The U.S. Fish and Wildlife Service lists hydrilla – or water thyme – as the “world’s worst invasive aquatic plant,” and one specific strain of hydrilla is drawing the attention of federal agencies, including the U.S. Army Engineer Research and Development Center (ERDC).
Connecticut River hydrilla has proven resistant to traditional control efforts and is threatening to take over New England’s Connecticut River.
“The rapidly spreading invasive plant poses considerable risk to nationwide navigation, flood control, hydropower, recreation and aquatic ecosystems and costs the nation billions in lost revenue,” Dr. Benjamin Sperry, a research biologist at ERDC and part of the Aquatic Plant Control Research Program (APCRP), said.
APCRP is a federally authorized research program directed to develop technology for managing non-indigenous aquatic plant species. The program is working closely with the New England District to better understand and treat this unique type of hydrilla.
In 2016, the genetically unique strain of hydrilla was identified in the Connecticut River in Glastonbury, Connecticut, and has since spread. The non-native weed only needs 1 percent of sunlight to flourish, and currently, there are no proven eradication solutions.
It has also spread into the river’s many coves, tributaries and boat basins. Since the Connecticut River hydrilla is unique from other known hydrilla strains, the plant’s biology is largely unknown at this time.
Surveys conducted along the river in 2019 and 2020 found that hydrilla had spread as far north as Agawam, Massachusetts. Those hydrilla surveys confirmed the plant is spreading and poses a significant risk of invading other regional waterbodies.
“Rapid development of a science-based hydrilla management solution is needed to mitigate these negative impacts and to slow the spread of the new genetic strain,” Sperry said.
Fragments of the plant, which are easily caught and transported by boats and boat trailers, can sprout roots and establish new populations. Fragments also float and are capable of dispersing via wind and water currents.
Researchers have studied its growth patterns and developed a detailed understanding of its weak points and effective treatment strategies. After completing laboratory testing of the efficacy of herbicides approved for safe aquatic use by the U.S. Environmental Protection Agency, researchers conducted field tests on the river during the summer of 2024.
ERDC developed site-specific treatment plans for those areas. Extensive monitoring is set for the next two years to observe hydrilla suppression and the response from native plants. In addition, threatened and endangered species will be monitored, along with the concentrations of herbicides in the water and other chemistry.
The treatment should have minimal-to-no impact to recreation on the Connecticut River and no long-term closures or restricted access are currently anticipated with this work. By treating the hydrilla to suppress its growth, the intent is to diminish these negative effects and in turn benefit the Connecticut River’s natural ecology and the local economy.
“Other methods to stop hydrilla to date have been minimally effective, and physical and mechanical methods do not stop further plant spread,” Sperry said. “Large-scale problems require complex solutions. Our end goal is to develop and provide verified and environmentally compatible management techniques to both guide hydrilla containment and control operations in the Connecticut River.”