Potassium Endothall Considerations

No single herbicide is appropriate for controlling all invasive aquatic plants (or nuisance growths of native aquatic plants), in all situations. A herbicide may perform differently depending on the waterbody, its use, the time of year—or even the time of day. Therefore, aquatic plant managers must have a thorough understanding of how each herbicide acts in Florida aquatic systems. The following parameters are evaluated when considering this herbicide to manage aquatic plants in a specific waterbody. Each parameter is linked to an explanation and examples are provided to demonstrate their relevance to developing comprehensive aquatic plant management strategies.

Table A:  Herbicide Use Patterns for Potassium Endothall

Target Plant Scientific Name Use Pattern Other Herbicides
Hydrilla Hydrilla Verticillata Frequent Potassium endothall alone or applied at lower rates in conjunction with low rate of bispyribac or penoxsulam
Hydrilla Hydrilla verticillata Occasional Potassium endothall applied with low rates of diquat or amine endothall for control in small areas, especially where disappation is anticipated
Hydrilla Hydrilla verticillata Occasional Potassium endothall applied after fluridone and other systemic herbicide applications to control remaining plants
Coontail Ceratophyllum demersum Occasional Potassium endothall alone or applied with low rate of diquat
Hygrophila Hygrophila polysperma Occasional Potassiom endothall alone or with low rate of amine endothall
Eurasian watermilfoil Myriophyllum spicatum Occasional Potassium endothall alone or with low rate of amine endothall
Water hyacinth Eichhornia crassipes Spot application Foliar potassium endothall application in potable waters
Water lettuce Pistia stratiotes Spot application Foliar potassium endothall application in potable waters
Crested water lily Nymphoides cristata Occasional Submersed application of potassium endothall or potassium and amine endothall followed by frequent spot applications of glyphosate to control regrowth

Table B:  Water Uses and Functions

Water Use Parameters Management Considerations
Downstream Uses and Needs Do not apply within 600 feet of a functioning potable water intake
Fish and Wildlife Mgmt.  
Vegetation planting Spotting may occur on comingled emergent plants from liquid foliar applications
Forage and prey No issues related to this tool
Fisheries Practically non-toxic – blue gill EC50 = 1071 ppm / rainbow trout EC50 = 363 ppm
Non-game wildlife No issues related to this tool
Endangered species No issues related to this tool
Waterfowl Non-toxic to waterfowl – mallard duck LD50 = 328 mg/kg (ppm)
Flood Control No issues related to this tool
Navigation and Access No issues related to this tool
  • No issues related to this tool for established plants
  • Consult label for possible irrigation restrictions for newly seeded or transplanted plants
Livestock Consumption No issues related to this tool for applications in Florida public waters
Potable Water
  • Do not apply within 600 feet from a functioning potable water intake
  • Applications within 600 feet of potable water intake can be made if:
    • The intake is shut down until approved laboratory analysis indicates endothall concentration below 0.1 ppm
    • Coordinate applications with water facility operator
Boating No issues related to this tool
Fishing No fish consumption restriction – does not bioaccumulate in fish
Hunting No issues related to this tool
Swimming No swimming restriction

