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 Plant Scientific Name Use Pattern Other Herbicides
Hydrilla Hydrilla verticillata Frequent Penoxsulam plus endothall* (followed by low concentration of penoxsulam for several months for long term control)
Hydrilla Hydrilla verticillata Occasional Penoxsulam alone for immature stands of actively growing hydrilla in quiescent waters
Water hyacinth Eichhornia crassipes Occasional Penoxsulam (in-water or foliar application)
Mosquito fern Azolla pinnata Occasional Penoxsulam (in-water or foliar application)
Duckweed Spirodela spp. Occasional Penoxsulam (in-water or foliar application)
Water meal Wolffia columbiana Frequent Penoxsulam
Salvinia Salvinia spp. Occasional Penoxsulam (in-water or foliar application)

Table B: Water Uses and Functions

Water Use Parameters Management 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 newly installed aquatic revegetation sites
Forage and prey No issues related to this tool
Fisheries No issues related to use patterns with this tool
Practically non-toxic to rainbow trout – LC50 >96 hr. > 44,000 ppm
Non-game wildlife No issues related to this tool
Endangered species No issues related to this tool
Waterfowl No issues related to this tool
Practically non-toxic in mallard duck dietary tests – LC50 >4,300 ppm
Flood Control May 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 Access No issues related to this tool
Irrigation 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 issues related to this tool
Potable Water No issues related to this tool
Boating No issues related to this tool
Fishing No issues related to this tool, little to no bioaccumulation in fish
Hunting No issues related to this tool
Swimming No issues related to this tool

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

Herbicide Parameters Management 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 / Inactivation Average half life in water of about 2-4 weeks in Florida waters
Microbial Minor breakdown pathway
Photolysis Broken down primarily by sunlight
Dissipation Soluble – 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
Liquid Available in liquid formulation
Solid Available in solid formulation
Mode of Action  
Systemic 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 Regulator Used 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 movement Need 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, 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 – 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 resuspension No issues related to this tool
Plant Physiology Parameters Management Considerations
Plant origin/ growth potential  
Native Frequent use for duckweed and water meal control
Non-native Frequent use for salvinia control
Invasive 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
Weather Daily
Apply when wind/wave action is low
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

Parameter Management Considerations
Cost Generics are not available
Anticipated Control  
Spatial 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
Duration 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 Control Relatively fast acting (7-14 days) for submersed plant control when applied with potassium endothall
Contractor/Equipment 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

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