Herbicide Considerations Reference Guide—Introduction
Topics In This Section
- Table A: Herbicide Use Patterns
- Table B: Water Uses and Functions
- Table C: Herbicide, Waterbody, Plant, and Climate Parameters
- Table D: Other Parameters
This reference guide defines the parameters considered by aquatic plant managers and FWC biologists when formulating management plans using herbicides in Florida waters. No single herbicide is appropriate for controlling all invasive or nuisance aquatic plant infestations. Therefore, aquatic plant managers must have a thorough understanding of how each herbicide acts in Florida aquatic systems, which plants it may or may not control, and what impacts may be expected under different environmental conditions.
There are 17 herbicide active ingredients registered for use in aquatic sites (i.e. for application directly to water) by the U.S. Environmental Protection Agency (EPA). The state agency designated to regulate pesticides used in Florida is the Florida Department of Agriculture and Consumer Services (FDACS), which uses the Florida Pesticide Law (Chapter 487, Florida Statutes) for this purpose. Before a new pesticide active ingredient is registered for use in Florida, it is reviewed by the multi-agency Florida Pesticide Registration Evaluation Committee. The FDACS Bureau of Compliance Monitoring has primary responsibility for enforcing those portions of the Florida Pesticide Law which govern chemical aquatic weed control programs. The primary emphasis of the law is to require pesticides be applied in accordance with the pesticide product label.
The Florida Fish and Wildlife Conservation Commission (FWC) is the lead agency for aquatic plant management in Florida. FWC issues permits and contracts for the control of aquatic plants, much of which is achieved using EPA and FDACS-registered herbicides. Each herbicide permitted for use by the FWC in Florida waters has a Considerations Page providing information specific to the use of that herbicide when formulating a management plan. The parameters considered for each herbicide are defined in the following tables. The information is presented for the invasive aquatic plants managed by FWC in Florida public waters. Please consult individual EPA Herbicide Labels for specific requirements related to each herbicide and for additional plants that the manufacturer suggests may be controlled.
On each Herbicide Considerations page, Table A indicates plants that are most often controlled by the herbicide in waters where aquatic plant control is permitted or funded by FWC, and the frequency at which each herbicide is applied each year statewide. Herbicide use patterns are described as frequent or occasional, and small-scale or spot applications. Also shown are other herbicides used in conjunction or rotation with each herbicide to enhance cost-effectiveness, selectivity, and herbicide-resistance management.
FWC biologists evaluate the uses and functions of each water body to evaluate impacts associated with various levels of aquatic plants. When necessary, herbicides are selected to conserve or enhance the uses and functions of the water body by managing aquatic plants. The uses of a water body strongly influence the herbicide choice and the timing of applications. Uses associated with most waters change throughout the year and control measures must adapt to these changes.
Downstream water use requirements may influence decisions such as herbicide selection or timing of drawdowns.
Examples: Lake Istokpoga water is used to irrigate millions of dollars of crops, many miles downstream of the lake. Any slow-acting systemic herbicide requiring extended exposure to control plants in Lake Istokpoga must also be compatible with crop irrigation, or an alternative irrigation source may be needed. Winter drawdowns to reduce the amount of herbicides needed to treat the entire water body may not be feasible if water is required for downstream irrigation during dry winter months.
Vegetation Planting: Control methods must be compatible with newly planted restoration sites. Plants for restoration should be selected considering ongoing aquatic plant management.
Examples: a broad spectrum herbicide should not be used in a recently revegetated area. Conversely, do not plant vegetation susceptible to a frequently applied herbicide in a water body.
Forage and Prey: When developing an aquatic plant management program, it is equally important to consider plants that benefit fish and wildlife.
Example: Hydrilla and water hyacinth control efforts on Lake Toho need to conserve the cattail or willows in which endangered Everglades snail kites nest; and conserve the submersed plants on which apple snails, the preferred food of snail kites, lay eggs and feed.
Fisheries: Timing of control efforts must not injure or adversely impact fish populations; especially native sportfish species or the fish on which they feed.
Examples: Control invasive plants like hydrilla before they form dense canopies that stunt sportfish populations or lower oxygen levels below fish tolerances. Don't use herbicides that are toxic to fish where fisheries are an important concern.
Nongame Wildlife: Select aquatic plant control strategies compatible with nongame species (especially endangered).
