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 Plant Scientific Name Use Pattern Herbicides
Torpedograss Panicum repens Frequent glyphosate
Cattail Typha spp. Occasional glyphosate
Tropical American water grass Luziola subintegra Occasional glyphosate
West Indian marsh grass Hymenachne amplexicaulis Occasional glyphosate

Table B: Water Uses and Functions

Water Use Parameters Management 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 Fairly broad spectrum herbicide for floating and emergent plants
Do not apply immediately adjacent to desirable emergent plants
No activity on submersed plants
Forage and prey No issues related to this tool
Fisheries Low toxicity – blue gill, rainbow trout, channel catfish, daphnia LC50 >100 mg/L
Non-game wildlife No issues related to this tool
Endangered species No issues related to this tool
Waterfowl 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
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 Consumption No 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  
Boating No issues related to this tool
Fishing No fish consumption restriction – does not bioaccumulate in fish
Hunting No issues related to this tool
Swimming No swimming restriction

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

Herbicide Parameters Management 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 / Inactivation Half life in water about 2 days – much greater on terrestrial soils (1-5 months)
Adsorption In soils, slow microbial metabolism is primary degradation pathway
Microbial Adsorbs 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  
Liquid Available in liquid formulation only
Solid Not 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 Regulator Not 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 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
Fairly slow acting herbicide used for shoreline applications and within secluded marshes where DO is less concern
Alkalinity, ph, 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
Fairly 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  
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  
Native Occasional use to control cattail
Non-native Occasional 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
Seasonal Selectivity is increased if torpedograss is controlled in winter when non-target plants like 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
Cost Generics available
Anticipated Control Amount  
Spatial Area 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 Control Symptoms in 2 or more weeks, complete kill in several weeks after application
Contractor/Equipment Foliar applications by backpack for spot control, airboat or ATV for moderate-scale applications, and helicopter for large-scale applications
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