Fluridone Considerations

*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 Plant Scientific Name Use Pattern Compatible 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 Parameters Management 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 Parameters Management 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 Parameters Management 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 Parameters Management 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 Parameters Management 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

Parameter Management 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
back to top