Imazamox 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 Imazamox

Target PlantScientific NameUse PatternHerbicides
HydrillaHydrilla VerticillataOccasional use as a growth regulatorImazamox
DuckweedSpirodela spp.OccasionalImazamox
SalviniaSalvinia spp.OccasionalImazamox
Frog’s bitLimnobium spongiaOccasionalImazamox
Wild taroColocasia esculentaFrequentImazamox
Alligator weedAlternanthera philoxeroidesOccasionalImazamox
Chinese tallowSapium sebiferum (Syn. Triadica sebifera)FrequentImazamox
Water pennywortHydrocotyle spp.FrequentImazamox
CattailTypha spp.FrequentImazamox
Primrose willowLudwigia spp. incl. Ludwigia grandifloraOccasionalImazamox
PhragmitesPhragmites australisOccasionalImazamox

Table B:  Water Uses and Functions

Water Use ParametersManagement Considerations
Downstream Uses and NeedsSee irrigation and potable water below
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 this tool
  • Practically non-toxic to fathead minnow - LC50-96hr >122 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 >5,500 ppm
Flood ControlNeed to minimize flow for at least 48 hours to allow good contact
Navigation and AccessNo issues related to this tool
IrrigationWait 24 hours before irrigating if applied within 100 feet of intake at concentrations >75 ppb
  • Approved for food crop irrigation after concentration drops below 75 ppb
  • No turf irrigation restrictions
Livestock ConsumptionNo issues related to this tool
Potable WaterNo restrictions for applications made ¼ mile or more from active potable water intake
  • Shut down water intake if application exceeds 50 ppb within ¼ mile of potable water intake
BoatingNo issues related to this tool
FishingFishing – no fishing restrictions
HuntingNo issues related to this tool
SwimmingNo swimming restrictions

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

Herbicide ParametersManagement Considerations
Herbicide RateMaximum label concentration for controlling hydrilla is 500 ppb per growing season
  • Generally applied at 24-32 oz / ac for wild taro and cattail control
  • Generally applied at 50-150 ppb hydrilla growth regulation
Breakdown / InactivationAverage half life in water of 14 days in Florida
PhotolysisBroken down by sunlight – primary pathway
DissipationSoluble - weakly adsorbed to clay and organic particles
  • May dissipate widely in submersed plant applications depending on water movement
  • Herbicidal activity only in areas where concentration is sufficient to control the target plant
LiquidAvailable as a liquid formulation
Mode of Action 
  • Quickly absorbed by leaves, shoots, and roots - translocated  to meristematic tissue
  • Shuts down plant growth almost immediately - control in 2-6 weeks
  • Inhibits the plant enzyme acetolactate synthase (ALS) - inhibits cell division and causes plant death
Plant Growth Regulator
  • Primarily used as herbicide for aquatic plant control at rates of 150-250 ppb
  • Functions as growth inhibitor or growth regulator at lower rates of 50-150 ppb
  • May provide 3-6 months growth suppression at lower rates
Herbicide resistance / tolerance
  • Resistance to ALS compounds confirmed in terrestrial species - none in aquatics
  • Rotate other compounds for successive large-scale hydrilla control applications, or
  • Use in combination with other active ingredients including potassium endothall or penoxsulam 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
  • Apply if possible when water level (volume) is lower
Water movementNeed at least 2-3 days of exposure for submersed plant control - similar to potassium endothall
Water chemistry 
Dissolved oxygen (DO)
  • Relatively fast  acting when used alone and in combination with potassium endothall for submersed plant control
  • DO sags not likely an issue, especially in fall-spring applications
pH, alkalinity, hardnessNo issues related to this tool
Nutrient content
Slow acting herbicide so nutrient release from decomposing plants in large-scale control is slow
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 – do not apply granular formulation to flocculent organic sediments
Potential for resuspensionNo issues related to this tool
Plant Physiology Parameters Management Considerations
Plant origin/ growth potential 
NativeUsed for pennywort, cattail and frog’s bit control
Non-nativeUsed for primrose willow and phragmites control
  • Frequent use for wild taro and Chinese tallow control
  • Controls water hyacinth through root uptake in submersed plant applications, and by direct foliar application
  • Increasingly applied to control large flower primrose willow and crested floating heart
Plant growth stage (target/non-target)Lower rates may have greater impact on young actively growing plants
Plant susceptibility (target/non-target)
  • Must be applied to actively growing target plants
  • Occasional use in combination with potassium endothall to control small to large-scale hydrilla in quiescent waters
    • Imazamox rate of 75-150 ppb / potassium endothall rate of 1-2 ppm
    • Control is relatively fast - 2-6 weeks
    • Controls some beneficial submersed plants like Illinois pondweed (Potamogetom illinoensis) during season of treatment
    • Does not control southern naiad (Najas guadalupensis)
  • Wild taro, alligator weed, and Chinese tallow control
    • Up to 1 gpa for foliar broadcast and up to 5% solution plus surfactant for spot control
  • Cattail control
    • 32 fl oz/a for selective, but slow (~3 months) control when mixed with bulrush
    • 64 fl oz/a for effective control of monocultures, but less selective
    • 32 fl oz/a + 32 fl oz/a glyphosate for excellent monoculture control, but non-selective
    • 32 fl oz/a + 4 oz/a flumioxazin or 6 fl oz/a carfentrazone for cattail, hyacinth, lettuce mix - selective in bulrush, pickerel weed, duck potato
  • Large flower primrose willow control
    • 32 fl oz/a + 4 fl oz/a carfentrazone - foliar applicaton
    • 400 ppb for submersed and rosette growth stage or emergent plants less than 15" tall
  • Good efficacy for foliar application to young water hyacinth @ 32 fl oz/a - 2-4 wk for control
  • Phragmites control @ 2% with 1% MSO backpack, and 1 gpa with 1% MSO broadcast
  • Crested floating heart
    • 128 fl oz/a + 4 oz/a flumioxazin or 6 fl oz/a carfentrazone for excellent control
    • 128 fl oz/a + 48 fl oz/a glyphosate
Potential for regrowth (target/non-target)
  • Effective for annual control of hydrilla when applied in combination 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 higher dose of imazamox
  • May provide more than 1 year control of floating plants during in-water applications
Climate Parameters Management Considerations
  • Daily
    • Apply when wind / wave action is calm
  • Seasonal
    • Optimum hydrilla control efficacy when applied during active plant growth periods
    • Extended hydrilla treatment window when applied with potassium endothall for fall or winter-spring applications
Light intensityLow intensity reduces submersed plant (hydrilla) ability to recover
Water temperature
  • Apply with water temperature generally above 50º F to ensure active growth of target plants
  • Plants must be actively growing for plant to take up herbicide
  • Apply throughout water column for hydrilla control if strong thermocline exists

Table D: Other Parameters

ParameterManagement Considerations
CostGenerics are not available
Anticipated Control Amount 
  • Acres
    • Systemic-type herbicide generally disperses outside treatment area depending on water movement
    • Acres of submersed plant control generally equals acres of plants to which imazamox or imazamox + potassium endothall is applied
  • Percent of water column
    • when used as a growth regulator for hydrilla, lowers surface canopy to mid water column depths
    • Growth regulation lasts several months per application
DurationSimilar to contact-type herbicide for hydrilla control - 3-8 months
Time to Achieve ControlRelatively fast acting  (2-6 weeks) for hydrilla  control; faster  when applied in combination with potassium endothall
  • Apply by hand gun and airboat for small acreages or by helicopter for larger acreages of floating and emergent plants

Last updated: 23 March 2015