Penoxsulam Considerations

No single herbicide is appropriate for controlling all invasive aquatic plants (or nuisance growths of native aquatic plants), in all situations. An 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 pinnataOccasionalPenoxsulam (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 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 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
  • Used alone, penoxsulam requires extensive contact time of 60-120 days for submersed plant control, depending on plant species and level of plant maturity
  • Using in combination with potassium endothall formulations reduces required contact time to 3-14 days for hydrilla control
Navigation and AccessNo issues related to this tool
IrrigationNo 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 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 12-30 ppb in combination with potassium endothall at 1-3 ppm for hydrilla control - (20 ppb penoxsulam is most common - higer rates needed for smaller plots to compensate for dissipation)
  • Applied at 20 ppb to control Azolla pinnata or Salvinia molesta in non-flowing water 
Breakdown / InactivationAverage half life in water of about 2-4 weeks in Florida waters
MicrobialMinor breakdown pathway
Broken down primarily by sunlight
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: most commonly potassium endothall or fluridone for hydrilla control
  • Used in combination with flumioxazin or carfentrazone for foliar applications to floating plants
Waterbody Parameters Management 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
Water movementNeed 3-14 days of exposure for optimum submersed plant control when combined with potassium endothall
Water chemistry 
Dissolved oxygen (DO)
  • Relatively 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, hardnessNo issues related to this tool
Nutrient contentNo issues related to this tool
Water transparency
  • Dark or tannic stained waters may decrease sunlight and prolong half-life
  • 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 at 10-20 ppb, and by direct foliar application of 4-5 oz / acre
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 potassium endothall to control hydrilla in higher energy lakes and reservoirs
    • Penoxsulam rate of 12-30 ppb / potassium endothall rate of 1.0 - 3.0 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 non-target native 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 carbohydrate reserves will require a higher dose or longer exposure 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 low
  • 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 intensity

Low intensity:

  • reduces submersed plant (hydrilla) ability to recover
  • extends penoxsulam half-life (longer half-life in dark waters or during winter months)  
Water temperature
  • Apply when water temperature generally above 50o  F, to ensure active growth of target plants
  • Enhanced efficacy when applied during active plant growth

Table D: Other Parameters

ParameterManagement Considerations
CostGenerics are not available
Anticipated Control  
  • Acres
    • 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; optionally 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+ months)
  • Provides up to annual hydrilla control when penoxsulam applied alone at low dose for several months of exposure
    • Slow plant decline - recovery generally from tuber / turion sprouting
  • When applied with potassium endothall, provides 3-8 months of control
    • Relatively fast plant decline (2-4) weeks
Time to Achieve ControlRelatively fast acting (7-14 days) for submersed plant control when applied with potassium endothall
  • 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)

* dipotassium salt of endothall

Last updated: 21 August 2014