Parks, Recreation and Historic Preservation

Press Release

June 08, 2023

(518) 486-1868 |

New York State Parks Announces Invasive Species and Harmful Algal Bloom Treatments at Harriman State Park

The New York State Office of Parks, Recreation, and Historic Preservation (State Parks) announced that a treatment of the aquatic invasive plant Hydrilla verticillata will begin at Lake Sebago in Harriman State Park on June 8, 2023. Hydrilla is a federally listed noxious weed and a prohibited plant under New York State invasive species guidelines. Hydrilla was first discovered in Lake Sebago in August 2022. State Parks will also begin preventive Harmful Algal Bloom (HAB) treatment applications to Lake Welch in Harriman State Park starting June 12, 2023. Last summer, a HAB affected all of Lake Welch resulting in the closure of the popular beachfront during peak swimming season.

To control Hydrilla in Lake Sebago, up to six applications of the aquatic herbicide fluridone will occur between June and November 2023 and a single application of the aquatic herbicide chelated copper will occur in July. Shoreline signage will also be in place around Lake Sebago.

There are no water use restrictions for boating, fishing, livestock watering, or drinking associated with the fluridone and chelated copper treatments. Public swimming is not permitted at Lake Sebago. An irrigation restriction will be in place for Lake Sebago from June to November 2023.  Treated water should not be used to irrigate greenhouse or nursery plants, turf grass, or any plants in the nightshade family during this time.

Extensive planning for this eradication project began shortly after discovery including an extensive aquatic plant survey and development of a 5-Year Treatment Plan. Similar treatment efforts for hydrilla have been successful in recent projects elsewhere in New York State. To learn more about the Lake Sebago Hydrilla Control Project visit the project's webpage.

State Parks will also commence algaecide application treatment to Lake Welch beginning June 13 with an additional treatment day also planned for June 27. The algaecide will help minimize the potential for a bloom to occur and provide treatment if a bloom does form. During the application of the algaecide and for a 24-hour period after application, swimming, boating and kayaking will be restricted at Lake Welch.

A preemptive treatment is also planned that will reduce the risk of another HAB like the one that closed the Lake in 2022. Additional steps taken at Lake Welch to minimize or prevent HABs include reinstalling solar powered ultrasonic devices to suppress the formation of HABs and continuously monitoring water quality data and the installation of a 2,100-foot-long boom to minimize the movement of a HAB bloom and allow for targeted treatment to occur. For more information, please see the following press release.

New York State Office of Parks, Recreation and Historic Preservation oversees more than 250 individual parks, historic sites, recreational trails, and boat launches, which were visited by a record 78 million people in 2020. For more information on any of these recreation areas, call 518-474-0456 or visit, connect on Facebook, or follow us on Twitter.