Examples: Do not apply copper herbicides where manatees congregate. Suspend control efforts in designated refuge areas within Crystal River, King's Bay, and Blue Springs along the St. Johns River during peak winter manatee presence. Curtail control efforts within 500 feet of active bird rookeries.
Endangered Species: In waters where endangered species have been identified, FWC develops management strategies to conserve or enhance the endangered species as well as their habitat and forage or prey. Control methods and timing are coordinated with the FWC Imperiled Species Management Program as well as the U.S. Fish and Wildlife Service.
Examples: Curtail hydrilla and water hyacinth control in and around warm-water springs designated as manatee aggregation areas during winter months in areas such as Crystal River and Blue Springs. Large-scale hydrilla management strategies in the Kissimmee Chain of Lakes and Lake Istokpoga accommodate snail kite nesting and foraging, as well as apple snail reproduction and availability to kites.
Waterfowl: When developing management plans, managers consider the impact of invasive plants and dense growths of native plants on waterfowl foraging and nesting habitat. The amount of plants controlled, control methods, and timing are important, especially in water bodies where waterfowl hunting is a key use of the system. In these waters, control operations are curtailed, to the degree practicable, between late September and January to accommodate waterfowl scouting and hunting.
Most Florida lakes are connected to one another via surface connections such as rivers and streams, canals, or ditches. These connections often have a structure to regulate the water level and rate of flow in and out of the water body. Plant managers must be aware of these attributes to ensure that aquatic plants do not interfere with this flow, and to ensure that the flow or discharge of water through Florida lakes is compatible with the timing of herbicide applications. Example: Water hyacinth and hydrilla must be under control in the Kissimmee Chain of Lakes (KCOL) before the onset of hurricane season. Unimpeded flow through these waters is important to prevent upstream municipalities such as St. Cloud and Kissimmee from flooding. Large-scale hydrilla control using systemic herbicides in the KCOL is difficult, since systemic herbicides require maintaining an appropriate dose for 2-3 months in a system where flood control gates may need to be opened with a few hours to a few days notice, flushing much of the costly herbicide investment out of the treatment area.
One of FWC's highest aquatic plant management priorities is to ensure that public access ramps, established boat trails and navigation channels remain open and unobstructed by aquatic plant growth. Managers must anticipate which plants may impair navigation and access, and must implement control measures before problems occur. Control strategies must be compatible with local water uses and the extent of control necessary to sustain navigation and access. Example: Slow-acting systemic herbicides may be appropriate to control hydrilla in confined coves or narrow channels connecting boat ramps to open water in low traffic areas. However, faster-acting contact-type herbicides may be more appropriate for controlling hydrilla in high-traffic areas; or when maintaining boat trails in more open areas where dissipation is a greater concern.
Florida lakes and rivers are used for irrigation, ranging from small backyard pipes that water lawns and gardens, to pumps and canals that supply water to millions of dollars of crops including sod, ferns, and vegetables. Where irrigation is a substantial use of the waterbody, managers select control strategies that reduce impacts to irrigation, plan management operations during periods when irrigation is less likely, or notify potential users to find an alternative irrigation source.
Examples: Managers do not generally apply herbicides like imazapyr with lengthy irrigation use restrictions (one or more months) in waters with residential or commercial crop irrigation uses. Managers inform lake residents when they should water their lawns or gardens from wells or municipal water sources when herbicides with a short waiting period for irrigation after application are planned for use. In large systems like Lake Istokpoga, where millions of dollars of crops depend on frequent irrigation water releases, hydrilla must be managed at low levels, through frequent small applications of contact-type herbicides that break down rapidly. Managers do not have the technology for systemic herbicides requiring long-term exposure without impacting irrigation waters.
Herbicide product labels specify a concentration above which livestock should not have access to treated water. In these cases, stakeholders seek alternative watering sources for the use restriction period, or alternative control methods are chosen.
There are approximately 15 functioning municipal drinking water intakes in Florida’s public lakes and rivers. Most herbicide labels specify a setback distance for functioning municipal drinking-water intakes. Use restrictions range from required sampling to shutting down intakes for a brief period. For herbicides that do not specify a drinking-water intake setback on the USEPA label, FWC (in conjunction with the Florida Department of Agriculture and Consumer Services, Department of Environmental Protection, and Department of Health), established a setback half a mile downstream and 2 miles upstream from functioning potable water intakes, inside which the herbicide may not be applied.
