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Croton River Hydrilla Control Project

DEC and partners have 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.

UPDATE: Croton River Herbicide Treatment ended on October 29, 2017. Daily sampling will continue at all locations until there are two consecutive days of herbicide levels below 1ppb at all locations. Once levels are below 1ppb sampling will occur only at the original sampling locations (Village of Croton drinking water wells and finished water) on a weekly basis for the remainder of the two months after the treatment end date. Visit the Water Sample Analysis Results page for drinking water information.

The next public stakeholder meeting will be held in May 2018, details to follow. Please see the archive of presentations made at past meetings.


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, the most recent of which was held January 24, 2018 to inform the public of the infestation and address concerns about management plans. Surveys conducted after the first treatment in October 2017 indicated an 80% decline in hydrilla abundance in the river.

Five Year Management Plan

Through collaboration with national hydrilla experts, the NYC Department of Environmental Protection (DEP), the NYS Department of Health Bureau of Water, NYSDEC 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 (PDF, 3.4 MB) [updated 4-21-17] 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. 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 2018

After a successful first year treatment in 2017, the hydrilla control program proposed for 2018 again involves the injection of the aquatic herbicide Sonar Genesis NYS Label (PDF, 199 KB), 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 60-120 days. The actual dosage and duration of the application will be determined by flow rates in the river, observed efficacy and label requirements. DEC will coordinate with NYCDEP to manage dam releases when possible, thereby maximizing herbicide efficiency. Please see the US Geological Survey website for current flow information.

An aquifer providing drinking water to the Village of Croton on Hudson is located beneath the Croton River, and because it is unknown if the herbicide will penetrate the aquifer, the proposed 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 41,250 ppb for one day or 1,050 ppb per day for a lifetime. Additional information can be found on the Sonar Genesis Material Safety Data Sheet (PDF, 1.7 MB).

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. Water samples will be collected from drinking wells daily for the first week of treatment, and weekly thereafter. Sample collections will continue at least two months after the treatment ends. Water analysis reports will be posted on the DEC website as soon as possible.

If fluridone is detected at 1 ppb in the finished water, the sample results will be posted as "normal" and the treatment will continue. If the results exceed 1 ppb and are less than or equal to 4 ppb, sampling and analysis will again occur on a daily basis. Results will be posted as "additional monitoring necessary." If detections exceed 4 ppb, the treatment plan will be modified or terminated.

2017 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. A detailed survey report developed by SOLitude Lake Management can be found here 2017 Croton River Aquatic Vegetation Final Report (PDF, 18 KB).

Water Use Restrictions

The 2018 treatment will begin in late May or 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, 0.5 MB).


The DEC Invasive Species Coordination Section will be required to obtain project permits which include Article 15 Aquatic Pesticide Permit and Article 24 Freshwater Wetlands Permit. The documents are 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 Need permit for Sonar Genesis was issued April 5, 2017 with approval of NYS Department of Health. A Wetland Permit was approved by the Village of Croton on Hudson on June 6, 2017. 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 and 24 permit approvals were received June 29, 2017. Land Use Permits from Westchester County and NYCDEP were received June 29, 2017.


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 macro invertebrate sampling and tuber sampling, collect water samples, and work with all stakeholders.

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
Cathy McGlynn, Aquatic Invasive Species Coordinator
Willow Eyres, Response & Management Coordinator
625 Broadway, Albany NY 12233

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