Users of aquatic herbicides must consider environmental conditions before each use. Such conditions can mean the success or failure of the job at hand.

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There are a variety of environmental conditions that can affect the efficacy of herbicide applications to both emergent (including floating) and submersed vegetation. They include:

Wind
To protect the applicator and adjacent vegetation, the spraying of emergent plants may be prohibited if wind speeds are above 16 km h-1 (10 mph) of if conditions are gusty. The use of appropriate nozzles and reduced spray pressure, or the addition of polymers can reduce the risk of drift by eliminating fine droplets. In addition, aerial applications may be prohibited if there is a temperature inversion because such conditions increase the risk of long-distance drift dispersal.

Rainfall
The efficacy of herbicide applications to emergent vegetation can be reduced if the application is quickly followed by rainfall or irrigation, or if a boat or its wake (or propeller-wash of an airboat) washes over floating plants. This effect is particularly noticeable with glyphosate, for which a rain-free period of at least 6 hours after treatment is recommended on the label, and for which significant reductions in efficacy have been detected following treatments with less than a 72 hour rain-free period (Willard et al. 1998). Over 12 adjuvants have been tested to see if they improve the efficacy of glyphosate when followed at various intervals by simulated rainfall, but only one provided slight improvements (Smith et al. 1999).

Characteristics of the leaf surface
The leaf surfaces of some emergent and floating plants are particularly resistant to wetting, either due to a hairy or waxy epidermis. For herbicides to be effective on these species, good coverage with a high volume of spray mixture is often necessary, and a surfactant that improves epidermal penetration may be crucial.

Water quality
Some herbicides such as diquat are readily bound to suspended matter and are thus less effective in turbid water. Turbid water would preclude their use in submersed treatments and might require the use of a non-local water supply for treatment of emergent vegetation. Glyphosate has also been found to be less effective if the diluent water is hard (e.g., (200-400 mg l-1 of Ca++) or rich in iron (280 mg l-1 Fe+++; Shilling et al. 1990a). For emergent treatments, alternative sources of more suitable water can be sought or water softening additives such as ammonium sulphate have been recommended (Shilling et al. 1990b).

Thermal stratification
The summertime occurrence of thermal stratification in lakes is typically associated with deep water bodies in temperate climates. A surface layer of warm and less dense water of 10-15 m depth (epilimnion), becomes established during calm weather over the deeper layer of cold dense water (hypolimnion). The stratum between these layers (the metalimnion) may exhibit a change in temperature of 10-15º C over less than 5 m depth (with a change of at least 1º C per meter depth), and this thermal discontinuity provides a highly resistant barrier to mixing of the upper and lower layers of water (Wetzel 1983).

Although the depth of the zone in which there is sufficient light for the growth of plants, the euphotic zone, varies with water color and turbidity, this zone is typically contained within the epilimnion. Thus, when whole-lake herbicide applications are being made to a thermally stratified system, it is only necessary to calculate the herbicide rate based on the desired concentration in the volume of the epilimnion rather than the volume of the whole waterbody. If the herbicide is applied in a manner that will not penetrate the metalimnion, such as liquid applied from shore or boat to the surface or with weighted hoses of length less than the depth of the epilimnion, the required amount of herbicide to achieve a given concentration in the euphotic zone can significantly be reduced (e.g., Table 5). Such savings could easily offset the effort needed to determine the batymetry of the lake and its pattern of thermal stratification. Thermal stratification can also influence the vertical mixing and efficacy of fast- acting herbicides in shallower or more tropical lakes, especially where submersed plants form dense canopies at the water surface. Diurnal studies of water temperature on beds of H. verticillata in north Florida, U.S., demonstrated that by late afternoon on a sunny day, water temperatures could be almost 6º C higher at the water surface than at a depth of 0.5m, with a 3º C difference established by 11 am (Getsinger et al. 1990). Although such thermal stratification was temporary, disappearing overnight and on cloudy days, it was sufficient to reduce the mixing of dye-treated surface water with the rest of the 2 m-depth water column for 24 to 48 hours. This lack of vertical mixing was also observed when dye and herbicide were applied as a surface spray to mats of M. spicatum in a more temperate lake in Minnesota, U.S. (Fox and Haller 1995). When plants need to be exposed to maximum concentrations of herbicides for 24 to 48 hours (e.g., diquat, copper, endothall) the consequences of such reduced mixing can be an effective kill of the tops of the plants but poor efficacy at the bases of plants, from with regrowth rapidly occurs. Some solutions to this problem are more practical (e.g., use of long weighted hoses, granules/pellets or sinking agents such as inverts) than others (e.g., heribicide applications at night or only on cloudy days).