Not all herbicides have drinking-water restrictions. For example, copper is occasionally applied to water hyacinth and water lettuce adjacent to drinking-water intakes. Copper is also used by many municipal water authorities in drinking-water reservoirs and sections of rivers near drinking-water intake pipes to control algae, for taste and odor issues in finished water.
Boating: Recreational boating is one of the most popular uses of Florida lakes and rivers. Herbicides are routinely applied to maintain access to, and navigation within, Florida public water bodies. There are no boating restrictions related to herbicides registered for aquatic use.
Fishing: Recreational fishing is an important component of most Florida public waters. As addressed above, aquatic plants provide habitat and forage for many fish species, but uncontrolled invasive plants such as water hyacinth and hydrilla can adversely impact fisheries and fishing activities. FWC biologists work with stakeholders to ensure that herbicide applications are scheduled to cause as little disruption to fishing as possible. Herbicides selected have few or no fishing or fish-consumption restrictions. Applications are scheduled before or after events such as tournaments or annual sportfish spawning.
Hunting: Waterfowl hunting is popular on a small but important number of Florida public waterbodies. As with fishing, the number of aquatic plants significantly affects the quality of hunting. FWC biologists develop strategies to selectively manage invasive aquatic plants while conserving or enhancing native species, and try to implement control before or after hunting season. Hunting season generally starts in late September with scouting, and ends in late January.
Swimming: Most herbicides registered for aquatic use have no swimming restrictions at the rates applied in Florida waters. Herbicide labels clearly indicate any swimming restrictions. FWC biologists select compounds with no swimming restrictions, or apply at times of year when swimming is not an important activity.
Table C identifies the many parameters considered—before this herbicide is used—to determine the feasibility of success (climate, plant growth, water quality, herbicide characteristics).
There is a common misperception that all herbicides are alike, but the reality is quite different. Different families of herbicides perform differently from each other, and herbicides within some families may react with plants in very different ways. In addition, the same herbicide may react differently within the same waterbody at different times of the year. The US Environmental Protection Agency (USEPA) and Florida Department of Agriculture and Consumer Services (FL DACS) registration processes focus on the benefits provided vs. the risks associated with applying herbicides to the aquatic environment. The Florida Fish and Wildlife Conservation Commission (FWC) funds research to improve our understanding of each herbicide compound under changing operational conditions in Florida public waters. This research improves cost-efficiency and selectivity, and ultimately reduces the overall amount of herbicides applied to Florida public lakes and rivers.
The herbicide label specifies the maximum rate at which each compound may be applied in Florida waters. Research and operational evaluations show that most compounds can be applied effectively at much lower doses, depending on environmental and plant growth conditions, ultimately reducing the overall amount of herbicides applied and reducing the costs associated with aquatic plant management.
To be effective, herbicides must be in contact with plant tissues at a prescribed concentration for a prescribed period of time. This concept is known as the dose. As soon as organic herbicides enter the aquatic environment, they begin to break down or become inactive or inaccessible; and their ability to control aquatic plants diminishes. This is a positive outcome which avoids chronic levels of pesticides in Florida waters. However, if a herbicide breaks down or becomes inactivated before it can accomplish its desired function, the result is wasted funds and the unnecessary introduction of a pesticide to Florida waters. Breakdown and inactivation are especially important concepts for submersed plant management, where herbicides may need to be in contact with plant tissues for several weeks. Plant managers must understand the conditions that lead to herbicide inactivation when selecting the type and timing of herbicide applications.
Microbial Most herbicides are broken down by microbes (bacteria) or photolysis (sunlight); and in some cases, by both. Herbicides that are broken down by photolysis generally last longer in the water during winter months when the sun’s rays are less direct, and in tannic or turbid waters where sunlight does not penetrate very deeply. Like photolysis, microbial degradation is generally more rapid in summer months when microbes are more active. Microbial degradation is also generally more rapid in waters in which a specific herbicide compound has been repeatedly applied.
Examples: Potassium endothall is degraded by microbial activity and therefore persists longer in the water during the winter months, when microbes are less active. This increased exposure allows managers to apply potassium endothall at a lower concentration in the winter for hydrilla control, translating to increased cost-efficiency, extended duration of control, and reduced amounts of herbicide used. Fluridone is broken down by photolysis and microbes, and must be sustained in the water at prescribed concentrations for 60-90 days for cost-effective and selective hydrilla control. In two waterbodies where fluridone was applied on several occasions, increased microbial degradation resulted in the half-life declining from an average of 30-35 days to 7-10 days. In these waters, where 2-3 applications of fluridone were once required for hydrilla control, as many as 10 applications would have been needed to achieve the same level of control. From a logistical and economic perspective, enhanced microbial degradation eliminated fluridone as a feasible control option on these waters.
