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Center for Aquatic and Invasive Plants Logo    Plant Management in Florida Waters

Center for Aquatic and Invasive Plants Logo    Plant Management in Florida Waters


*Important: See Reference Guide Beforehand

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 Fluridone

Target PlantScientific NameUse PatternCompatible Herbicides
Hydrilla Hydrilla verticillata Occasional Alone for moderate size plots
Hydrilla Hydrilla verticillata Spot applications Applied in small confined areas

Table B: Water Uses and Functions

Water Use ParametersManagement Considerations
Downstream Uses and Needs
  • See irrigation and potable water parameters below
Fish and Wildlife Mgmt.  
Vegetation planting
  • Avoid applications within or near newly planted aquatic revegetation sites
Forage and prey
  • No issues related to this tool
  • No fishing or fish consumption restrictions
Non-game wildlife
  • No issues related to this tool
Endangered species
  • No issues related to this tool
  • No issues related to this tool
Flood Control
  • No issues related to this tool
Navigation and Access
  • No issues related to this tool
  • Rate dependent water use restrictions – see label for specific information
    • < 10 ppb concentration in treated water – no precautions for established tree, row crops, or turf
    • > 5 ppb – do not irrigate tobacco, tomatoes, peppers and newly seeded crops or grass
Livestock Consumption
  • No water use restriction
Potable Water
  • No restrictions when applying fluridone at < 20 ppb
  • Do not apply at concentration > 20 ppb within 1/4 mile from functioning potable water intake
  • Coordinate applications within 1/4 mile of an active potable water intake with water facility manager
  • No issues related to this tool
  • Fishing – no fishing restriction – little to no bioaccumulation in fish
  • No issues related to this tool
  • No swimming restrictions

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

Herbicide ParametersManagement Considerations
Herbicide Rate
  • Generally applied for hydrilla control at concentration in water of 5-20 ppb
    • Label allows rates up 150 ppb
    • Water and crop tolerance established by EPA at 150 ppb
  • Hydrilla exhibits different levels of susceptibility to fluridone in Florida public waters
    • Conduct genetic test to determine effective rate needed for each waterbody
Breakdown / Inactivation
  • Half-life in water of 20 days or more
    • Varies with light intensity
    • Half-life longer in clay or organic sediments
  • Broken down microbially – lesser pathway
    • Some areas experience enhanced microbial breakdown
      • After repeated use of fluridone
      • Half-life may be as little as 7-10 days
    • Too short to sustain concentration for cost-effective hydrilla control
  • Mildly binds to clay and organic particles
  • Broken down by sunlight – primary pathway
  • Not sensitive to hydrolysis
  • May disperse widely throughout treated waterbody due to long half-life
  • Available in liquid / aqueous formulation
  • Available in various clay pellet formulations
    • Reported rates of 7-30 days for peak release of herbicide from pellet
    • Release is generally faster in sandy soils
    • Release is generally faster in flowing water
  • Quick-release pellets applied directly to root zone of non-target eel grass (Vallisneria americana) may have unintended impacts
Mechanism of Action
  • Classified in WSSA Resistance Grouping #12
    • Carotenoid Biosynthesis Inhibitors
Mode of Action  
  • Absorbed only by underwater tissues
    • Does not move within the plant, but is not a contact-type herbicide
  • Inhibits phytoene desaturase, which leads to decreased levels of carotenes
    • Carotenoids are chlorophyll protecting pigments
    • Unprotected chlorophyll photo-oxidizes leading to plant starvation
    • Eventually leads to decreases in chlorophylls, photosynthesis, and carbohydrate stores
      • Symptoms include bleached, white tissue, sometimes pinkish
    • Slow acting – concentration must be maintained from 60-90 days or more
Plant Growth Regulator
  • Has been applied as a plant growth regulator in Florida waters
  • Low dose fall applications (5ppb) suppress tuber and turion production in hydrilla
Herbicide resistance
  • Resistance confirmed in hydrilla in Florida
    • Repeated use killed susceptible clones and selected for less susceptible biotypes
  • Conduct genetic test prior to application to determine susceptibility of current hydrilla population
  • Rotate other compounds for subsequent hydrilla control operations or
  • Use in combination with another herbicide active ingredient
Waterbody ParametersManagement Considerations
Water depth
  • Important to target and maintain prescribed dose
  • Accurate bathymetry is imperative for hydrilla and other submersed plant control
Water volume
  • Important to target and maintain prescribed dose
  • Accurate bathymetry is imperative for hydrilla and other submersed plant control
Water movement
  • Maintaining a prescribed dose 60-90 days or more may be required for submersed plant control
    • Shorter for young actively growing plants – longer for mature, robust plants
Water chemistry  
Dissolved oxygen (DO)
  • Very slow acting for hydrilla control
    • Dissolved oxygen sags are usually not an issue
pH, alkalinity, hardness
  • No issues related to this tool
Nutrient content
  • Slow acting – nutrients released from dying plants over extended period, therefore, no issue
Water transparency
  • No issues related to this tool
Sediment characteristics  
  • Sand/Clay – slight absorption in deep flocculent clay sediments
  • Organic –slight absorption to suspended organic particles
Potential for re-suspension
  • No issues related to this tool
Plant Physiology ParametersManagement Considerations
Plant origin / growth potential  
  • Not used to control native plants in FWC-funded program
  • Not used to control non-native plants in FWC-funded program

