Imazapyr 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 and become reddish at the tips of the plant. Plant death and decomposition will occur gradually over several weeks to months. Imazapyr should be applied to plants that are actively growing. If applied to mature plants, a higher concentration of herbicide and a longer contact time will be required. Imazapyr is used to control the invasive plants common reed (Phragmites australis), purple loosestrife (Lythrum salicaria) and Japanese knotweed (Polygonum cuspidatum). Native species that are also controlled include cattails (Typha spp.), water lilies (Nymphaea sp.), pickerelweed (Pontederia cordata), duckweeds (Lemna spp.) and arrowhead (Sagittaria spp.).
It is important to note that repeated use of herbicides with the same mode of action can lead to herbicide-resistant plants, even in aquatic plants. More resistant weeds have developed to the ALS inhibitor herbicides than to other herbicide types, and so this mechanism of action may be more susceptible to developing resistance. 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.