Adsorption: Some herbicides have a strong positive electrical charge and adhere quickly and nearly inextricably to clay or organic particles in the water or sediments. This is important when choosing a herbicide to control plants in turbid waters or when applying herbicides in shallow water where sediments may be disturbed by the herbicide-application equipment. Adsorption is also an important consideration for floating or emergent plant control if the herbicide-tank water is taken from turbid lakes, or from rivers with suspended organic or clay particles. The herbicide may be deactivated in the mixing tank even before it is applied to the plants.
Examples: Diquat is not applied to control hydrilla in shallow waters where the boat wake, propeller, or trailing hoses could disturb and resuspend organic sediments and deactivate the herbicide. Glyphosate is readily adsorbed to organic particles, so herbicide spray-tanks are filled from alternative water sources when applying glyphosate to floating or emergent plants in highly turbid waters.
Dissipation is a consideration primarily with submersed plant control. Herbicides can disperse or dissipate from the control site in a variety of ways including herbicide solubility in water, water flow, boat wake or wave action, tidal influences, or convectional mixing within the waterbody. Generally, the more contact time the herbicide needs and the smaller the control site, the greater the chance that the herbicide can dissipate from the site before controlling the target plants. Fast-acting contact herbicides are usually better choices for controlling submersed plants in spot infestations at boat ramps, narrow boat trails, or slowly flowing waters. Managers can take advantage of dissipation in large-scale submersed plant control operations or confined areas, by applying herbicides to a few sites and allowing water movement to further disperse or mix the herbicide throughout the intended plant-control zone.
Many herbicides are available in liquid (aqueous) or solid formulations. Liquid formulations are generally used for foliar applications to control floating or emergent plants. Solid formulations include clay or polymer-based granules or pellets. Many herbicides that control submersed plants are available in both liquid and solid formulations. While liquid formulations are generally easier and less costly to apply, solid formulations can increase cost-effectiveness or selectivity in certain situations.
Example: Potassium endothall is often applied by helicopter to increase large-scale hydrilla control efficiency. The liquid formulation can cause browning or spotting when applied to commingled non-target emergent plants such as bulrush or pickerelweed. The polymer granule formulation allows potassium endothall to be applied by helicopter through bulrush stands, controlling hydrilla with little-to-no impact on the bulrush. Solid formulations allow the herbicide to be applied directly to the root zone and can be used to deliver herbicides through a strong thermocline. Managers must be aware of sediment type and thickness when applying solid herbicide formulations as they can sink into thick, flocculent sediment deposits and become inactive before controlling the target plant.
The terms Mode of Action and Mechanism of Action for aquatic herbicides are often used interchangeably, but there is a difference. Both terms provide insight into how an herbicide works and what to expect when a susceptible plant is treated. The Mechanism of Action refers to the actual biochemical site of herbicide activity; generally referring to an enzyme or co-factor. The Mechanisms of Action for herbicides registered for use in Florida waters are listed under Background on the Aquatic Herbicides Registered for Use in Florida. In some cases, the actual ‘site’ is unknown. The Mode of Action describes the symptoms that occur after herbicide application that lead to plant injury or death.
Mode of Action
Herbicides are also categorized as contact or systemic mode of action.
Contact: Contact-type herbicides kill tissues of susceptible plants on contact and are relatively fast-acting. Contact herbicides are considered when plants need to be controlled quickly; and for submersed plant control when dissipation may be a problem for slower-acting systemic compounds. More caution is required when applying contact herbicides to large acreages of submersed plants in warm or hot weather, since dissolved oxygen (needed to buffer plant decomposition) is usually lower then.
Systemic: Systemic herbicides translocate from leaves or stems to various tissues in the plant (including growing tips and the underground root system) and are generally slower-acting. Exceptions are the auxin or auxin-like compounds, including 2,4-D and triclopyr, when applied to control water hyacinth or frog’s bit. Systemic compounds applied to control submersed plants such as hydrilla may require several weeks or months to kill the plants. The herbicidal activity of systemic compounds may be accelerated if used at lower rates combined with reduced rates of contact-type herbicides.
