Penoxsulam was registered with the EPA for aquatic use in 2009. The active ingredient is 2-(2,2-difluoroethoxy)—6-(trifluoromethyl-N-(5,8-dimethoxy[1,2,4] triazolo[1,5,-c]pyrimidin-2-yl)) benzenesulfonamide). It is a liquid (Galleon SC™) and is used for large-scale control of submerged, emergent and floating-leaf vegetation.

Aquatic Use and Considerations

Penoxsulam 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. Penoxsulam should be applied to plants that are actively growing; mature plants require a higher concentration of herbicide and a longer contact time.

Penoxsulam must remain in contact with plants for around 60 days. A supplemental “bump” treatment may be needed to maintain the herbicide concentration for the required contact time. Because of this long contact period, penoxsulam is likely to be used for larger-scale or whole-lake treatments and should not be used where rapid dilution can occur such as spot treatments or moving water. 

Penoxsulam may be used to treat the invasive Eurasian watermilfoil (Myriophyllum spicatum). In other parts of the country, it is valuable as a rotational herbicide against the invasive plant hydrilla (Hydrilla verticillata). Desirable native species that may also be affected include sago pondweed (Stuckenia pectinata), Illinois pondweed (Potamogeton illinoensis), 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.

Industrial Vegetation Management Knowledge Base 

Biology and Control of Aquatic Plants 
Penoxsulam Chemical Fact Sheet

Note: Products may not be registered for use in your state or locale. Check to be sure a specific use pattern is approved in your area before use. Check product labeling or your local state agency for more information. Most current product labels are available by visiting the product manufacturers website or at