Skip to main content

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 Penoxsulam

Target PlantScientific NameUse PatternCompatible Herbicides
Hydrilla Hydrilla verticillata Occasional Alone or with potassium endothall
Water hyacinth Eichhornia crassipes Occasional Alone or with flumioxazin in mixed with water lettuce

Table B: Water Uses and Functions

Water Use ParametersManagement Considerations
Downstream Uses and Needs
  • No crop tolerance established, except for rice
  • Do not apply to waters used for food crop irrigation until concentration drops below 1 ppb
  • No turf irrigation restrictions for penoxsulam concentration < 30 ppb
Fish and Wildlife Mgmt.  
Vegetation planting
  • Avoid applications within or adjacent to newly planted aquatic revegetation sites
Forage and prey
  • No issues related to this tool
  • No issues related to use patterns with this tool
    • Rainbow trout 96-hr LC50 > 102 ppm
    • Bluegill sunfish 96-hr LC50 > 103 ppm
Non-game wildlife
  • No issues related to this tool
Endangered species
  • No issues related to this tool
  • Bobwhite quail Oral LD50 > 2,025 mg/kg bw
  • Mallard duck Oral LD50 > 1,900 mg/kg bw
Flood Control
  • May be difficult to maintain effective concentration in flow-through flood control waters
  • Used alone, penoxsulam requires contact time of 60-120 days for submersed plant control
    • Depending on plant species and level of plant maturity
  • Combining with potassium endothall reduces required contact time to 3-14 days to control hydrilla
Navigation and Access
  • No issues related to this tool
  • No crop tolerance established, except for rice
  • Do not apply to waters used for food crop irrigation until concentration drops below 1 ppb
  • No turf irrigation restrictions for penoxsulam concentration < 30 ppb
Livestock Consumption
  • No restrictions related to this tool
Potable Water
  • No restrictions related to this tool
  • No issues related to this tool
  • No restrictions related to this tool, 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
  • Maximum label concentration for controlling hydrilla is 150 ppb per growing season
Breakdown / Inactivation
  • Average half-life in water of about 2-4 weeks in Florida waters
  • Slow microbial breakdown – minor pathway
  • Weakly adsorbed to clay and organic particles in water
  • Adsorbs to dry soils and broken down by microbes
  • Broken down primarily by sunlight in water
  • Half-life 20-50 days
  • Not sensitive to hydrolysis
  • May dissipate widely with long exposure requirement and extended presence in water column
  • Herbicidal activity only in areas where concentration is sufficient for controlling the target plant
  • Combine with potassium endothall for submersed plant control in moving or open water
    • Significantly reduces required exposure time to control submersed plants
  • Available in liquid formulation
  • Available in solid formulation
Mechanism of Action
  • Classified in WSSA Resistance Grouping #2
    • Acetolactate enzyme (ALS) inhibitor
Mode of Action  
  • Absorbed by foliar tissues
    • Moves to areas of new growth
  • Prevents the formation of essential branched chain amino acids in plants – ALS
    • Without these essential amino acids, the plant cannot make proteins, enzymes, etc.
    • Plant cannot continue growing and eventually starves and dies
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 to ALS compounds confirmed in terrestrial species
  • No evidence of penoxsulam resistance in Florida waters
  • Rotate with other compounds for successive large-scale applications, or
    • Use in combination with another active ingredient
      • Most commonly potassium endothall or fluridone for hydrilla control
  • Used in combination with flumioxazin or carfentrazone for foliar applications to floating plants
Waterbody ParametersManagement Considerations
Water depth
  • Amounts used are depth dependent for submersed plant control – see label for chart
Water volume
  • Accurate bathymetry is required to calculate prescribed dose for submersed plant control
  • Apply if possible when water level (volume) is lower
    • Reduces the amount of herbicide required
Water movement
  • Avoid using alone for small-scale, open-water submersed plant control
    • Required exposure period may be too long to sustain herbicide in the control zone
    • Consider combining with potassium endothall
  • Need 3-14 days of exposure for optimum submersed plant control when combined with potassium endothall
Water chemistry  
Dissolved oxygen (DO)
  • Slow acting when used alone
  • When applied with potassium endothall or other contact type herbicides for submersed plant control, use caution for large control sites and in warmer water to avoid oxygen sags
  • DO sags not likely an issue in fall or spring through early summer applications
pH, alkalinity, hardness
  • No issues related to this tool
Nutrient content
  • No issues related to this tool
Water transparency
  • Dark or tannic stained waters may decrease sunlight and prolong half-life
  • Turbidity not an issue related to this tool
Sediment characteristics  
  • Sand/Clay – no issues related to this tool
  • Organic – no issues related to this tool
Potential for re-suspension
  • No issues related to this tool
Plant Physiology ParametersManagement Considerations
Plant origin / growth potential  
  • Not used for native aquatic plant control in FWC funded management programs
  • Not used for non-native aquatic plant control in FWC funded management programs

