Eno River Hydrilla Management Project

Pleasant Green Hydrilla infestation(2014)
Pleasant Green Hydrilla Infestation(2014)


History of Hydrilla within the Eno River Watershed

The Eno River
Eno River State Park

The Eno River is part of the Neuse River Basin. The Eno begins in the northwest reach of Orange County, flows eastward through Durham County and enters Falls Lake. From its origin to Falls Lake, the Eno River flows for approximately 28 miles and encompasses a ~150 square mile watershed area.

A Hydrilla plant
Hydrilla verticillata




In 2005, hydrilla was discovered in a section of the river within the Eno River State Park boundaries. Prior to 2005, the only known occurrence of hydrilla in the Eno watershed was at Lake Orange. Lake Orange has a history of hydrilla dating back to 1992. In 1994 and again in 1998, triploid grass carp were stocked to remove this invasive plant and hydrilla was thought to have been eradicated from Lake Orange. 

Tab/Accordion Items

Eno River Hydrilla Management Task Force

The Eno River Hydrilla Management Task Force (ERHMTF) was formed in 2013 in partnership with government agencies, academic institutions, and non-profit organizations with the goal of controlling and eventually eradicating hydrilla. The partnership is composed of aquatic vegetation management experts, environmental managers, stakeholders and environmental stewards.  The ERHMTF serves as a project oversight group and is composed of four committees (Outreach, Research and Monitoring, Funding, and Public Health). The idea of assembling a task force emerged in November of 2012 when several agency representatives met to discuss the continuing and worsening spread of the invasive weed hydrilla within the Eno River.  The ERHM TF began to coalesce to include the many partners now represented and has met roughly twice per year since 2013. 

Task Force Planning & Development

From 2005-2015 staff from the Eno River State Park monitored the river annually, documenting the presence and observing the spread of hydrilla in the river.  More extensive surveys were conducted between 2013 and 2014. Those surveys included sections of the Eno that were outside of the park boundaries. The objective of that survey work was to delineate the infestation, providing environmental managers with a complete picture of where Hydrilla is within the Eno.   

Some non-herbicide control methods were explored such as in 2011 a team of volunteers spent a day hand pulling hydrilla from the river. This approach proved to be extremely challenging and ultimately unpractical. In 2014, the NC Wildlife Resources Commission coordinated a release of tagged sterile grass carp in conjunction with the deployment of tracking equipment. Grass carp are very effective as a biocontrol agent for hydrilla management in ponds and lakes. The outcome of that study showed that grass carp were effective against the hydrilla, but did not remain in the section of the river where they were released.

Ultimately, it was decided that herbicide would be the best control agent against the hydrilla infestation. Before herbicide application began, toxicology studies were performed at NC State University to determine native mussel and native snail sensitivity to the herbicides being considered.  This research showed that there would be no impact on the mussel and snail survivorship, growth or reproductive abilities at the concentration levels needed to suppress hydrilla. 

Summary of Eno River Hydrilla management activities

During the years 2015-2019, a section of the Eno River was treated with an aquatic-use herbicide. The treatment was accomplished using a sophisticated metering system. The specific section of the Eno that was treated was modified over the five years, but generally, the treated section was defined by Lake Ben Johnson dam and Roxboro Road. This marked the first time a flowing body of water was treated using this method in North Carolina. The annual treatments have been very successful.

The herbicide treatments did not prohibit or impact in any way, recreational activities such as wading and swimming nor did it impact fishing or fish consumption. The treated water remained fit for consumption by animals (livestock, domestic pets, wildlife, etc.).

Monitoring efforts were conducted throughout the project; no negative impacts to the environment have been observed/reported to date.   

  • City of Durham
  • City of Raleigh
  • Durham County
  • Eno River State Park
  • NC Dept. of Agriculture
  • NC Division of Parks and Recreation
  • NC Natural Heritage Program
  • NC Wildlife Commission
  • NC Division of Water Resources
  • NC State University
  • Orange County
  • Town of Hillsborough
  • Triangle Fly Fishers
  • US Army Corps of Engineers
  • US Fish & Wildlife Services

2023 Summary

The 2nd year of treatment since the pause began on time in 2023. The treatment began May 15th and ended August 31st. Two injectors were again used to meter the herbicide into the river. Reach surveys conducted in the fall found no Hydrilla in the treated area. This was also the first year that formal surveys were conducted of private ponds in the upper part of the watershed.  A total of 28 ponds were surveyed and Hydrilla was identified in 3 of them.   The Task Force is planning to continue the treatment of the river, surveying private ponds and begin management of the private ponds in 2024. 

2022 Summary

After a 2 year pause to assess the treatment's efficacy, herbicide treatments resumed in 2022. As in previous years, two injectors were used to meter fluridone into the river. The treatment area was from the tailrace of the Lake Ben Johnson dam in Hillsborough to the HWY 501 bridge in Durham.  Due to logistical issues and environmental conditions the treatment began later in the season.  The target treatment window was May 15th to August 31st. The treatment this year began June 27th and ended October 5th. Due to the late start and low flows throughout the summer the reach surveys following the treatment found that hydrilla still remained in the river.  This was the first time Hydrilla was found in the treated area since the project began. The same area will be treated in 2023.