Example: Penoxsulam herbicide may require a dose of 40 ppb for as long as 100-120 days to control hydrilla. Contact time may be reduced to as little as 14 days if penoxsulam is applied at 20 ppb in conjunction with 1-2 ppm of potassium endothall.
Plant growth regulators (PGRs) slow the growth, or interrupt other physiological functions of, susceptible plants. Some of the herbicide compounds approved for use in Florida waters can be used as PGRs when applied at lower than recommended rates for herbicidal activity.
Examples: Imazamox applied at half the recommended herbicide rate will suppress hydrilla growth for up to several months. Low rates of fluridone can suppress hydrilla tuber formation while not killing the plants. PGRs can be used to interrupt hydrilla growth for a short period, preventing it from forming surface mats until large-scale control measures can be applied at a more appropriate time. However, most PGR activity in herbicides registered for aquatic use is associated with families of herbicides. This factor may increase herbicide tolerance or resistance issues. If the standing crop (including root crowns) is not killed, then plants may quickly develop a higher tolerance or even resistance to that compound.
Repeated use of the same herbicide can lead to diminished effectiveness of that compound through increasing tolerance or resistance of the plant to the herbicide, or through enhanced microbial degradation. Relatively few herbicide compounds are registered to control aquatic plants, so it is important to use strategies that avoid resistance and prolong the effectiveness of these tools. When herbicide use is necessary, good stewardship (e.g., using different compounds for sequential applications or applying more than one herbicide active ingredient) can prevent or delay increased tolerance and enhanced microbial degradation of herbicides.
Tolerance and resistance: Herbicide tolerance may result in the need to apply increased amounts of herbicides which, in addition to increasing control costs, may reduce protection for native plants. Both of these outcomes can lead to the functional loss of the herbicide to the management program. The development of herbicide tolerance was well documented in Florida during the early 2000s with fluridone and hydrilla, as well as diquat and some duckweed species. More recently, hydrilla tolerance to potassium endothall has been verified in at least two sites in Florida public waters. Managers must remember that most new herbicide compounds registered for use in Florida waters are from families of herbicides in which resistance has been documented in terrestrial plant control. FWC is working with researchers to develop rotational strategies and to identify combinations of herbicides that will provide cost-effective and selective control.
Examples: FWC has developed a database to track herbicide use for large-scale aquatic plant control in order to apply different management strategies where possible for subsequent control operations. FWC is working with researchers to apply contact-type herbicides together with newly registered systemic compounds for large-scale hydrilla control. Various rates and sequences of systemic compounds applied together are also under evaluation. Combinations include potassium endothall applied with penoxsulam, or before and after fluridone applications; diquat applied simultaneously with flumioxazin; and penoxsulam applied sequentially with imazamox or fluridone.
Enhanced Microbial Degradation: Many herbicides are degraded by common bacteria present in all Florida waters. This breakdown is a beneficial process unless degradation occurs before the herbicide can kill the target plant. Enhanced microbial degradation is not well-understood, but appears to have occurred in at least two Florida waterbodies after repeated fluridone use for large-scale hydrilla control. In both of these waters, the fluridone half-life was reduced from an average of 30 days to approximately 7 days. This phenomenon persisted for more than one year in each waterbody. Managers also know that potassium endothall is broken down much more rapidly in warm summer waters than in cooler mid-winter conditions. Therefore, large-scale potassium endothall treatments require much less herbicide to control hydrilla in winter than in summer. The longevity of the compound in water during winter months also translates to more thorough and extended hydrilla control resulting from cool-weather applications.
While herbicide properties are fairly constant, water body parameters change; some daily, some seasonally. Managers need to understand and adapt to current conditions.
Water Depth: Water depth is an important consideration for herbicide application systems that draw water out of the water body, especially in shallow depths with flocculent sediments that could bind or otherwise inactivate herbicide active ingredients. Managers also weigh the benefit of controlling a submersed plant such as hydrilla in water depths less than three feet. If this is a new infestation, eradication may be an incentive. However, if tubers are established, regrowth to the surface by this rapidly growing plant may take as little as 1-2 months.
Water Volume: Water volume is important for submersed plant control since optimum efficacy depends on herbicide concentration in the water column within the proposed control area. Consequently, FWC has undertaken a program to map accurate bathymetry in all Florida public lakes in which hydrilla has been recorded. Using this information and GPS technology, managers can calculate current water volumes for small-area or large-scale herbicide applications. Lowering water volumes (dewatering) prior to herbicide applications may substantially reduce the amount and expense of herbicides applied. However, possible impacts of dewatering to all uses and functions of the water in that system must be considered along with the capability to refill the waterbody after the herbicide application.
