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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 Diquat

Target PlantScientific NameUse PatternCompatible Herbicides
Hydrilla Hydrilla verticillata Frequent Mostly used in combination with potassium endothall; occasionally used alone or in combination with flumioxazin, bispyribac, or topramezone
Water lettuce Pistia stratiotes Frequent Used alone
Water hyacinth Eichhornia crassipes Frequent Used alone or in combination with 2,4-D
Cuban club-rush Cyperus blepharoleptos Frequent Usually in combination with 2,4-D
Wright’s nutrush Scleria lacustris Occasional Used alone
Feathered mosquitofern Azolla pinnata Occasional Used alone
Giant salvinia Salvinia molesta Spot / eradication Used alone

Table B: Water Uses and Functions

Water Use ParametersManagement Considerations
Downstream Uses and Needs
  • Water-use restrictions depend on rate
    • 1-3 day restriction for ornamental plant watering
    • 1-5 day restriction for irrigating crops
Fish and Wildlife Mgmt.  
Vegetation planting
  • Avoid applications within newly installed aquatic revegetation sites
Forage and prey
  • Diquat dibromide has a low bioaccumulation potential
  • Slight to moderate toxicity to fish
    • 8-hour LC50 is 12.3 mg/L for rainbow trout and 28.5 mg/L for Chinook salmon
    • Do not use in waters used for fish protein concentrate or fish meal production
Non-game wildlife
  • No issues related to this tool
Endangered species
  • No issues related to this tool
  • Moderately toxic to birds
    • Mallard duck oral LD50 is 564 mg/kg
Flood Control
  • No issues related to this tool
Navigation and Access
  • No issues related to this tool
  • Water-use restrictions depend on rate
    • 1-3 day restriction for ornamental plant watering
    • 1-5 day restriction for irrigating crops
Livestock Consumption
  • 1-day water-use restriction if treated waterbody is the sole source for watering
Potable Water
  • Rate-dependent setbacks of 350-1600 feet from functioning potable water intake
    • See label for specific information
    • Within these distances, shut off water intake for 1-3 days depending on rate, or
    • Until concentration falls below 0.02 ppm
  • Coordinate with water facility manager for applications within 1600 feet of an active potable-water intake
  • No issues related to this tool
  • No fish consumption restriction
  • Does not bioaccumulate in fish
  • No issues related to this tool
  • No swimming restriction

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

Herbicide ParametersManagement Considerations
Herbicide Rate
  • Maximum label rate for controlling floating plants is 0.75 gpa
    • Generally applied at 0.25 to 0.5 gpa
  • Applied at concentrations up to 0.37 ppm for submersed plant (hydrilla) control
Breakdown / Inactivation
  • Almost immediately bound to soil or vegetation
  • Tightly bound residues are not biologically available, so the herbicide is deactivated on soil
    • Bound residues are resistant to microbial degradation
    • Soil dissipation half-life exceeds 3 years
  • Broken down slowly by microbes if not bound to sediments
  • Binds tightly to clay and organic material
    • Do not apply in muddy or turbid waters for submersed plant control
    • Avoid tank mixing with muddy or turbid water before applying to emergent foliage
  • Broken down slowly by sunlight if not bound to sediments
  • Not sensitive to hydrolysis
  • Dissipation half-life in water is 1-2 days as diquat is bound to sediment and deactivated
  • Available in liquid formulation
  • Not available in solid formulation
Mechanism of Action
  • Classified in WSSA Resistance Grouping #22
    • Photosystem I inhibitor
Mode of Action  
  • Fast acting, contact-type herbicide: rapidly absorbed by plant leaves
  • Interferes with cell respiration, the process by which plants take in oxygen
  • Plant tissues are killed too quickly to allow translocation to other parts of the plant
Plant Growth Regulator
  • Used as herbicide for aquatic plant control applications in FWC programs
  • Functions as growth inhibitor or growth regulator at lower rates
Herbicide resistance
  • Resistance confirmed in Landoltia species of duckweed in Florida
  • Rotate or combine with other compounds where possible
    • Used in combination with endothall for submersed plant control
    • Combine with 2,4-D for mixed water hyacinth / water lettuce populations
Waterbody ParametersManagement Considerations
Water depth
  • Avoid disturbing organic or clay sediments in shallow waters during submersed plant control (binds tightly and is inactivated in turbid waters)
Water volume
  • Use at rates up to 0.37 ppm for submersed plant control
  • Amounts used are depth-dependent – see label for chart
Water movement
  • At least 24 hours of contact are required for submersed plant control
Water chemistry  
Dissolved oxygen (DO)
  • Fast-acting when used in combination with endothall for submersed plant control
  • Use caution in large-scale treatments in warm water to avoid dissolved oxygen depletion
  • Treat only 1/3 to 1/2 of the water body at one time – wait 14 days between treatments
pH, alkalinity, hardness
  • No issues related to this tool
Nutrient content
  • Fast-acting contact type herbicide
  • Nutrients may be released from decomposing plants in large-scale treatments
Water transparency
  • Color/tannic content: no issues
  • Turbidity: avoid mixing with, or applying to turbid waters
    • Binds tightly to clay and organic particles
Sediment characteristics  
  • Sand: no issues related to this tool
  • Clay: binds tightly to, and is inactivated in waters with suspended clay particles
  • Organic: binds tightly to suspended organic particles and organic sediments
Potential for re-suspension
  • Avoid stirring flocculent sediments during submersed plant applications
    • Especially in shallow water
  • Avoid drawing clay or organic turbid water for tank mixes using diquat
Plant Physiology ParametersManagement Considerations
Plant origin / growth potential  
  • Illinois pondweed (Potamogeton illinoensis), southern naiad (Najas guadalupensis)
    • Occasionally controlled for access or navigation
    • 1.0 ppm potassium endothall + 0.37 ppm diquat
  • Water fern (Salvinia minima)
    • Occasional control for access or navigation in confined areas
    • 0.5-0.75 gpa

