Landscape image of Fence Rock Lake. A blue lake surrounded by green trees and vegetation with a blue sky with some clouds.

Fence Rock Lake, Guilford - 2023

Survey map of Fence Rock Lake from 2023. Different colors indicate different plant species.
2023 Aquatic Plant Survey Map of Fence Rock Lake
Transect Data | Water Data

Fence Rock Lake is a private 17-acre waterbody located in Guilford, Connecticut. Because of the presence of invasive Brazilian waterweed (Egeria densa), which is unusual in CT, the lake has been surveyed, by CAES OAIS (formally CAES IAPP) since 2009.  From 2009 to 2013, the plant spread to the majority of the littoral zone and was present on 60 percent of the transect points.

In 2014, the lake was treated with the herbicide Reward® using a bottom injection technique. In the summer of 2015, a small population of Brazilian waterweed remained, so the lake was retreated. Our 2016 survey and 2017 surveys found no Brazilian waterweed. In 2019, CAES OAIS performed a survey of Fence Rock Lake and found Brazilian waterweed in a small patch by the private beach. In 2021, it had spread to multiple locations throughout the lake, however, still at levels much lower than in 2013. In 2022, the lake was again treated with Reward®. Compared to 2021, Brazilian waterweed was substantially reduced and was only found in three small locations.

In 2023, CAES OAIS performed another survey to track Brazilian waterweed and document the entire plant community. Fifteen native aquatic plant species were found, two more than in 2021. Most plants were found near the shoreline. Notable differences from 2021 include more watershield (Brasenia schreberi) around the rocky area separating the two coves of the lake and fewer plants in deeper areas. Unlike previous years, there were no populations of slender naiad (Najas flexilis), pondweeds, or coontail (Ceratophyllum demersum) in the deeper areas near Transect 2; however, there were still deep-water plants in other parts of the lake. Watershield remained the most common species in the lake, occupying much of the shoreline. Two invasive wetland plants, purple loosestrife (Lythrum salicaria) and phragmites (Phragmites australis), were also found in a few locations on the shoreline. Overall, Fence Rock Lake has a diverse abundance of native species, and 2023 conditions were similar to 2021 with a decrease in invasive Brazilian waterweed.

Species recorded in the 2023 survey of Fence Rock Lake
Scientific Names
Arrowhead Low watermilfoil Snailseed pondweed
Berchtold's pondweed Quillwort Spikerush
Brazilian waterweed* Phragmites* Watershield
Bur-reed Primrose-willow Waterwort
Cattail Purple loosestrife* White water lily
Humped bladderwort Slender naiad Yellow water lily

Previous Surveys: 2021, 2019, 2017, 2016, 2013, 2012, 2011, 2010, 2009