Water Movement: Selection of herbicide type and application strategies must account for water movement that may dissipate herbicides during the course of an application program. Water moves in several ways including gravity flow, wind-generated waves, boat wakes, tidal activity, internal convectional mixing, and potentiometric flow into sink holes or out of springs.
Examples: Applications of contact herbicides such as potassium endothall and diquat may be more appropriate for hydrilla control in flowing or moving water than systemic herbicides such as fluridone or penoxsulam. Managers postpone applications of contact and systemic herbicides in flood control systems for several days before or after significant rainfall events to allow the water flow to return to average.
Dissolved Oxygen: In water, dissolved oxygen (DO) is essential to support fish and other organisms, and to buffer the microbial decomposition of organic materials. Generally, cooler water holds more dissolved oxygen than warmer water. Actively growing submersed plants produce oxygen during photosynthesis, especially on bright sunny days; but consume dissolved oxygen during respiration at night and on cloudy days. Managers prefer to control submersed plants such as hydrilla using systemic herbicides in late winter or early spring, after hydrilla begins active growth, but before it fills the water column and covers vast areas. Plants die slowly over a longer period, there are fewer decomposing plants, and more oxygen is available for plant decomposition. Managers face a dilemma when invasive plants such as water hyacinth and hydrilla require control during hot summer months. Dissolved-oxygen levels may already be too low to support fish. Microbial decomposition of controlled plants may further suppress oxygen levels; but not controlling plants will allow them to expand and cause even lower oxygen levels during respiration.
Example: Extensive fish kills have been reported on Lakes Rousseau, Rodman, Kissimmee, and Winder when hydrilla covered the surface during hot summer months and several cloudy or rainy days increased oxygen consumption (through respiration and decomposition of additional organic material flushed into the system).
Alkalinity, pH, and Water Hardness: These parameters should be measured before using several herbicide compounds as they may influence cost-effectiveness of applications or toxicity to non-target organisms.
- Waters with high alkalinity or hardness may bind and inactivate glyphosate, and should not be used in the mixing tank.
- Copper toxicity is much greater to fish in high pH waters.
- Flumioxazin should be avoided for submersed plant control in high pH waters, since it has a reported half-life of only 16 minutes in waters with a ph of 9 or greater. Managers must be aware that in dense hydrilla growth, pH may fluctuate widely during the day (related to the amount of carbon dioxide in the water.)
Nutrient Content: Nitrogen and phosphorus (in the sediments and water column) are the most important nutrients related to algae and aquatic plant growth and management. Generally, invasive aquatic plants are less likely to become problems in nutrient-poor, oligotrophic waters. Eutrophic, or nutrient-rich waters are more likely to support rapid aquatic plant growth that can lead to management problems. Hypereutrophic (or highly enriched) waters may support perpetual algae blooms that restrict light to the extent that submersed plant growth is stunted or inhibited. Extensive stands of invasive plants may mask the true trophic state of a waterbody through biological and physical processes.
Example: Water in dense hydrilla mats that cover hundreds or thousands of acres is usually clear because nutrients are sequestered in hydrilla and periphyton (algae growing on hydrilla), and wind cannot stir the sediments to create turbidity and release nutrients to fuel planktonic algae blooms.
Water Transparency: Water transparency, the amount of light penetrating into the water column, is especially important in the control and regrowth of submersed plants. Water transparency is influenced by biological, chemical, and physical parameters including planktonic algae content, tannins, turbidity, and water depth. When controlling hydrilla in tannic or turbid waters, herbicide use may be reduced by applying enough herbicide to cause stressed plants to decline below the photic zone. Insufficient light further stresses plants and delays their recovery. Conversely, managers must assess costs and benefits of controlling hydrilla in shallow, clear waters where plant recovery may take only a few weeks or months.
Example: Lake Rousseau has two primary water sources: tannic waters from the Withlacoochee River and clear waters from Rainbow Springs. The duration of hydrilla control in Lake Rousseau is much longer during wet periods (when the Withlacoochee River is the primary water source, and secchi transparency may be 2-3 feet), than in dry periods (when the primary water source is the Rainbow River and transparency may exceed 12 feet.)