  • Hydrilla
    • Penoxsulam alone ~30 ppb for young stands of actively growing hydrilla in quiescent waters
      • Stops hydrilla growth immediately
      • Need 60-120 days of exposure for control
      • Extended exposure will also control water hyacinth via root uptake
    • Penoxsulam ~20 ppb + potassium endothall 1-2 ppb
      • Higher rates needed for smaller plots to compensate for dissipation
      • Follow with low concentration of penoxsulam for several months for long term control
  • Water hyacinth
    • Control water hyacinth through root uptake in submersed plant applications at 10-20 ppb, or
    • By direct foliar application of 2-4 oz / acre
    • Penoxsulam 2-4 oz / ac + flumioxazin 2 oz /ac for water hyacinth / water lettuce mix
      • Selective control in bulrush
      • Water lettuce control in < 1 wk – hyacinth control ~ 2+ weeks
  • Feathered mosquitofern (Azolla pinnata) and Giant salvinia (Salvinia molesta)
    • Penoxsulam alone
      • Containment or eradication applications
      • Applied at 20 ppb in non-flowing waters
Plant growth stage (target/non-target)
  • Lower rates may be applicable for young actively growing plants
  • Higher rates / longer exposure may be necessary for mature plants or warmer waters
Plant susceptibility (target/non-target)
  • Good efficacy for small to large-scale areas of hydrilla in quiescent waters
  • Combined with potassium endothall to control hydrilla in higher energy lakes and reservoirs
    • Control is faster than with penoxsulam alone
    • Hydrilla mats begin to collapse in as little as two weeks
    • Seasonal control of non-target native submersed plants such as Illinois pondweed (Potamogeton illinoensis)
    • May temporarily impact some emergent plants
      • Spikerush (Eleocharis), soft-stem bulrush (Schoenoplectus), pickerelweed (Pontederia) sp.
      • Thick lush growth usually recovers next growing season
  • Minimize non-target effects by applying when native plants are dormant
Potential for regrowth (target/non-target)
  • Effective for annual control of hydrilla, especially when applied with potassium endothall
    • Regrowth mostly from sprouting turions/tubers
    • Extent of control varies with season and plant growth stage – up to 1-year control
    • Older plants with more carbohydrate reserves require a higher dose or longer exposure
  • May provide more than 1-year of control of floating plants especially water hyacinth
    • For in-water or foliar applications
Climate ParametersManagement Considerations
Weather Daily

  • Apply when wind/wave action is low to minimize dissipation


  • Apply alone for hydrilla control during periods of active growth in quiescent waters
  • Combine with potassium endothall for year-round hydrilla applications in Florida waters
Light intensity
  • Low intensity
    • Reduces submersed plant (hydrilla) ability to recover after application
    • May extend penoxsulam half-life (longer half-life in dark waters or during winter months)
Water temperature
  • Apply when water temperature generally above 50°F to ensure active growth of target plants
  • Enhanced efficacy when applied during active plant growth

Table D: Other Parameters

ParameterManagement Considerations
  • Generics are not available
  • Herbicides with similar mechanism of action or similar efficacy on different target plants are available
Anticipated Control Amount  
  • Surface area
    • Disperses widely outside treatment area depending on water movement and length of exposure
    • Acres of submersed plants controlled generally equals acres of plants to which penoxsulam + potassium endothall is applied
  • Percent of Water Column – need to treat entire water column
  • When applied with potassium endothall, provides 3-8 months of control
    • Relatively fast plant decline (2-4 weeks)
    • Optionally follow by low dose penoxsulam application once hydrilla biomass is reduced
    • Typically, in 1-2 months
  • Annual control when hydrilla exposed to low dose of penoxsulam for several months
    • Slow plant decline – recovery generally from tuber / turion sprouting
Time to Achieve Control
  • Hydrilla growth shuts down soon after exposure
    • May require 60-120 days exposure to achieve complete control
  • Fast acting (7-14 days) for submersed plant control when applied with potassium endothall
  • Water hyacinth control in 3+ weeks after application
  • Apply by hand gun and boat for small acreages of floating plants
  • Apply by hoses trailing from boat, or by helicopter for submersed plant control
    • Especially large acreages of hydrilla (500+ acres)
back to top