Invasive aquatic plants may be managed using one or several of the eight herbicidal compounds that are registered by the EPA and Florida Department of Agriculture and Consumer Services for use in Florida waters.

The choice of herbicide depends on a number of factors including:

• the kind and amount of target plant
• the kind of water body: lake or river, its volume and water flow
• the uses of the water body: recreational, commercial, wildlife
• growth stage of the target and non-target vegetation
• wind speed and direction, temperature of the water and the air, oxygen levels in the water, pH, and other environmental variables
• amount of suspended organic clay particulates in the water
• type and depth of organic material in the sediments
• the kinds and numbers of personnel and equipment required to do the job
• economic and environmental costs
• amount of time available to apply the herbicides and before the plants must be controlled

Aquatic herbicides are utilized in aquatic plant management programs where the benefits of their use, i.e. invasive weed control, far outweigh potential environmental and human health risk, and the herbicides are evaluated with other methods of weed control in terms of availability, cost, and efficacy.

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The major advantages of herbicide use over mechanical means of aquatic weed control include:

• herbicides can provide selective control of target vegetation, leaving non-target plants and animals behind
• herbicides are easier to apply to large areas from boats and from the air
• herbicides can be used for weed control in areas that are inaccessible to harvesters
• herbicides are generally cost-effective

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In addition to negative public perceptions and concerns about the human and environmental effects of pesticide use, there are several disadvantages to aquatic herbicide use that water managers need to consider. These include:

• temporary control--not eradication
• some water-use restrictions with some compounds

Aquatic plant managers, by necessity, have to consider cost, effectiveness, water use, water flow, public perception, species present, and a myriad of other factors while developing a weed management plan specific to a water body.

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HERBICIDE SELECTION AND APPLICATION METHODS
Having established that a herbicide application is the appropriate management plan for a particular aquatic weed problem, and knowing which herbicides are permitted for use in Florida, if there remains a choice, the next step is to select the optimal herbicide and application method. Several factors can influence these selections: specifications on the label, extent of area to be treated, water-use restrictions, desired level of selectivity, environmental conditions, and cost-effectiveness.

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Specifications on the label
The EPA-approved label on the herbicide container "is the law" and it must be followed. Included on the label is information about which kinds of aquatic sites may be treated using the specific herbicide.

Also, the herbicide will be effective only on certain species of aquatic weeds, and this must be taken into consideration as well.

For information about what kinds of herbicides may be used in what kinds of aquatic sites, download this UF/IFAS-Extension publication, Labelled Aquatic Sites for Specific Herbicides, by V.V. Vandiver.

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

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Extent of area to be treated
The extent of the area of vegetation to be treated will have a major influence on whether small-scale techniques (e.g., backpack sprayer, hand-gun from truck or boat) or extensive methods (e.g., aircraft) will be necessary. Although it might appear that large areas could be treated more cheaply with smaller-scale methods over a longer period of time (e.g., multiple days of boat application compared to a single day of aircraft use), the continued growth of the vegetation in the areas yet to be treated (especially with floating plants where the area of infestation is increasing) can keep prolonging the smaller-scale treatments beyond cost-efficiency.

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The importance of herbicide exposure time and concentration
The exact relationship between exposure time and concentration will vary with herbicide and weed species but the required exposure period increases as the herbicide concentration drops below the maximum permitted level. It is not clear whether there is a "minimum threshold concentration" for all herbicides but there are practical and economic limits as to how long a low concentration can be maintained.

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Cost-effectiveness
If all other factors have not been enough to be able to choose between different herbicides, then the cost-effectiveness will usually be the final determining factor. Cost-effectiveness would include all aspects of application cost to achieve a certain level and duration of control, beyond just the price of the herbicide. This may be influenced by the method and frequency of application (e.g., some herbicides may need to be applied several times to achieve a year of control compared to others that only need to be used once), and the actual area of vegetation controlled, which may be larger or smaller than the actual area treated.

For example, a single mechanical harvest of Hydrilla verticillata in Florida costs at least $500/hectare (ha) and 2 to 3 harvests may be required annually. Fluridone, applied at 8-10 micrograms per liter controls the entire hydrilla plant, often providing control for 2 or 3 years, at a cost of $50-$250/ha/year. Similar economic data are available for mechanical and herbicidal control of Myriophyllum spicatum in the northern United States. Single mechanical harvesting costs of M. spicatum in Delavan Lake, Wisconsin cost from $730 (1997) to $1400/ha (1999), compared to herbicide costs of $250/ha/year of weed control provided by fluridone ($750/ha fluridone applied in 1997 provided weed control through 1999), and two years of weed control with 2,4-D granular ester at a cost of $795/ha in 1997 provided weed control through 1998 for a cost of $398/ha/year.

For information about herbicide costs, download this UF/IFAS-Extension publication, Approximate Herbicide Costs for Canals, Ditch Banks, Ditches, Lakes, Ponds, Rivers and Rights-of-Way Sites, V.V. Vandiver.

The use of herbicides is usually chosen over mechanical harvesting in areas inaccessible to harvesters. The most efficient harvesters are the larger machines which can carry greater loads to the offloading sites. These large machines cannot operate in stump and log fields in flooded reservoirs. Also, studies have shown that a large proportion of many weeds, particularly Eichhornia crassipes and Pistia stratiotes, often occur in water too shallow (<1m deep) for large harvester machines to operate.

Other factors that may preferentially influence the selection of a herbicide include: public acceptability of specific herbicides, tolerance of water-use restrictions, speed of vegetation death (e.g., fast removal of an acute problem versus slower plant death and decomposition with reduced dissolved oxygen), and compatibility of chemicals that might be applied in mixture.

Part of the preceeding was excerpted from, Role of Herbicides in Aquatic Plant Management Programs, by W.T. Haller and A.M. Fox, 2002.