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Croton River Hydrilla Control Project

DEC and partners developed a plan to control Hydrilla verticillata (hydrilla) in the Croton River. Hydrilla grows and spreads rapidly and is one of the most difficult aquatic invasive plants to control and eradicate in the United States. Infestations can have negative impacts on recreation and tourism, as well as severe consequences for aquatic ecosystems.

HERBICIDE TREATMENT UPDATE: Croton River herbicide treatment began on June 7, 2021 and will continue for 90-120 treatment days (dependent on weather and plant condition). Surface water spill from the New Croton Reservoir will be monitored again in 2021 for viable hydrilla fragments. Village of Croton drinking water wells and finished water will continue to be sampled twice weekly at all locations, as long as fluridone levels are below 1 ppb. If levels are equal to or exceed 1 ppb, but are less than 4 ppb sampling will occur three times per week. If fluridone levels equal or exceed 4 ppb the project plan will be terminated or modified. After two consecutive samples at "not detected" at all drinking water wells sampling will end. Visit the Water Sample Analysis Results page for drinking water information.

A public stakeholder meeting was held virtually on Tuesday, May 4, 2021. 2021 project details can be found here Croton River Hydrilla Control Project Brochure (PDF). Information covered at these meetings from the past four years can be found in the archive of presentations.

DIDYMO NOTE: In 2018 Didymo (Didymosphena geminata), also known as "rock snot," was found in the Croton River near Silver Lake Park (approximately 1.6 miles downstream from the New Croton Reservoir Dam). Didymo has been reported in the Croton Watershed in the past (West Branch). It is a microscopic algae that prefers cool, clear, nutrient-poor waters and is primarily spread through human activity. Didymo can cling to waders, boots, boats, clothing, lures, hooks, fishing lines, and other equipment. Didymo is likely native to New York, but has only recently become a nuisance due to changing environmental conditions. Learn more about didymo.


Hydrilla was discovered in the Croton River in October 2013 and later found in Croton Bay during a site survey in 2014 (Towns of Cortlandt and Ossining, Westchester County, NY). This survey also revealed that hydrilla is well-established in the Croton River and the New Croton Reservoir. While hydrilla remains in the Croton River and Bay, it threatens habitats in the Hudson River and its tributaries. Fortunately, the results of the survey conducted in 2017 indicate that hydrilla has not yet spread outside of the Croton River and New Croton Reservoir. Public meetings have been held biannually to inform the public of the infestation and address concerns about management plans.

Five Year Management Plan and Annual Project Updates

Through collaboration with national hydrilla experts, the NYC Department of Environmental Protection (DEP), the NYS Department of Health Bureau of Water, DEC agency staff, Village of Croton on Hudson, and environmental stakeholders including Lower Hudson PRISM, Scenic Hudson, Riverkeeper, Saw Mill River Audubon, and Hudson River Sloop Clearwater, a Five Year Management Plan has been developed to guide management decisions using "adaptive management", and defines partnerships and goals, and demonstrates DEC's commitment to the project. The plan is available for public review as is the annual project report which supplements the Five Year Management Plan and provides information about how the project is being adaptively managed. Other successful hydrilla control projects in New York were studied closely when determining appropriate management options regarding the Croton infestation, the Eno River project in North Carolina (leaves DEC website), the Cayuga Inlet project (leaves DEC website) and the Tonawanda Erie Canal project (leaves DEC website) in particular.

Resources about Herbicide Use (Fluridone) and Environmental and Ecological Impacts

"We conclude that the use of fluridone according to label instructions does not pose any effect to human health." Washington State Department of Ecology, Environmental Impact Statement Appendix E (see below)

According to the US EPA Human Health Benchmarks for Fluridone (see below), a person would be adversely impacted by fluridone if he or she consumed 34,500 ppb (parts per billion) of fluridone in a single day or 962 ppb per day over the course of a lifetime.

Proposed Plan for 2021

After successful treatments in 2017, 2018, 2019 and 2020 the hydrilla control program proposed for 2021 again involves the injection of the aquatic herbicide Sonar Genesis NYS Label (PDF), also known as fluridone, into the river just below the New Croton Dam at a concentration of 2.0-4.0 parts per billion (ppb) for 90-120 days. The actual dosage and duration of the application will be determined by flow rates in the river, observed efficacy and label requirements. DEC has received its Article 15 Permit and initial notification of riparian owners is completed. DEC will coordinate with NYCDEP to manage dam releases when possible, thereby maximizing herbicide efficiency. Please see the US Geological Survey website (leaves DEC's website for current flow information.

Because an aquifer providing drinking water to the Village of Croton on Hudson is located beneath the Croton River the proposed herbicide concentrations are far below the acceptable 150 ppb and 50 ppb thresholds for drinking water as established by the US Environmental Protection Agency (EPA) and the NYS Department of Health (DOH), respectively. Furthermore, USEPA Human Health Benchmark studies demonstrate that for a person to show adverse health impacts from fluridone, one would need to consume 34,500 ppb for one day or 960 ppb per day for a lifetime. Additional information can be found on the Sonar Genesis Material Safety Data Sheet (PDF).