Sediment type
When using herbicides that can be deactivated by turbidity, clays, or organic matter (e.g., diquat, fluridone – Mossler et al. 1993), care must be taken not to stir up soft sediments during applications with trailing hoses, or by boat movement through shallow water. Pellets and granules that sink into deep soft sediments are unlikely to provide and effective release of herbicide to kill plants in the water column, especially if the herbicide is bound to organic matter. Thus, for example, fluridone pellets are best used where there is a hard sand substrate.

Water movement
Depending upon whether whole-system or spot-treatments are intended, water movement can be a help or a hindrance to effective weed control. Water movement generated by wind action is the least predictable both in magnitude and direction, and can significantly disperse herbicides from open-water plots, especially in shallow lakes. Tidal movement is typically bidirectional and predictable but can be complicated by freshwater inflows from springs and rivers (Fox et al. 1991a). Gravity flow is unidirectional but can vary with a variety of factors such as rainfall, opening and closing of water control structures, differential importance of surface water and groundwater, etc.

If it is possible to stop (e.g., closing water control structures) or divert flow (e.g., into other channels or through a polythene tube, Leslie and Van Dyke 1991) then herbicides can be applied as if to a static water system. However, as was discovered in the canal systems of south Florida which are carved out of highly porous rock, stopping flow down a canal does not guarantee that water will not be exchanged between the groundwater and channel, especially if the flow of groundwater is perpendicular to the direction of the canal. Such herbicide loss and dilution can significantly reduce the efficacy of herbicides in the canals, and can increase the risk of herbicide residues being drawn into irrigation wells that are near the canal (A.M. Fox and W.T. Haller, unpublished data).

When variations in water movement can be predicted or measured, it is possible to develop application techniques that exploit this to disperse the herbicide over a large target area, but to be effective the weeds must be exposed to a sufficient concentration of herbicide for long enough to be phytotoxic (see Box “The importance of herbicide exposure time and concentration”). For fast-acting herbicides like diquat, endothall, 2,4- D, etc., there is only a small margin of error in their exposure and concentration relations, such that the herbicide concentration must be maintained at close to the maximum concentration for a fairly specific length of time, and if the concentration or exposure time falls much below expectations, less than complete control will result. Thus, it is important to estimate water volumes and discharge rates accurately, and to reduce any factors that would dilute, dissipate, or degrade the herbicide in the target area. With the development of accurate metering pumps that automatically adjust herbicide delivery to match water velocity, it has been possible to use endothall in high-flow situations at concentrations much lower than the maximum label rate.

The discovery that fluridone could be effective at concentrations of 10% or less of the maximum permitted rate of 150 µg l-1 provided that the duration of exposure was increased to several weeks rather than days, vastly extended the capabilities of that herbicide in flowing water (Fox et al. 1996). Large-scale river treatments were developed by applying fluridone over 10-15 weeks upstream of the target area at a rate that maintained the water concentration at 10-15 µg l-1 of fluridone. Such treatments depend upon having daily measurements of water discharge in the river so that the herbicide rates could be adjusted to maintain a constant concentration (Haller et al.1990; Fox et al. 1994). Immediate analysis of fluridone residues from water samples by enzyme-linked immunosorbent assay has improved this ability to tailor extended herbicide applications in both flowing water and large lake systems (Demmi 1998).

As preservative treatments have been developed for water in large or non-linear flow systems, fluorescent dyes have frequently been used to anticipate the maximum potential half-life and dispersal pattern of a herbicide (Fox et al. 1991b, Turner et al. 1994). Concentrations of the conservative dye rhodamine WT (Turner et al. 1991) can be measured in the field. When applied concurrently with a herbicide, this dye has allowed researchers to distinguish between herbicide losses due to water movement and dilution (losses also indicated by the dye) and reductions in herbicide due to degradation and plant uptake (Fox and Haller 1993, Langeland et al. 1994). Such information has proven valuable not only for optimizing herbicide use in moving water and anticipating how far downstream an application will be effective, but in providing credible explanations for observed reductions in herbicide concentrations in residue studies required for herbicide registration (e.g., Getsinger et al. 2000).

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

A BIT MORE...
Here is another page on this web site, Environmental Factors That Affect Herbicide Application, by K.A. Langeland and D.D. Thayer.