Imazapyr Considerations

*Important: See Reference Guide Beforehand

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 Plant Scientific Name Use Pattern Compatible Herbicides
Torpedograss Panicum repens Occasional Alone or with glyphosate
Paragrass Urochloa mutica Occasional Alone or with glyphosate
Cuban club-rush Cyperus blepharoleptos Occasional Alone or with glyphosate
Tropical American watergrass Luziola subintegra Occasional Alone or with glyphosate
Cattail Typha spp. Occasional Alone
Tussocks Mixed herbaceous and woody plants Occasional Alone or with glyphosate or 2,4-D

Table B: Water Uses and Functions

Water Use Parameters Management Considerations
Downstream Uses and Needs
  • Extensive (120-day) irrigation label restriction limits imazapyr use in FWC management programs
    • Applied mostly within the marshes of Lake Okeechobee and TM Goodwin / Broadmoor Marsh
  • Care taken when deciding to apply near desirable trees as residual soil activity is possible
Fish and Wildlife Mgmt.  
Vegetation planting
  • Broad spectrum herbicide for emergent and floating plants
  • Do not apply immediately adjacent to desirable emergent plants
  • No activity on submersed plants
Forage and prey
  • No issues related to this tool
  • Low toxicity 96hr LC50
    • Bluegill = 148 mg/L
    • Rainbow trout = 117 mg/L
    • Daphnia = 132 mg/L
Non-game wildlife
  • No issues related to this tool
Endangered species
  • No issues related to this tool
  • Non-toxic to waterfowl
    • Mallard duck LD50 > 5,000 ppm
Flood Control
  • No issues related to this tool
Navigation and Access
  • No issues related to this tool
  • 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 ppm
  • Do not irrigate for at least 24 hours following application to allow for dissipation
Livestock Consumption
  • No issues related to this tool
Potable Water
  • Do not apply within 1/2 mile upstream of a functioning potable water intake
  • Do not apply within 1/2 mile of an active potable water intake in a standing body of water
  • 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
  • No issues related to this tool
  • No fish consumption restriction
    • Does not bioaccumulate in fish
  • No issues related to this tool
  • No swimming restriction

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

Herbicide Parameters Management Considerations
Herbicide Rate
  • Max rate = 1.5 lb ae (96 fl oz) / ac / yr
  • Generally applied at 64 oz imazapyr + 96 oz glyphosate / acre for invasive grass control
  • Must be applied with spray adjuvant
    • Non-ionic or silicone surfactant
    • Methylated seed or vegetable oil
Breakdown / Inactivation
  • Half-life in water about 2 days
    • Much greater on terrestrial soils (1-5 months)
  • In soils, slow microbial metabolism is primary degradation pathway
  • Adsorbs to soil at < pH5
  • More mobile and available to plants in soil > pH5
  • Rapidly degrades by photolysis in water
    • Degrades faster in spring and summer
  • Not sensitive to hydrolysis
  • Not used for submersed plant control so not applied directly to water
  • May persist longer and therefore be more mobile in soils > pH5
  • Available in liquid formulation only
  • Available in liquid formulation only
Mechanism of Action
  • Classified in WSSA Resistance Grouping #2
    • Acetolactate synthase (ALS) enzyme inhibitor
Mode of Action  
  • Quickly absorbed by leaves and shoots
  • Moves to areas of new growth
  • Shuts down plant growth almost immediately
    • Acts like contact herbicide in this regard
    • Control may take 2-6 weeks
  • Prevents the formation of essential branched chain amino acids in plants – ALS
    • Without these essential amino acids, the plant cannot make proteins, enzymes, etc.
    • Plant cannot continue growing and eventually starves and dies
  • May be exuded from roots and control plants with intermixed root systems
Plant Growth Regulator
  • Not used as a plant growth regulator in Florida aquatic plant control applications
Herbicide resistance
  • Resistance to ALS compounds confirmed in terrestrial species
    • No resistance reported in Florida aquatic plants
  • Rotate or combine with glyphosate where appropriate
Waterbody Parameters Management Considerations
Water depth
  • No issues related to this tool, not used for submersed plant control
Water volume
  • No issues related to this tool, not used for submersed plant control
Water movement
  • No 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
  • Slow acting herbicide
  • Used for shoreline applications or within secluded marshes where DO is less concern
pH, alkalinity, hardness
  • May 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
  • Slow acting herbicide used for shoreline applications and within secluded marshes
Water transparency
  • No issues related to this tool, not used for submersed plant control
Sediment characteristics  
  • Sand/Clay – no issues related to this tool
  • Organic – no issues related to this tool
Potential for re-suspension
  • 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  
  • Cattail and tussocks
    • Apply imazapyr at 64 oz / ac to control cattail or mixed herbaceous and woody tussocks
    • Most often applied in TM Goodwin Waterfowl Management Area (WMA)
  • Occasional spot applications on non-native plants

  • Torpedograss, paragrass, Cuban club-rush, Tropical American watergrass
    • Apply imazapyr at 64 oz /ac or
    • Apply imazapyr at 16 oz /ac + 96 oz / ac glyphosate
    • Most often applied within Lake Okeechobee levee or TM Goodwin WMA
      • Imazapyr and imazapyr + glyphosate are relatively non-selective
      • Used to control most invasive grass and woody species to improve waterfowl habitat
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
  • Broad spectrum herbicide will control most annual and perennial grasses and broadleaf plants
Potential for regrowth (target/non-target)
  • Up to 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

  • Rainfast one hour after application
  • Non-target damage due to drift is a concern
    • Best applied in winds between 3-10 mph
    • Caution in winds below 3 mph – variable wind or temperature inversion may exist


  • Selectivity is increased if torpedograss is controlled in winter
    • When non-target plants (e.g. buttonbush) are dormant
Light intensity
  • No issues related to this tool
Water temperature
  • No issues related to this tool

Table D: Other Parameters

Parameter Management Considerations
  • Generics available
Anticipated Control Amount  
  • Area of control is generally confined to the area to which imazapyr is directly applied to target emergent or floating plants
  • 2-5 years of control reported for torpedograss and cattail
  • Regrowth usually from incomplete control of extensive underground rhizomes, from existing seed bank, or pioneering plants
Time to Achieve Control
  • Symptoms in 2 or more weeks
  • Complete kill in several weeks after application
  • Foliar applications by backpack for spot control
  • Airboat or ATV for moderate-scale applications
  • Helicopter for large-scale application
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