Turbidity: Some herbicides, such as diquat and glyphosate, bind tightly and quickly to organic and clay materials, either in the sediments or suspended in the water column. When using herbicides that may bind to organic or clay particles, it is important to not mix them with lake water with high suspended particle content in the spray tank. It is equally important not to disturb flocculent organic sediments (by application or by wind or wave action).
Managers must consider sediment type and thickness, especially when selecting herbicide formulations for submersed plants. Applying pelletized or granular herbicide formulations may improve selectivity or increase herbicide contact with the plant, particularly in the root zone. However, some herbicides may bind with organic sediments; or pellet formulations may sink deep within flocculent sediments, diminishing their effectiveness.
Sediment Composition: Generally, sediments are composed of sand, clay, or organic material; usually one of these is predominant in a proposed control area. A hard sand substrate or sand with a thin lens of clay or organic material has little impact on herbicide performance.
Sediment Depth: Thick layers of clay or organic sediments, especially flocculent material, can diminish herbicide performance in several ways. Pellets and granules can sink deep into flocculent sediments, preventing herbicide release into the water column. Adsorption to clay or organic material occurs with some liquid-formulation herbicides, including diquat and fluridone. Care should be taken to prevent trailing hoses from penetrating deep into flocculent sediments when applying liquid-formulation herbicides for submersed plant control.
Potential for Sediment Resuspension: Wind-generated waves and boat wakes in shallow waters may resuspend materials that could diminish performance of some liquid-formulation herbicides that adhere to clay or organic particles. Large-scale applications of diquat or fluridone to control submersed plants may need to be postponed for several days after a significant wind event in waters with flocculent organic sediments, to allow turbidity to settle out. For control of submersed plants in shallow waters, managers should take care not to disturb flocculent sediments when applying herbicides such as diquat that adsorb to clay and organic particles; or choose an alternate control method. Similarly, do not disturb organic or clay sediments when drawing lake water for herbicide mixing tanks to control emersed or floating plants using herbicides such as diquat or glyphosate; these herbicides adsorb to clay and organic particles.
Plant Origin / Growth Potential: Plant growth potential is related to physiological properties of the plant as well as environmental conditions. Generally, invasive plants grow much faster than their native counterparts and warrant greater attention from a resource-management perspective. This section addresses native, non-native and invasive plants that may be managed using this herbicide under FWC management programs.
Examples: Water hyacinth is reported to double its coverage in as little as two weeks during the growing season in Florida. Hydrilla can grow several inches per day. In each case, seemingly innocuous levels of these plants can cover the water surface in a single growing season if not monitored and managed. Some native plants, such as pickerelweed and cattail, thrive in shallow water; they warrant close monitoring when water levels recede during droughts or drawdowns in reservoirs.
Plant Growth Stage: All herbicides registered for aquatic use in Florida waters must be applied to actively growing plants to be effective; none are pre-emergent compounds. Herbicide quantities and associated management costs can be reduced by applying them to target plants early in the season when growth is at its peak and before plants can build carbohydrate reserves. This is especially important for hydrilla. Hydrilla requires less light than most native plants and therefore grows longer in the fall and winter after native plants have senesced. It also begins to grow earlier during the spring while many native plants are still dormant. Conversely, allowing hydrilla to form mats at the surface before attempting control in mid to late summer allows hydrilla to form dense light- and oxygen-limiting canopies over native plants; and requires more herbicide. Additionally, the duration of hydrilla control is usually less, and the potential to impact more actively growing native plants is increased.
Plant Susceptibility: The FWC has contracted millions of dollars of research to develop strategies to control invasive plants while conserving or enhancing native vegetation. Plant susceptibility to herbicides is influenced by the type and amount of herbicide applied, growth stage of target and non-target plants, and the timing of the herbicide application. Example: Hydrilla control is generally more selective when herbicides are applied in winter and spring (when hydrilla is actively growing and most native plants are dormant) vs. summer months (when native plants are actively growing and more herbicide is needed to control more robust and slower-growing hydrilla).
Potential for Regrowth: If invasive plants are established and eradication is not feasible, FWC management goals are generally to maximize the amount and duration of control of invasive plants in the treatment area while conserving or enhancing native plants. Occasionally non-target native plants may be spotted, wilted, or brown; or the standing crop may be temporarily reduced when controlling commingled water hyacinth, water lettuce, or hydrilla. Examples include curling or twisting of spatterdock leaves when applying 2,4-D to control intermixed populations of water hyacinth; and browning of adjacent grasses when applying diquat to control commingled water lettuce. In each case, the floating invasive plants eventually overwhelm the mixed native vegetation. Despite temporary impacts from herbicide applications, native plants quickly recover in the absence of competition from the controlled invasive plants.
