of Hydrilla

What is hydrilla?

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)

Why is hydrilla management important?

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.

What is USACE doing to help manage hydrilla in the Connecticut River?

The U.S. Army Corps of Engineers, and its Engineer Research and Development Center’s (ERDC) Aquatic Plant Control Research Program, will lead 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 project will investigate hydrilla’s growth patterns, water exchange dynamics in the Connecticut River, and evaluate herbicide efficacy in laboratory conditions in 2023 to guide operational scale field demonstrations of herbicide efficacy in 2024.

How will hydrilla be treated?

The U.S. Army Corps of Engineers will be developing site-specific 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 in fall 2024 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.

Where will the demonstration project occur?

The exact locations of where the hydrilla treatment would occur has not been selected yet, but the eight sites being considered are Keeney Cove, Mattabesset River, Portland Boat Works, Dart Island State Park, Chapman Pond, Chester Boat Basin, Selden Cove, and Deep River.

Fact Sheets

Dye Study

Public Meetings

2024 Herbicide Application Schedule


Environmental Documents

Additional Resources

Connect with the USACE Team

Contact Phone Number

(978) 318-8833