To assess impacts of the treatment, pre and post treatment surveys are proposed for aquatic plants, mussels, and insects, as well as continued testing for the cyanobacteria associated with hydrilla and research on the native aquatic plant water celery (Vallisneria americana) regarding genetics and susceptibility. Assessments of the drinking water will be done routinely as per the defined sampling and analysis protocol.

At the start of treatment water samples will be collected from drinking wells and entry point twice weekly. If fluridone is detected at below 1 ppb in the finished water, the sample results will be posted as "normal" and the treatment will continue. If the results are equal to or greater than 1 ppb and are less than or equal to 4 ppb, sampling and analysis will occur three times a week. Results will be posted as "additional monitoring necessary." If detections exceed 4 ppb, the treatment plan will be modified or terminated.

Once treatment ends as long as concentrations are equal to or exceed 1 ppb samples will be collected at the drinking water wells and entry point three days a week. Once the concentration is below 1 ppb samples will be collected from the drinking water wells and entry point on a weekly basis until two consecutive samples with readings of "non-detect" are found at all sample locations. Water analysis reports will be posted on the DEC website as soon as possible.

Hydrilla Survey Results

The results of the 2016 aquatic plant survey in the Croton River show that hydrilla has become more dense and more abundant in the upper portion of the river, particularly around Black Rock Park, Silver Lake Beach, and Paradise Island. Fortunately, hydrilla in the lower portion of the river was less abundant than in the 2014 survey, possibly an indication that the dry summer and brackish water have kept the hydrilla from establishing further. The survey conducted at the end of the treatment in 2017 indicated significant declines in hydrilla abundance throughout the river.

Detailed survey reports developed by SOLitude Lake Management:

Water Use Restrictions

The 2021 treatment began in early June and there will be no water use restrictions for swimming, boating, fishing, and these activities can all continue as normal. However, the chemical label does identify a restricted use at concentrations above 1 ppb for greenhouse plants and 5 ppb for turf grass. Updates on treatment will be posted on the website of Village of Croton website and this DEC webpage. A summary of the information contained on this webpage can be found on the Frequently Asked Questions about Fluridone factsheet (PDF).


The DEC Invasive Species Coordination Section has received its 2021 Article 15 Aquatic Pesticide Permit and its Article 24 Freshwater Wetlands Permit is valid until 2022. The documents were reviewed by DEC Region 3 staff. The Article 15 application process involves the formal notification of landowners along the Croton River and the installation of signage. Learn more about the DEC Aquatic Pesticides Program.

While the 1981 Programmatic Environmental Impact Statement for Aquatic Vegetation Control and the 2014 Amended Findings Statement covers a majority of the project components, it was determined that a coordinated project review was required to satisfy SEQR. As lead agency, the DEC prepared an Environmental Assessment Form (EAF) which was reviewed and approved by the Village of Croton on Hudson's Water Control Commission. A Negative Declaration was issued April 26, 2017 for the five year project.

A Special Local Needs permit (PDF) for Sonar Genesis was issued December 14, 2018 with approval of NYS Department of Health. A Notice of Intent was completed for coverage under the SPDES General Permit GP-0-16-005 on June 19, 2017. A Notice of Complete Application was published in the Croton Gazette on June 8, 2017. NYSDEC Article 15 permit approval was received May 11, 2018. NYSDEC Article 24 permit approval was received June 29, 2017. Land Use Permits for five years from Westchester County and NYCDEP were received in April 2018.

Project Manager

In April 2018 DEC hired a full-time Project Manager for the Croton River Hydrilla Control Project. Nicole White is based in Westchester County and works closely with the Village of Croton and DEC contractors during the field season. Nicole is also involved in local education and outreach and attends the public stakeholder meetings.


As a component of the Five Year Management Plan, DEC has retained SOLitude Lake Management (NYS applicator license # 16506) for the duration of the project (2017-2021) to implement the hydrilla control program, conduct aquatic plant and tuber sampling, collect water samples, and work with all stakeholders. Macroinvertebrate sampling was conducted by the staff of SUNY Oneonta May 2017-May 2018. Researchers from the University of Maryland and the University of Maryland Center for Environmental Science have been contracted to propagate and genotype native water celery (Vallisneria americana) plants from the Croton River. In addition, they will be testing water celery's sensitivity to different concentrations of the herbicide fluridone.

Future Work

Hydrilla is notoriously difficult to manage because it reproduces in several ways. Control projects can take several years of treatment to be considered successful. Detailed reports, updated information and water analysis reports will be made available on this webpage. Please check back regularly for current news.

Contact Information

Please contact us with any questions or if you need additional information.

NYSDEC Bureau of Invasive Species and Ecosystem Health

Nicole White, Project Manager
Cathy McGlynn, Aquatic Invasive Species Coordinator
625 Broadway, Albany NY 12233

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