How herbicide is applied is almost as important as which herbicide is applied when it comes to the outcome of a treatment. The main aquatic herbicide application techniques include foliar, surface-spray or -broadcast, and subsurface injection, according to Scott Jones, small impoundment Extension specialist at the University of Arkansas at Pine Bluff. Which approach to use depends mainly on the species of plants present, density of plant growth, depth of water and choice of herbicide.
“Foliar applications are those where chemical is sprayed to coat the leaves of plants,” Jones said. “This is generally the preferred approach for aquatic plants that are growing mostly out of the water, like water primrose and alligator weed, or fully to partially floating on the surface, like duckweed and water lilies.”
In most cases, a non-ionic surfactant must be added to the spray solution for foliar applications to be fully effective, he said. Non-ionic surfactants help the chemical spread more evenly and penetrate into the plant.
“Be sure to limit boat wake, prop wash and avoid treating on windy days as products washed off the leaves following foliar applications may be rendered ineffective,” Jones said. “Application can be done by aircraft when using herbicides labeled for aerial application, or from spray rigs on shore or mounted to boats.”
Surface spraying, or broadcast spreading, refers to distributing herbicide to the surface of water with the intent of it settling down into weed beds below, he said. This is generally the preferred approach when low density submersed weeds are being treated in shallower water, up to about 5 or 6 feet deep.
“As long as the weeds are not growing too densely and the water is not too deep, this is a quick approach to cover large areas,” Jones said.
Granulated herbicides settle to the bottom where they dissolve and begin killing weeds from the base of stems upward; they are not affected so much by depth but can still be intercepted near the top of dense weed beds, he said. Surfactant is not needed for application to submersed weeds. Application can be done by aircraft when using herbicides labeled for aerial application, or from spray rigs on shore or mounted to boats.
“Subsurface injection refers to using weighted hoses to deliver herbicide near the bottom in deep water and/or when weed growth is dense,” Jones said. “Deep water tends to become thermally stratified during summer. This creates layers of differing water density that can interfere with herbicide applications.”
Some herbicides, if applied to the water surface, may not be dense enough to sink below the thermocline, a layer of decreasing water temperature and increasing water density, he said. This may only burn off the tips of plants, stimulating rapid regrowth resulting in a failed treatment.
“Many species of submersed weeds can become very dense, potentially creating surface mats or canopies that can intercept herbicide applied to the surface from reaching the base of the weed bed,” Jones said. “This too, when using a surface spray technique, may only burn the mat or canopy with little to no effect to the deeper plant segments.”
Injecting herbicides to the base of weed beds with weighted hoses kills plants from the bottom-up. This improves the effect of contact herbicides by killing the base of the stems first and shortens the distance of travel for systemic herbicides that are designed to be absorbed into plants and transported to roots, he said. Subsurface injection requires weighted hose spray rigs mounted to motorized boats.
“As good stewards of our land and water, we should always use the least amount of chemical necessary in the most prudent manner available,” Jones said. “Possibly more important to many, aquatic herbicides can become exceedingly expensive–let’s use them wisely.”
For more information about herbicide application techniques, contact Jones at (870) 575-8185 or joness@uapb.edu.
