Center For Aquatic And Invasive Plants

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.

These considerations are not meant to replace or contradict the EPA approved label.
Always read and follow the pesticide label for each product used.

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
Fisheries	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
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 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
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	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)
Microbial	In soils, slow microbial metabolism is primary degradation pathway
Adsorption	Adsorbs to soil at < pH5 More mobile and available to plants in soil > pH5
Photolysis	Rapidly degrades by photolysis in water Degrades faster in spring and summer
Hydrolysis	Not sensitive to hydrolysis
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	Available in liquid formulation only
Mechanism of Action	Classified in WSSA Resistance Grouping #2 Acetolactate synthase (ALS) enzyme inhibitor
Mode of Action
Systemic	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
Stewardship
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
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 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
Composition	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
Native	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)
Non-native	Occasional spot applications on non-native plants
Invasive	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 Seasonally 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
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, 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 Helicopter for large-scale application