Parrotfeather (Myriophyllum aquaticum) Identification and Management in Waterways

Introduction

Parrotfeather (Myriophyllum aquaticum) is an invasive aquatic plant species that grows in ditches, ponds, lakes, and other freshwater habitats. Due to its aggressive nature, parrotfeather will often take over freshwater settings by creating a thick mat covering that prevents sunlight for native aquatic organisms and inhibits regular water flow (Figure 1). Originally from the Amazon River Basin in South America, parrotfeather has adapted to thrive in subtropical climates such as the United States (Wersal et al., 2022). Although parrotfeather has been in the United States since 1890, it was not observed in Utah until 2020 (Wersal et al., 2022). Salt Lake and Cache are the only counties that have reported parrotfeather. However, due to its ability to spread quickly, it is essential that this noxious weed is identified, treated quickly, and otherwise managed.

Identification

Parrotfeather is a perennial heterophyllous plant, meaning that it has both emergent and submerged leaves (Gettys, 2020). Emergent stems can be up to 1 foot long out of water, while submerged stems can grow up to 16 feet (Washington State Noxious Weed Control Board [WANWCB], 2022; Figure 2). Due to the length of submerged stems, runners and stems intertwine, creating a dense mat that covers waterways (WANWCB, 2022; Figure 1).

Emergent leaves are grayish-green and have a feather-like appearance (Wersal et al., 2022; Figure 3). Leaves are whorled, stiff, and have 10 leaflet pairs (20 linear divisions). Submerged leaves are darker and can even be reddish-orange (Wersal et al., 2022; Figure 4). Flowers, located in the leaf axils of emergent stems, are usually white (WANWCN, 2022).

Management

Prevention

Prevention (Table 1) is the first and preferred approach when managing any noxious weed. Being aware of and able to identify aquatic noxious weeds like parrotfeather is crucial for successful prevention. Observe waterways frequently to ensure that streams, ditches, and canals are not infected. Be aware of all plants within waterways to help distinguish when a weed, such as parrotfeather, enters the system. If you have questions about aquatic plant or weed identification, please call your local Utah State University (USU) Extension office or county weed department.

Cultural Methods

If maintained for long enough to dry the soil completely, drawdown can be an effective cultural control of parrotfeather. Drawdowns need extra attention, as a large infestation can retain moisture under the biomass, and the root crown can survive. Also, rain and snowfall can provide enough moisture for the plant to survive (Gettys et al., 2020).

Mechanical Methods

Parrotfeather can be removed by seining (with a net to collect the plant) or by raking it from the pond (AquaPlant, 2021). However, any fragments or remaining parrotfeather roots can re-establish quickly. Because of this phenomenon, mechanical methods are not advised unless an area is completely infested with parrotfeather to avoid inadvertently increasing the infestation (WANWCN, 2022; Table 1).

Table 1. Summary of Non-Herbicide Options for Controlling Parrotfeather

Control method	Notes
Prevention	Teach yourself and others how to properly identify parrotfeather. Observe waterways frequently. Be able to identify all plants within waterways to distinguish when aquatic noxious weeds enter the system.
Mechanical: Small infestation	Mechanical methods are not suggested for small infestations as the removal will most likely cause larger infestations.
Mechanical: Large infestation	Only use mechanical methods, such as raking or seining, in completely infested areas, as mechanical removal in areas with small infestations can lead to larger infestations in waterways.
Biocontrol	Grass carp (freshwater cyprinid fish) is not a suitable biological control method. No sustainable biocontrol method has been found.

Biological Methods

Some research has suggested that grass carp can be a reasonable biological control when stocked at a range of 7 to 15 fish per surface acre in private ponds (AquaPlant, 2021). Other research sources have stated that parrotfeather is unpalatable to grass carp and is not a reasonable control method (WANWCN, 2022). Grass carp are not native to Utah and require a permit from the Utah Division of Wildlife Resources to be added to any waterway. Additionally, as no sustainable biocontrol method has been found for parrotfeather control, introducing carp can lead to changes in aquatic ecosystems, potentially causing more harm than benefit (Table 1).

Chemical Control

Before using any herbicide, always read the label; it is against the law and unsafe to do otherwise. While reading the label, pay special attention to safety requirements, restrictions of use, directions for use, and disposal requirements.

The herbicide label will dictate what areas can be treated with a particular product and restrict options for use under specific site conditions (Table 2).

At the target location, be aware of your surroundings. Slope, vegetation nearby, wind, and temperature can all have a major effect on applying chemical herbicides. When using herbicides to treat parrotfeather, it is important to check the target area’s waterways for other hazards that may prevent the safe and effective use of herbicides.

Table 2. Summary of Herbicide Options for Controlling Parrotfeather in Waterways

Active ingredient	Trade name	Commentsa
2,4-D	(several)	Foliar application provides consistent control.
2,4-D butoxyethyl ester	Navigate	Granular formulation. In-water application.
Triclopyr	Renovate 3	Both foliar and in-water applications provide consistent control.
Triclopyr + 2,4-D	AquaSweep	Both foliar and in-water applications provide consistent control.
Imazapyr	Habitat, Arsenal, Polaris	Foliar application provides consistent control.
Florpyrauxifen-benzyl	ProcellaCOR	Foliar application.
Carfentrazone + 2,4-D	Stingray + 2,4-D	No pre-mixture is available. Excellent control of small infestations. Foliar applications of carfentrazone-ethyl alone will not provide long-term control.
Flumioxazin + 2,4-D	Clipper or Flumigard + 2,4-D	No pre-mixture is available. Excellent control of small infestations. Foliar applications of flumioxazin alone will not provide long-term control.

^aNote. All foliar applications must have enough foliage above water for sufficient herbicide uptake.

Disclaimer

Parrotfeather is an aquatic plant, and using herbicides will require applications over water (Table 2). An applicator performing aquatic herbicide treatments must have the appropriate State of Utah and federal licenses and permits.

Picture Credits

USU Extension and authors provided all photos.

References

• AquaPlant: A Diagnostic Tool for Pond Plants and Algae. (2021). How to control parrot's-feather. Texas A & M Extension. Retrieved October 7, 2024, from https://aquaplant.tamu.edu/management-options/parrotfeather/
• Gettys, L. A., Haller, W. T., & Petty, D. G. (Eds.) (2020). Biology and control of aquatic plants: A best management practices handbook. Aquatic Ecosystem Restoration Foundation. https://aquatics.org/bmp.html
• Washington State Noxious Weed Control Board (WANWCB). (2022). Parrotfeather [Fact sheet]. https://www.nwcb.wa.gov/weeds/parrotfeather
• Wersal, R. M., Baker, E., Larson, J., Dettloff, K., Fusaro, A. J., & Redinger, J. (2024). Myriophyllum aquaticum (Vell.) Verdc. [Fact sheet]. Nonindigenous Aquatic Species Database, U. S. Geological Survey. Retrieved October 7 , 2024, from https://nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=235

October 2024
Utah State University Extension
Peer-reviewed fact sheet

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Authors

Jacob Hadfield, Cody Zesiger, Mike Pace, Mirella Ortiz, and Corey Ransom

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