Controlling weeds in water can be difficult whether targeting unwanted vegetation in a row crop drainage ditch or a traditional waterbody such as a lake, pond, bayou or river. Selecting the proper active ingredient is key to saving money, eliminating a wasted application, minimizing negative impacts to nontarget organisms and conducting a legal pesticide application. In agricultural settings, weed management typically extends from the crop field through the ditch bank, stopping before the waterline. In contrast, aquatic weed management begins at the waterline and continues into the waterway.

The misuse of row crop labeled herbicides in aquatic canals or other waterways adjacent to cropping systems is a concern for the agricultural industry. There is a convenience factor when spraying terrestrial weeds and using the same herbicides to manage the aquatic plants close to irrigation or drainage ditches. Often, growers purposely spray both upland and aquatic weeds with the row crop-labeled herbicides. However, regardless of applicator awareness of an off-labeled application, or if this is the first-time hearing about the need to use site-specific products, spraying plants growing in the water with agricultural products is illegal if not labeled specifically for aquatic site use. Unless the product label allows the applicator to treat both weeds growing in agriculture and aquatic sites, a separate application with an appropriately labeled herbicide is required. Besides the legality of the misused herbicides, several of the nonaquatic herbicides are toxic to fish, invertebrates or aquatic organisms, or contain the same active ingredients as those registered for aquatic use but are formulated with adjuvants or inert ingredients which pose harm to aquatic organisms.

Herbicide labels contain language directing the applicator to use the product on specific application sites. Although some labels allow usage across multiple cropping systems (e.g., aquatics, right-of-way, horticulture, turf, etc.) most labels have limited-to-no flexibility where the herbicide can be utilized. In other words, an aquatic herbicide cannot be used to treat the commonly found aquatic pest, torpedograss, even if present in an agricultural field. Often, growers, homeowners, natural resource agencies and other applicators desire to use less expensive products, one already purchased and/or already tank mixed for a nonlabeled site. Most aquatic-labeled herbicides are selective or safe to use around aquatic species such as fish, wildlife, insects and other invertebrates. Many terrestrial herbicides are frequently used illegally over water.

Another situation concerning the legal use of herbicides around water involves crawfish ponds. While the site might not be in rice production during the off-season (fall to winter) and contains standing water, an aquatic herbicide cannot be used to treat weeds such as pickerelweed, duck potato, cattail, giant cutgrass or other emergent plants since the location is classified as a rice field or crawfish pond. Aquatic herbicides can be used on most lakes, rivers, bayous, ponds or other waterbodies, but not permitted for crawfish pond use, and/or if rice is present, or if the pond is in rotation with rice production. Therefore, even if a traditional aquatic weed is present, managing weeds in these ponds can only be done with herbicides labeled for rice and/or crawfish production.

Don’t assume the product you’re using on your crop is also labeled for use over water to control aquatic vegetation. Remember: the label is the law. Consult all product labels prior to purchase and application to ensure a legal and effective application will be achieved. Table 1 provides the aquatic herbicides labeled for use in Louisiana and the primary application method.

Table 1. Herbicides labeled for aquatic use in Louisiana.

^aMetsulfuron is registered in Louisiana under a FIFRA Section 24(c) Special Local Need (SLN) label only for the control of giant salvinia (Salvinia molesta).

For more information regarding weed control in aquatic environments, see the Louisiana Suggested Chemical Weed Control Guide.

If you have questions about how to safely and legally use herbicides to manage weeds in your row crops, as well as weeds in or around the water in your ditches, contact Christopher Mudge at Christopher.R.Mudge@usace.army.mil, Matthew Foster at MFoster@agcenter.lsu.edu or Connor Webster at LWebster@agcenter.lsu.edu.

P4026
10/28/2025
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