Imazapyr 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 Imazapyr

Target PlantScientific NameUse PatternHerbicides
TorpedograssPanicum repensFrequentglyphosate
CattailTypha spp.Occasionalglyphosate
Tropical American water grassLuziola subintegraOccasionalglyphosate
West Indian marsh grassHymenachne amplexicaulisOccasionalglyphosate

Table B:  Water Uses and Functions

Water Use ParametersManagement Considerations
Downstream Uses and Needs
  • Do not apply within 1 mile upstream of active irrigation water intake
  • Do not apply within 1/2 mile upstream of functioning potable water intake
Fish and Wildlife Mgmt. 
Vegetation planting
Forage and preyNo issues related to this tool
FisheriesLow toxicity - blue gill, rainbow trout, channel catfish, daphnia LC50 >100 mg/L
Non-game wildlifeNo issues related to this tool
Endangered speciesNo issues related to this tool
WaterfowlNon-toxic to waterfowl - mallard duck LD50 >5,000 ppm
Flood ControlNo issues related to this tool
Navigation and AccessNo issues related to this tool
Irrigation
  • Do not apply within 1 mile of active irrigation water intake (1/2 mile downstream of active intake in flowing water)
  • Do not use treated water for irrigation for 120 days after application or until residue levels from laboratory analysis fall below 1.0 ppb
Livestock ConsumptionNo issues related to this tool
Potable Water
  • Do not apply within 1/2 mile of a functioning potable water intake
  • Applications within 1/2 mile of potable water intake can be made if:
    • The intake is shut down for a minimum of 48 hours
    • Coordinate applications with water facility operator
Recreation 
BoatingNo issues related to this tool
FishingNo fish consumption restriction - does not bioaccumulate in fish
HuntingNo issues related to this tool
SwimmingNo swimming restriction

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

Herbicide ParametersManagement Considerations
Herbicide Rate
  • Generally applied at 2quarts imazapyr + 7.5 pints glyphosate / acre
  • Do not apply more than 6 pints per acre per year
  • Must be applied with spray adjuvant
    • Non-ionic or silicone surfactant, methylated seed or vegetable oil, or invert emulsion
Breakdown / InactivationHalf  life in water about 2 days - much greater on terrestrial soils (1-5 months)
AdsorptionIn soils, slow microbial metabolism is  primary degradation pathway
MicrobialAdsorbs to soil at pH<5 - more mobile and available to plants in soil >pH5
Photolysis  Rapidly degrades by photohydrolysis in water
Dissipation
  • Not used for submersed plant control so not applied directly to water
  • May persist longer and therefore be more mobile in soils >pH5
Formulation 
LiquidAvailable in liquid formulation only
SolidNot available in solid formulation
Mode of Action
 
Systemic
  • Foliar application - translocates in xylem and phloem to meristematic tissue
  • Prevents synthesis of branched-chain amino acids
  • May be exuded from roots and control plants with intermixed root systems
Plant Growth RegulatorNot used as a plant growth regulator in Florida aquatic plant control applications
Stewardship 
Herbicide resistance
  • No tolerance or resistance issues reported in Florida
  • Rotate or use with glyphosate where appropriate
Waterbody Parameters Management Considerations
Hydrology 
Water depth
No issues related to this tool, not used for submersed plant control
Water volumeNo issues related to this tool, not used for submersed plant control
Water movementNo issues related to this tool, not used for submersed plant control
Water chemistry 
Dissolved oxygen (DO)
  • No issues related to this tool, not used for submersed plant control
  • Fairly slow acting herbicide used for shoreline applications and within secluded marshes where DO is less concern
Alkalinity, ph, hardnessMay be more mobile when controlling torpedograss on non-aquatic soils >pH 5
Nutrient content
  • No issues related to this tool, not used for submersed plant control
  • Fairly slow acting herbicide used for shoreline applications and within secluded marshes
Water transparencyNo issues related to this tool, not used for submersed plant control
Sediment characteristics 
Composition
  • Sand/Clay – no issues related to this tool
  • Organic – no issues related to this tool
Potential impacts from sediment resuspension
  • No issues related to this tool
  • Not used for submersed plant application
  • Does not readily adsorb to suspended material in tank mix water
Plant Physiology Parameters Management Considerations
Plant origin/ growth potential 
NativeOccasional use to control cattail
Non-nativeOccasional use to control alligatorweed, tropical American water grass
Invasive  
  • Frequent use to control torpedograss away from irrigation water intakes
  • Occasional use to control West Indian marsh grass
Plant growth stage (target/non-target)
  • Need actively growing plants for herbicide uptake
  • Same rates must be applied to control young or mature plant growth
Plant susceptibility (target/non-target)
  • Apply to actively growing target plants
  • Fairly broad spectrum herbicide will control most annual and perennial grasses and broadleaf plants  
Potential for regrowth (target/non-target)
  • Generally 2-5 years of torpedograss control allowing other species to colonize
  • Sagittaria and water lily species usually recover from seed bank in areas where dense torpedograss is controlled
Climate Parameters Management Considerations
Weather 
Daily considerations
  • Rainfast one hour after application
  • Non-target damage due to drift is a concern
    • Best applied in winds between 3-10 mph
    • Avoid applications in winds below 3 mph - variable wind or temperature inversion may exist
SeasonalSelectivity is increased if torpedograss is controlled in winter when non-target plants like buttonbush are dormant
Light intensityNo issues related to this tool
Water temperatureNo issues related to this tool

Table D: Other Parameters

ParameterManagement Considerations
CostGenerics available
Anticipated Control Amount 
SpatialArea of control is generally confined to the area to which imazapyr is directly applied to target emergent or floating plants
Duration
  • 2-5 years of control reported for torpedograss and cattail
  • Regrowth usually from incomplete control of extensive underground rhizomes or from existing seed bank or pioneering plants
Time to Achieve ControlSymptoms in 2 or more weeks, complete kill in several weeks after application
Contractor/EquipmentFoliar applications by backpack for spot control, airboat or ATV for moderate-scale applications, and helicopter for large-scale applications

Last updated: 28 November 2011