2021 Summary

During the 2nd year of the treatment pause, surveys and inspections continued to measure the efficacy of the treatments of previous years. The reach surveys that were started in 2020 were done again in 2021.  Hydrilla was also found in Sevenmile Creek, which is upstream of the treatment area. This area will continue to be monitored and treated, as needed.  In addition to the surveys, treatments continued upstream in Lake Orange, and it’s associated sub-impoundments. This year, we saw an increase in Hydrilla distribution and density in the river. With the increasing hydrilla population and significant progress made to address Hydrilla in private ponds in upper part of the watershed, the ERHM task force decided that they would resume treatments in 2022.

2020 Summary

After 5 years of treating the Eno River with fluridone, treatments were paused for 2 years to assess the efficacy of the herbicide on hydrilla in the river. Inspecting the site for hydrilla propagules (i.e. tubers) was made difficult and inefficient due to the river bottom being very rocky in many places. Instead, the river was divided into sections, or reaches, that different partners would be responsible for monitoring.  The survey area was from the confluence of the East and West Fork Eno Rivers to Penny’s Bend.  The surveys relied on visual reconnaissance and were conducted by kayaking/wading through the river. The reach surveys showed the treatments decreased the density and range of hydrilla but hadn't removed it. Treatments were also paused to begin determining how to address Hydrilla in private ponds in the upper part of the watershed.

2019 Summary

An herbicide treatment (fluoridone) started on June 17th and continued through August 15th.  The herbicide was metered into the river using an injector system, which was routinely readjusted to match the flow of the water.  The section of the Eno that was treated is defined by the Lake Ben Johnson dam in Hillsborough and downstream to Lawrence Road bridge in Hillsborough.  This is a shorter stretch of the river compared to previous treatment targets.
No Hydrilla was observed/reported within the treatment area in 2019.


2018 Summary

An herbicide treatment started on May 24th and continued through August 15th.  Following the promising results from previous years, the herbicide application and methods were repeated.  The section of the Eno that was treated is defined by the Lake Ben Johnson dam in Hillsborough and downstream to the Roxboro Street bridge in Durham.  The herbicide (fluridone) was applied by two injection systems.  Each system precisely metered the herbicide product and was routinely adjusted to match the flow of the river.  Monitoring efforts occurred throughout the season; no negative impacts to the environment were observed/reported.  Water samples pulled from the treatment area on August 28th were analyzed for herbicide levels and all samples were found to have <1 part per billion (ug/L) fluridone.  This is equivalent to 'no detect' levels. 
No Hydrilla was observed/reported within the treatment area in 2018. 


2017 Summary

An herbicide treatment started on June 30th and continued through August 15th.  Since the first two years of managing Hydrilla with herbicide had been very successful the ERHM Task Force decided to expand the treatment area in 2017.  The area that was treated in 2015-2016 was included in the treatment.  The change in 2017 was that the treatment area was expanded upstream to just below the Lake Ben Johnson dam.  Similar to 2016, two injection systems were used to meter herbicide into the river. 


2016 Summary

The treatment area and herbicide product remained consistent with the 2015 treatment. The herbicide product was metered into the river using two injector systems. The addition of a second injector system (located approximately midway through the treatment area) allowed for better control of the target concentration and allowed for a reduction in the total quantity of herbicide needed. The herbicide treatment did not prohibit, or impact in any way, recreational activities such as wading and swimming nor did it impact fishing or fish consumption. The treated water remained fit for consumption by animals (livestock, domestic pets, wildlife, etc.).


2015 Summary

An aquatic-use herbicide was used to treat a section of the Eno.  A liquid formulation of fluridone was metered into the water at a single location.  The treatment began in June and ended in August.  The treatment effectively controlled the growth of Hydrilla throughout the ~16 mile long stretch.  The treatment area defined as the Lawrence Road bridge (just downstream of Hillsborough) and down to N. Roxboro Street (north of Durham).

Hydrilla (Hydrilla Verticillata)

Key characteristics to look for:

  • Leaflet Number: 3-10 leaves per whorl
  • Leaf Margin: Finely serrated
  • Leaf/Leaflet Shape: Oblong
  • Leaf Length: 1/3– 1/2 inch

Hydrilla facts:

  • A fragment of hydrilla (as small as 1" long) can grow into a new plant
  • Hydrilla is an exotic species (native to Asia)
  • Hydrilla is considered one of the worst aquatic weeds in the United States
  • It is illegal to transport hydrilla; federal and state laws prohibit the culture, sale, distribution, etc.
  • Hydrilla grows as a submersed plant, only parts of the plant will reach the surface of the water

Effects of Hydrilla:

  • It has the potential to negatively impact fish communities
  • Interferes with recreation (boating, fishing and swimming)
  • Harmful to native aquatic communities
  • Reduces water flow
  • Loss of property values and tourism
  • Harbors toxic cyanobacteria that leads to bird deaths


What can be done to slow the spread of Hydrilla?

  • Educate yourself and others on Hydrilla
  • Report sightings of Hydrilla to Eno River State Park staff
  • Don't be a vector....  Clean, Drain, Dry is a great management practice, see below:
  1. Remove, brush and wash any visible mud, plants, other debris before transporting equipment/gear.
  2. Check your fishing pole, gear and bait bucket for plants, mud or debris.
  3. Clean your canoe/kayak storage compartments, rigging, etc.
  4. Drain water from buckets, live wells, bilge, etc.
  5. Let equipment completely dry out prior to using at another site.
Stop Aquatic HitchHikers





Drew Gay (Aquatic Weed Specialist)

Eno River State Park

This page was last modified on 05/10/2024