  • Occasionally used to control susceptible hydrilla
  • Used in waters in which recent genetic test shows hydrilla susceptibility from 3-15 ppb
  • Monitoring using HPLC or other acceptable method during fluridone exposure period to confirm correct concentration is sustained
  • May require multiple applications to target area to sustain prescribed concentration up to 90 days
  • Sustain prescribed concentration until visual observations confirm that hydrilla will not recover
  • Used in combination with contact-type herbicides (primarily potassium endothall)
    • Control with fluridone – follow up control of surviving hydrilla with potassium endothall
    • Control with contact-type herbicide to reduce biomass – manage regrowth with fluridone
    • Combine with systemic herbicide (primarily penoxsulam)
    • Reducing rates of each herbicide increases selectivity
Plant growth stage (target/non-target)
  • Shorter exposure periods may be applicable for young actively growing plants
Plant susceptibility (target/non-target)
  • Resistance confirmed in hydrilla
  • Cost-effective/selective control of hydrilla that is genetically confirmed as susceptible at 3-15 ppb
    • Apply to actively growing target plants
  • Selectivity varies widely depending on:
    • Growth stage
      • Surface matted hydrilla is more difficult to control
      • Slower growth rate, so lower herbicide uptake
      • Mature hydrilla is more difficult to control due to higher carbohydrate reserves
    • Herbicide dose
      • Test for and monitor to maintain the lowest effective dose for hydrilla control
      • Spatterdock (Nuphar lutea) and pondweeds (Potamogeton spp.) are susceptible to fluridone at most rates applied to control hydrilla – expect injury
    • Formulation
      • Pellets may allow for longer exposure of herbicide
      • Release herbicide through time
      • Avoid applying pellets at high rates directly to roots of eelgrass (Vallisneria americana)
    • Time of year
      • Native plants are generally dormant during fall and winter applications
        • Hydrilla may be actively growing
        • Increasing selectivity
Potential for regrowth (target/non-target)
  • Provides long-term control of susceptible hydrilla – up to 12-18 months
  • Lengthy exposure results in death of standing crop
    • Also controls sprouting tubers for several months
  • Low dose inhibits tuber and turion production during fall/winter applications
Climate ParametersManagement Considerations
Weather Daily

  • Avoid applications during high wind and wave conditions to minimize herbicide dispersal


  • Hydrilla control efficacy and selectivity is greatest for fall, winter, early spring applications
  • Shorter winter daylight period reduces herbicide photolysis
    • Leads to extended fluridone half-life
Light intensity
  • Photolysis is primary degradation pathway
  • Increased light intensity leads to shorter half-life
Water temperature
  • Avoid application of aqueous formulation if strong thermocline exists

Table D: Other Parameters

ParameterManagement Considerations
  • Generics are available
  • Relatively high fluridone cost per pound of active ingredient is mitigated through:
    • Low use rates, long-term control, and high ratio of acres controlled vs. acres treated
  • Other herbicides are available for rotation or combination
Anticipated Control Amount  
  • Slow action systemic herbicide generally disperses widely outside treatment area
  • Acres of hydrilla controlled can exceed acres of plants to which herbicide is directly applied
  • Controls susceptible hydrilla up to 12-18 months
Time to Achieve Control
  • Slow acting – hydrilla must be exposed to appropriate fluridone concentration for 60-90 days
  • Apply by boat or aerially by helicopter for larger acreages of submersed plants
  • Apply by hoses trailing from boat for submersed plant control
  • Laboratory analysis must be available to:
    • Determine hydrilla susceptibility prior to application
    • Monitor fluridone concentration in water to sustain prescribed concentration
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