Penoxsulam 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 Penoxsulam

Target PlantScientific NameUse PatternOther Herbicides
HydrillaHydrilla verticillataFrequentPenoxsulam plus endothall* (followed by low concentration of penoxsulam for several months for long term control)
HydrillaHydrilla verticillataOccasionalPenoxsulam alone for immature stands of actively growing hydrilla in quiescent waters
Water hyacinthEichhornia crassipesOccasionalPenoxsulam (in-water or foliar application)
Mosquito fernAzolla carolinaOccasionalPenoxsulam (in-water or foliar application)
DuckweedSpirodela sppOccasionalPenoxsulam (in-water or foliar application)
Water mealWolffia columbianaFrequentPenoxsulam
SalviniaSalvinia sppOccasionalPenoxsulam (in-water or foliar application)

Table B:  Water Uses and Functions

Water Use ParametersManagement Considerations
Downstream Uses and NeedsNo crop tolerance established
  • 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 plantingAvoid applications within newly installed aquatic revegetation sites
Forage and preyNo issues related to this tool
FisheriesNo issues related to use patterns with this tool
  • Practically non-toxic to rainbow trout –LC50 >96 hr. > 44,000 ppm
Non-game wildlifeNo issues related to this tool
Endangered speciesNo issues related to this tool
WaterfowlNo issues related to this tool
  • Practically non-toxic in mallard duck dietary tests – LC50 >4,300 ppm
Flood ControlMay be difficult to maintain effective concentration in flow-through flood control waters
Navigation and AccessNo issues related to this tool
IrrigationNo crop tolerance established
  • 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 ConsumptionNo issues related to this tool
Potable WaterNo issues related to this tool
BoatingNo issues related to this tool
FishingNo issues related to this tool, little to no bioaccumulation in fish
HuntingNo issues related to this tool
SwimmingNo issues related to this tool

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
  • Applied at 20 ppb in combination with endothall applied at 1-3 ppm for hydrilla control
  • Applied at 20 ppb to control Salvinia molesta in non-flowing water 
Breakdown / InactivationAverage half life in water of about two weeks
MicrobialBroken down microbially, primary pathway
Minor breakdown pathway
DissipationSoluble - weakly adsorbed to clay and organic particles
  • May dissipate widely due to long exposure requirement time - extended presence in water column
  • Herbicidal activity only in areas where concentration is sufficient for controlling the target plant
LiquidAvailable in liquid formulation
SolidAvailable in solid formulation
Mode of Action
  • Slow acting - absorbed by leaves, shoots, and roots - translocated to meristematic tissue
  • inhibits plant enzyme acetolactate synthase (ALS) - inhibits cell division causing plant death
Plant Growth RegulatorUsed as herbicide for aquatic plant control applications in FWC programs
  • Functions as growth inhibitor or growth regulator at lower rates
Herbicide resistance / tolerance
  • Resistance to ALS compounds confirmed in terrestrial species
  • Rotate other compounds for successive large-scale applications, or
  • Use in combination with other active ingredient: potassium endothall, fluridone or imazamox for submersed plant control
Waterbody Parameters Management Considerations
Water depth
Amounts used are depth dependent – see label for chart
Water volume
  • Accurate bathymetry is required to calculate prescribed dose for submersed plant control
  • Use at rates up to 50 ppb for submersed plant (hydrilla) control
  • Apply if possible when water level (volume) is lower
Water movementNeed 5-10 days of exposure for optimum submersed plant control when combined with endothall
Water chemistry 
Dissolved oxygen (DO)
  • Relatively slow acting even when used in combination with endothall for submersed plant control
  • DO sags not likely an issue in fall or spring through early summer applications
pH, alkalinity, hardnessNo issues related to this tool
Nutrient contentNo issues related to this tool
Water transparency
  • Color/tannic content – no issues related to this tool
  • Turbidity – no issues related to this tool
Sediment characteristics 
  • Sand/Clay – no issues related to this tool
  • Organic – no issues related to this tool
Potential for resuspensionNo issues related to this tool
Plant Physiology Parameters Management Considerations
Plant origin/ growth potential 
  • Frequent use for duckweed and water meal control
  • Frequent use for salvinia control
  • Frequent use for hydrilla control
  • Controls water hyacinth through root uptake in submersed plant applications, and by direct foliar application
Plant growth stage (target/non-target)Lower rates may be applicable for young actively growing plants 
Plant susceptibility (target/non-target)
  • Good efficacy for hydrilla control – small to large-scale areas of hydrilla in quiescent waters
  •  Used in combination with endothall to control hydrilla in higher energy lakes and reservoirs
    • Penoxsulam rate of 20 ppb / endothall rate of 1.5 - 2.0 ppm
      • Higher rates may be necessary for mature plants or warmer waters
    • Control is relatively fast
      • Faster than with penoxsulam alone
      • Hydrilla mats begin to collapse in as little as two weeks
  • Seasonal control of some beneficial submersed plants such as Illinois pondweed (Potamogeton illinoensis)
  • May temporarily impact spikerush (Eleocharis), soft-stem bulrush (Schoenoplectus) and pickerelweed (Pontederia) species
    • 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
    • Hydrilla control varies with season and plant growth stage – up to 1 year control
    • Older plants with more carbohydrates reserves will require a higher dose of penoxsulam
  • May provide more than 1 year of control of floating plants especially water hyacinth
    • For in-water or foliar applications
Climate Parameters Management Considerations
  • Daily
    • Apply when wind/wave action is calm
  • Seasonal
    • Use penoxsulam alone for hydrilla control during periods of active growth in quiescent waters
    • Use penoxsulam in combination with potassium endothall for year-round hydrilla applications in Florida waters
Light intensityLow intensity reduces submersed plant (hydrilla) ability to recover
Water temperature
  • Apply when water temperature generally above 50 degrees F, to ensure active growth of target plants
  • Enhanced efficacy when applied during active plant growth
  • Apply above and below strong thermocline to provide contact throughout water column
    • Use long and short trailing hoses

Table D: Other Parameters

ParameterManagement Considerations
CostGenerics are not available
Anticipated Control  
  • Acres
    • Systemic-type herbicide generally disperses widely outside treatment area depending on water movement
    • 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
  • Primary use – apply in combination with potassium endothall; followed  by low dose penoxsulam applications once hydrilla biomass is reduced – typically in 1-2 months
    • Relatively fast plant decline and long-term control (12-16 months)
  • Provides up to annual hydrilla control when penoxsulam applied alone at low dose for several months of exposure
    • Slow plant decline
  • When applied with potassium endothall, provides 3-8 months of control
    • Relatively fast plant decline (2-4) weeks
Time to Achieve ControlRelatively fast acting for submersed (several days) plant control when applied with potassium endothall
  • Apply by hand gun and airboat for small acreages of floating plants
  • Apply by hoses trailing from airboat, or by helicopter for submersed plant control, especially large acreages of hydrilla (1000+ acres)

* dipotassium salt of endothall

Last updated: 03 July 2014