Table C:  Herbicide, Waterbody, Plant, and Climate Parameters

Herbicide Parameters Management Considerations
Herbicide Rate
  • Generally applied at 1-5 ppm for submersed aquatic plant control:
    • 1-3 ppm for hydrilla and Eurasian water milfoil control
    • 3-5 ppm for coontail control
    • 3 ppm potassium endothall + 0.3 ppm amine endothall to control crested floating heart
    • 5 ppm for hygrophila suppression
Breakdown / Inactivation Half-life is generally 3-7 days
Microbial Primary degradation pathway – degradation rate is water-temperature dependant
Adsorption Does not adsorb onto soils
Not broken down via photolysis
  • Highly soluble so disperses in water
  • Higher rates may be necessary when applying in narrow bands or small areas where water movement may be expected (i.e. reservoirs)
Liquid Available in liquid formulation
Solid Available in solid (granular polymer) formulation
Mode of Action
Contact herbicide
  • Interferes with protein and lipid synthesis; disrupts cell membrane and respiration
  • Somewhat mobile in plant tissues
Plant Growth Regulator Not used as a plant growth regulator in Florida aquatic plant control applications
Herbicide resistance / tolerance
  • Increased tolerance identified in hydrilla at two sites in Florida
  • Potassium endothall combined with diquat proved effective in controlling tolerant hydrilla
  • Rotate or use with bispyribac, diquat, fluridone, penoxsulam, or imazamox where appropriate
Waterbody Parameters  Management Considerations
Water depth
No issues related to this tool, when used  for floating and emergent plant control
Water volume
  • See label chart when applying for submersed plant control
  • Accurate bathymetry is essential to calculate appropriate concentration
Water movement Apply higher rates or combine with diquat for control where water movement is anticipated
Water chemistry  
Dissolved oxygen (DO) Use caution for larger applications to control dense hydrilla surface mats in warm water to avoid DO depletion
pH, alkalinity, hardness No issues related to this tool
Nutrient content
  • Relatively fast-acting herbicide
    • Hydrilla mats generally decline in 1-3 weeks after application
    • Nutrients may be released from decomposing plants in large treatments
Water transparency
  • Hydrilla may recover more quickly in clear waters
  • May get extended submersed plant control in tannic or turbid waters where low light levels inhibit recovery
Sediment characteristics  
  • Sand/Clay – no issues related to this tool
  • Organic – no issues related to this tool, does not adhere to organic material
Potential for resuspension No issues related to this tool, does not adhere to organic material
Plant Physiology Parameters  Management Considerations
Plant origin/ growth potential  
Native Occasional use for coontail control
Non-native Occasional use to control Eurasian water milfoil and crested water lily
  • Spot control control water hyacinth and water lettuce
  • Frequent use to control hydrilla
  • Occasional use to control hygrophila
Plant growth stage (target/non-target)
  • Need actively growing plants for herbicide uptake
    • Use lower rates to control young actively growing plants such as hydrilla
    • Increase selectivity by controlling hydrilla in fall / winter when native plants are senescing or dormant
  • Higher rates are required to control mature surface-matted hydrilla
    • Plants usually have higher carbohydrate reserves
    • Plants may not be as actively growing
    • Microbes degrade endothall more quickly in warm water, requiring higher dose
Plant susceptibility (target/non-target)
  • Apply to actively growing target plants
  • Apply to entire water column at 3-4 ppm for crested water lily control
    • Hydrilla control – use endothall alone or combine with other active ingredients
    • Apply at 1.5-2.0 ppm when used alone for large-scale hydrilla control
    • Use 3 ppm or higher for spot or band application to account for dissipation
    • Control robust hydrilla with endothall; apply fluridone to control regrowth
    • Apply fluridone and spot control surviving hydrilla with endothall
    • Apply generally at 1.5 ppm + 20 ppb penoxsulam
    • Apply 1.0-2.0 ppm endothall + diquat up to 0.37 ppm for rapid control
  • Selective control of hydrilla growing among eelgrass (Vallisneria americana)
Potential for regrowth (target/non-target)
  • Hydrilla regrowth depends on impact on root crowns, water clarity and depth
    • Regrowth is slower if root crowns killed; limited to tuber/turion sprouting
  • Native southern naiad (Najas guadalupensis) and Illinois pondweed (Potamogeton illinoensis) are controlled by potassium endothall, but usually recover
Climate Parameters  Management Considerations
  • Daily
    • Need at least 24 hours of contact for hydrilla control
      • Dependant on dose and water temperature
    • Windy conditions may increase dissipation in spot or band applications
  • Seasonal
    • Less herbicide may be required in cooler months
      • Microbial breakdown is slower so herbicide is active for longer period
      • Increased uptake by actively growing plants in late winter / early spring
Light intensity No issues related to this tool – not broken down via photolysis
Water temperature Potassium endothall degradation can occur more rapidly in warmer water (>80º) which could result in reduced efficacy

Table D: Other Parameters

Parameter Management Considerations
Cost Generics are not available
Anticipated Control Amount  
  • Does not disperse as widely as herbicides with longer half-lives such as  fluridone, penoxsulam, imazamox
  • Area of hydrilla control is similar to area to which endothall is directly applied
  • Control duration depends on dose, extent of control, water clarity, and depth
    • If root crowns are killed, regrowth is slower; from tubers/turions only
Time to Achieve Control Symptoms in 7-10 days and control in 2-3 weeks
  • Subsurface application by airboat for small or moderate scale hydrilla control
  •  Apply aerially by helicopter for larger acreages