PLANT MANAGERS WANT TO CONTROL invasive plants, without harming the native plants that grow in the same area. Therefore, if they choose to use aquatic registered herbicides, they must apply these herbicides selectively.

As described below, field applicators target specific non-native plants through the use of specially formulated herbicides, seasonal timing of herbicide application, long periods of treatment of reduced herbicidal concentrations, and/or preemptive spot treatments before invasive plants grow out of control.

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Here is information for herbicide applicators, taken from the Aquatic Herbicide Applicator Manual (Langeland, 1998), which provides technical details on applying herbicides.

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Several herbicides are specially formulated to target specific plant types. For example, phenoxy herbicides, such as 2,4-D, are commonly used to control broadleaf plants such as the non-native water hyacinth. Because of their chemistry, these phenoxy herbicides do not harm beneficial native grasses like Paspalum and Panicum spp. Therefore, 2,4-D can be used to selectively remove water hyacinth and enhance native grass growth.

However, repeated 2,4-D use is toxic to bulrush (Scirpus), a highly beneficial native plant whose stems provide fish habitat and whose seeds are utilized as food by waterfowl. When water hyacinth is mixed with bulrush, the more costly and slightly less effective herbicide diquat is used to selectively control the invasive exotic. Diquat will cause spots on bulrush stems, but will not kill this valuable native plant.

For information about which herbicides are labelled for which aquatic sites, download this UF/IFAS-Extension publication, Labelled Aquatic Sites for Specific Herbicides, by V.V. Vandiver.

For information about which plants are controlled by which herbicides, download this UF/IFAS-Extension publication, Weed-Herbicide Cross Reference, by V.V. Vandiver.

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Special herbicides and adjuvants (additives which enhance herbicide usage) are also used to achieve specificity in a variety of environmental conditions. In still water, herbicide and adjuvant formulations such as granules, pellets, inverted emulsions, polymers, and viscous formulations (such as diquat alginate) can increase the localized impacts of spot treatments. This makes selective application of herbicides much more effective against the target invasive plants.

Use of faster-acting herbicides, such as endothall, diquat, and 2,4-D, is more successful for selectively controlling target aquatic species than is the use of herbicides which act more slowly against the target plants. Slower-acting herbicides are more likely to eventually spread from the site of application.

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Selectivity is also accomplished with manual herbicide applications, or spot treatments where herbicides are selectively applied by using a small backpack sprayer and hand-gun. Spot treatments more precisely target invasive plants than do herbicide applications from hoses trainling from an airboat or from aerial applications.

Manual spot treatment is most effective in small-scale projects like removing torpedo grass, water hyacinth, or water lettuce invading native shoreline vegetation. The use of appropriate sprayer nozzles, pressure, and/or adjuvants such as polymers that reduce spray drift, improves the selectivity of such applications.

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Applying herbicides at the appropriate stage in a plant's life cycle (seasonal timing) can also increase plant selectivity and reduce rates of herbicide use or the need for multiple treatments in a season.

Proper timing may also lessen impacts on non-target vegetation. This is because the differences in life-cycles between the target and non-target plants can render different effects by the herbicide. For example, large scale hydrilla control is mostly conducted using fluridone herbicide in winter and early spring when hydrilla is actively growing, but while most native plants are dormant. Winter treatments also allow less fluridone to be used since hydrilla growth slows in the summer, requiring more herbicide.

Additionally, there is more dissolved oxygen in the water during cool winter months. Therefore, the slow-acting fluridone allows thousands of acres of hydrilla to be controlled at one time without a significant dissolved oxygen reduction in the water that likely would be caused if a fast-acting contact herbicide like endothall or diquat were used instead.

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Herbicidal concentration and duration of treatment are also useful for selectively controlling target plants. Label rates of the amine salt formulation of endothall (3-5 parts per million) are toxic to the beneficial native strap-leaf sag (Sagittaria kurziana) that grows co-mingled with hydrilla in Wakulla Springs. However, by applying endothall at a lower rate of 1.5-2.0ppm or less for a longer period of time (up to 48 hours), hydrilla may be controlled for several months while reducing damage to the native sag and allowing it to recover and expand.

OVERALL, PLANT MANAGERS EMPLOY a variety of selective application methods that are specific to the environment and the target plants. For each situation, the unique recipes of chemicals, timing, and application methods often enable field applicators to effectively manage invasive species while safeguarding native plant populations.

For more on all these topics, go back to the main Herbicidal Control page.