Imazamox is the common name of the active ingredient ammonium salt of imazamox (2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-(methoxymethl)-3-pyridinecarboxylic acid. It was registered with EPA in 2008, and is currently marketed for aquatic use as Clearcast™. It is a liquid formulation that is applied to submerged vegetation by broadcast spray or underwater hose application and to emergent or floating leaf vegetation by broadcast spray or foliar application. There is also a granular version (Clearcast 2.7G™).

Aquatic Use and Considerations

Imazamox is a systemic herbicide that moves throughout the plant tissue and prevents plants from producing a necessary enzyme, acetolactate synthase (ALS), which is not found in animals. Susceptible plants will stop growing soon after treatment, but plant death and decomposition will occur over several weeks.

Imazamox is used for treating emergent vegetation such as common reed (Phragmites australis) and flowering rush (Butomus umbellatus). Imazamox may also be used to treat the invasive curly-leaf pondweed (Potamogeton crispus). Imazamox is a relatively new herbicide that has not been extensively field tested, so there is some uncertainty regarding the sensitivity of non-target species. Desirable native species that may be affected could include other pondweeds (P. nodosus, P. zosteriformis, P. foliosus, P. illinoensis, P. pusillus, P. gramineus, P. diversifolius, P. perfoliatus, P. amplifolius), water shield (Brasenia schreberi) and some bladderworts (Utricularia spp.). Higher rates of imazamox will control Eurasian watermilfoil (Myriophyllum spicatum), but would also have a greater impact on native plants.

If used as a post-emergence herbicide, imazamox should be applied to plants that are actively growing. It can also be used during a drawdown to prevent plant regrowth and on the emergent vegetation.

Repeated use of herbicides with the same mode of action can lead to herbicide-resistant plants. Herbicide resistance has now been found in at least one aquatic nuisance plant species. In particular, ALS inhibitor-resistant weeds have appeared at a higher rate than other herbicide types in terrestrial uses. In order to prevent herbicide resistance, avoid using the same type of herbicides year after year, and when possible, use non-herbicide methods of control instead.

Industrial Vegetation Management Knowledge Base 

Biology and Control of Aquatic Plants 
Imazamox Chemical Fact Sheet

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