  • Hydrilla – where suspended organics or clay are not a concern
    • Poor or inconsistent control of hydrilla when used alone
    • 0.37 ppm diquat + 1 ppm potassium endothall (2 ppm endothall for mature hydrilla)
      • Rapid knockdown of hydrilla mat
      • Requires shorter retention time and faster control than endothall alone
      • Resistance management strategy
      • Cutting trails or small exposed bands
      • 0.37 ppm diquat + 0.10 ppm flumioxazin
    • Use in clear, low pH waters due to short flumioxazin half-life in pH > 7
    • Very active on water lilies
    • Resistance management strategy
  • Floating plants (water hyacinth / water lettuce)
    • 1-2 qt/ac for smaller water hyacinth – up to 3 qt/ac for mature “Bull” hyacinth
      • Spray to wet – need complete coverage
    • 1-2 qt/ac for water lettuce
    • 1 qt diquat + 2 qt 2,4-D/ac for hyacinth/lettuce mix and resistance management
  • Cuban club-rush and Wright’s nutrush
    • 2 qt/ac diquat or 1 qt diquat + 2 qt 2,4D/ac
  • Giant salvinia (Salvinia molesta) and Feathered mosquitofern (Azolla pinnata)
    • 2 qt/ac
Plant growth stage (target/non-target)
  • Apply to actively growing target plants
    • Lower rates may be applicable for young, actively growing plants
Plant susceptibility (target/non-target)
  • Resistance confirmed in Landoltia species of duckweed in Florida
  • Good efficacy alone or combined with 2,4-D for water hyacinth/water lettuce control
  • Use in place of 2,4-D to control water hyacinth mixed with bulrush (Schoenoplectus spp.)
    • Repeated use or higher doses of 2,4-D are toxic to bulrush
    • Diquat may spot or brown bulrush stems, but they quickly recover
  • Controls some beneficial plants like southern naiad, Illinois pondweed, strapleaf sagittaria (Sagittaria kurziana) Spatter-dock (Nuphar spp.), jointed spikerush (Eleocharis interstincta)
  • Minimize non-target effects by applying when native plants are dormant
    • Or before target plants mature or covers large areas
Potential for regrowth (target/non-target)
  • Longer term control when applied to actively growing immature hydrilla
  • Temporary injury reported for soft-stem bulrush in greenhouse studies
Climate ParametersManagement Considerations
Weather Daily

  • Need at least 30 minutes of exposure for floating or emergent plants
  • Need at least 24 hours of contact time for submersed plant control with endothall


  • Several months of hydrilla control when applied in fall or winter
  • 1-3 months of control when applied to mature hydrilla in warm water
Light intensity
  • Low intensity may reduce submersed plant ability to recover after application
Water temperature
  • Apply with water temperature above 50o F to ensure active growth of target plants
    • Plants must be actively growing for plant to take up herbicide
    • Do not apply for hydrilla control if strong thermocline exists
      • Or use deep and shallow trailing hoses to treat the entire water column

Table D: Other Parameters

ParameterManagement Considerations
  • Generics are available
  • Alternate herbicides with equivalent efficacy may be available for some species
Anticipated Control Amount
  • Contact-type herbicide – generally does not disperse far outside submersed plant treatment area
    • Acres of submersed plants controlled equals area to which diquat (+ endothall) is applied
  • Need to treat the entire water column
  • Provides good water hyacinth/water lettuce control
    • Regrowth is generally from seed or new plants drifting from outside control zone
  • Hydrilla control varies (1 to 6 months) based on water temperature and plant growth stage
Time to Achieve Control
  • Fast-acting
    • Floating plant symptoms within hours after application – control in several days to one week
    • Submersed plant control in 1-2 weeks after application
  • Apply by hand gun from airboat for small acreages of floating plants
  • Apply aerially by helicopter for larger acreages of submersed plants
  • Apply with endothall by trailing hoses from airboat for submersed-plant control, especially smaller acreages of hydrilla (1 to 200 acres)
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