Water stargrass
Overview and Concerns
Water stargrass (Heteranthera dubia) is a species of perennial aquatic plant in the Pontederiaceae family. It lives in freshwater such as lakes, rivers, and irrigation canals; preferring calmer waters. In the Lower Yakima River, water stargrass is prolific in many stretches of the lower 47 miles, below Prosser Diversion Dam. Water stargrass forms a bank to bank monoculture and can grow from stream bottom to the water surface and then continue growing horizontally forming a canopy at the water surface. It also forms dense vegetative mats over stream substrates covering gravels cobbles.
Water stargrass causes a suite of issues including:
- River displacement from total volume of water stargrass
- Increased sedimentation from reduced streamflow
- Changes to bottom substrates and gravel quality that reduce salmonid spawning habitat
- Changes to fish migration
- Changes in water quality including fluctuations in dissolved oxygen levels and pH from stargrass respiration and photosynthesis
- Plugged or or block irrigation screens and fish ladders
- Reduced recreation opportunities and challenges to boating and fishing
Studies
Multiple studies have been completed or are on-going on stargrass and the Lower Yakima River.
A 2009 report[1] by the USGS and Benton Conservation District detailed a 2004-2007 assessment of eutrophication in the lower 116 miles of the Yakima River. This was one of the first studies that characterized the proliferation of water stargrass in the Lower Yakima River. See PDF
A 2011 assessment of the Lower Yakima River includes a chapter on water stargrass[2]. See PDF
A 2020 Master's Thesis on water stargrass in the Lower Yakima River[3]. See PDF
The USGS and Benton Conservation District released a 2025 draft report on stargrass and water quality in the Lower Yakima River from 2018-2020 See PDF. The data is available on the USGS website, link.
Stargrass Control Efforts
In the last few years Benton County Conservation District has ramped up efforts to control water stargrass in the Lower Yakima River. Management and Control Techniques for Water Stargrass are discussed in this report by the Benton Conservation District[4] Link
Starting in 2024, key partners came together to form the Water Stargrass Coalition and develop a shared strategy for addressing the negative impacts of water stargrass on the Lower Yakima. In the spring of 2025, the leadership group of the Yakima Basin Integrated Plan focused on highlighting the importance of reducing stargrass impacts and securing significant financial investments to accelerate work on the ground. As part of this effort, the Roza Irrigation District produced a good overview of stargrass control strategies and efforts in the Lower Yakima River, see PDF. The coalition is implementing several pilot projects in the summer/fall of 2025 and is working on developing a longer-term work plan to coordinate activities and secure support and funding for expanding control work and associated monitoring and research in the future.
Hand
The most effective approach to manage stargrass is to remove the plant material at the roots. The simplest and most straight forward method is hand pulling with or without tools. Several hand-pulling projects have been completed over the last decade, and have shown that treatments can improve water quality, reduce future stargrass growth, and restore conditions that support fall chinook spawning. but they effort would needed to be ramped up considerably to make an improvement.
Mechanical
Mechanical harvesters cut the plant above the sediment and remove the cut plant material from the water.
Mechanical harvesting began in 2021. With approximately 16 acres harvested in 2024. The focus of removal locations has been in the Yakima Delta area, the Wanawish Dam pool, and Prosser Dam pool. A mechanical harvester reaches to a depth of about 3 feet and harvest would need to occur multiple times during the summer.
Herbicides
Potential aquatic herbicide treatments are being assessed. Potential herbicides being considered are only endothall, imazamox, carfentrazone-ethyl, and fluridone.
Flows Management
There is the potential that flow management strategies could help manage water stargrass. High flow events or managed pulse flows in the spring and early summer could reduce early water stargrass biomass growth. The mechanism is likely through bed scouring and erosion that help uproot the plant and reduce establishment and also reduces early plant growth. Although flow management has potential, a pilot study and monitoring would be an important step to better understand this control method.
Websites
Benton Conservation District - Water Stargrass Management. Link Videos
Mid-Columbia Fisheries Enhancement Group - Water Stargrass. Link
State of Washington Water Research Center Spring 2025 Seminar Series- Water Stargrass Video YouTube Link
Media Coverage
| Type | Source | Title | Date | Notes |
|---|---|---|---|---|
| News | KIMA News | WDFW project targets invasive aquatic plant to improve habitat in Yakima River | September 4, 2025 | |
| Blog | WDFW | Collaboration in the Yakima Basin: Project kicking off in September will remove water stargrass, improve habitat in Yakima River | August 28, 2025 |
Citations
- ↑ Wise, D. R., M. Zuroske, K. D. Carpenter, and R. L. Kiesling. 2009. Assessment of Eutrophication in the Lower Yakima River Basin, Washington 2004-07. Page 108. U.S. Geological Survey, Investigations Report 2009–5078.
- ↑ Appel, M., R. Little, H. Wendt, and M. Nielson. 2011. Assessment of the Lower Yakima River in Benton County, Washington. Page 182. Benton Conservation District.
- ↑ Pelly, A. C. 2020. Overabundant macrophyte growth alters ecosystem function in a lowland river. Washington State University.
- ↑ Pelly, A., M. Appel, and R. Little. 2021. Management and Control Techniques for Water Stargrass in the Lower Yakima River. Benton Conservation District.