Like a monster in a horror movie franchise, the hydrilla that long plagued Lake Thoreau before lying low over the past couple of years has now returned.
Though past management efforts have seen some success, the invasive aquatic plant, whose full scientific name is Hydrilla verticillata, has resurfaced in “significant quantities” throughout the lake this summer, Reston Association announced on Monday (July 28).
Historically, RA managed hydrilla by stocking the lake off of South Lakes Drive with sterile grass carp, a type of herbivorous freshwater fish that has been used for aquatic weed control in the U.S. and Europe.
That approach appeared to be effective until the summer of 2020, when the plant spread rapidly to the point it became an issue for boating and other recreational uses, according to the homeowners’ association.
“In an effort to respond quickly, RA applied herbicide late in the summer — but the mass die-off of vegetation released nutrients into the water, triggering one of the worst cyanobacteria (blue-green algae) blooms in recent memory,” RA recalled in a press release.
The decision to treat the lake late in the summer, when high temperatures can fuel algal blooms, was criticized by some RA members at the time, and organization leaders acknowledged that they needed to explore better options for controlling aquatic plants.
With help from a consultant, RA increased its budget for lake management and developed a new strategy for Lake Thoreau that involved both grass carp and herbicide applied in “low” doses early in the growing season, which typically begins with sprouts emerging in the spring and concludes with the growth of tubers — buds that store food and help hydrilla reproduce — in the fall.
According to RA, crews treated Lake Thoreau with herbicide in 2021 and 2022 before pausing treatments in 2023, when the hydrilla appeared to be getting under control and the population of grass carp matured enough to support a “more natural, long-term biological approach.”
While the strategy initially worked with no hydrilla emerging in 2023 and “most of 2024,” RA staff and residents began seeing isolated plants earlier this year.
“There was initial hope that the carp were keeping the growth trimmed short,” RA said. “However, the hot, sunny days combined with Lake Thoreau’s unusually clear water have created ideal conditions for hydrilla to rebound — and it is now approaching ‘top-out’ levels across much of the lake’s shallow areas.”
Careful not to repeat the mistakes of 2020, RA says it will let the hydrilla die back naturally in the winter, with its watershed team removing plants mechanically when feasible.
The team will also continue monitoring the plant’s growth, and RA intends to collect community input to inform its management strategy for next year.
“RA is committed to protecting the health and usability of Lake Thoreau,” the association said. “If you’re experiencing localized hydrilla problems near your property, contact the RA Watershed team for assistance. In the meantime, thank you for your vigilance and continued partnership in stewarding Reston’s beautiful lake environments.”
Lake Thoreau is the only one of RA’s four manmade lakes to encounter issues with hydrilla this year, communications director Cara O’Donnell told FFXnow.
Small amounts of blue-green algae were reported at lakes Audubon and Anne in early July.
According to the Virginia Institute of Marine Science, hydrilla was introduced to the U.S. in the 1960s and was first spotted in the Potomac River in 1982, spreading since then throughout the upper Chesapeake Bay. Its ability to rapidly grow and reproduce means it crowds out native species in freshwater bodies, and it can be disruptive for irrigation and boating.