Similarly, growth of beneficial native eel grass may temporarily slow when applying potassium endothall to control hydrilla that is overgrowing it, but eel grass thrives once the hydrilla competition is removed. In this case, the healthy stand of eel grass may slow the recovery of hydrilla. Managers are often reluctant to control established hydrilla growing in less than 3 feet of clear water. Regrowth to the water surface from sprouting tubers may only take a few weeks and repeated applications may not be a cost-effective expenditure of public funds.
Daily: When wind speeds approach 10 mph, foliar herbicide applications are postponed or moved to calmer areas to prevent drift issues. Submersed herbicide applications are rescheduled if wind and wave conditions are forecast for the date of application. High wind conditions can increase herbicide dissipation out of the target control area, resuspend organic sediments in shallow waters and inactivate some herbicides, and create high wave conditions dangerous for boat operation.
Managers must also keep a close eye on rainfall forecasts. Most foliar applications require at least a few hours of contact with plant tissues before they are washed off by rainfall. Submersed herbicide applications must be rescheduled if high levels of rainfall are forecast that could flush herbicides from the control area. Large contact-type herbicide applications for submersed plant control may need to be rescheduled if prolonged overcast weather is forecast that might diminish dissolved oxygen content in the water.
Seasonal: Large-scale submersed plant control is scheduled in the drier, cooler months of the year for several weather-related reasons. Oxygen content is higher in winter and plant mass is much lower. High-rainfall events which could flush herbicides are less likely from November through April than from May through October. Many Florida waters are important flood-control conduits. Hydrilla must be under control in these systems before tropical storm season arrives in early June.
Light Intensity: Light intensity is one of the most important parameters influencing submersed plant growth. Without sufficient light, plants simply cannot photosynthesize and do not grow.
Examples: The anticipated duration of hydrilla control is much less in clear or shallow waters where tubers sprout in the photic zone and immediately begin robust growth toward the water surface. In highly tannic or turbid waters, low herbicide doses may be sufficient to stress hydrilla and reduce it below the photic zone, reducing its ability to photosynthesize; thus prolonging control. For herbicides that are degraded via photolysis, cost-effectiveness is usually improved in tannic or turbid waters vs. clear waters and in winter vs. summer months when light is less intense and photodegradation is reduced.
Water Temperature: Water temperature influences aquatic plant management decisions in a variety of ways. Plants must be actively growing for herbicides to be effective, and submersed plants like hydrilla generally do begin active growth until the water temperature is above 50oF. Cool water holds more dissolved oxygen than warm; therefore, managers try to schedule as much control as possible in winter or spring to avoid large-scale summer herbicide applications when there is less dissolved oxygen in the water to buffer plant decomposition. Similarly, if hydrilla is not managed and fills the water column and forms vast mats at the water surface during the summer, it creates conditions for extensive fish kills. Hydrilla produces oxygen during photosynthesis in daylight hours, but consumes oxygen at night and on cloudy days.
Strong thermoclines may develop in summer months, preventing mixing in the water column. Therefore, pellet herbicide formulations or deep water hoses may be needed to deliver herbicides below the thermocline and control aquatic plants in deeper waters. Microbial activity is greater in warmer waters. Herbicides degraded by microbes may need to be applied at higher rates in summer compared to fall through spring to ensure a sufficient herbicide dose long enough to control target plants.
Aquatic plant management costs must be evaluated at several levels, especially for invasive plants. Labor and material costs must be evaluated to control the current level of plants. The introduction of generic compounds has lowered herbicide costs somewhat. Economic and environmental values impaired by uncontrolled plant growth need to be considered. Finally, if control is delayed and plants establish across a broad area, future management costs must be estimated: both short-term costs to regain control, and long-term repeated maintenance. Example: It may be possible to eradicate hydrilla, even in large lakes, for as little as a few hundred or a few thousand dollars if it is discovered quickly and is controlled before establishing tubers. However, if hydrilla is allowed to establish, especially in large lakes, then eradication and containment may no longer be possible or practical. Long-term maintenance may require the application of thousands of pounds of herbicides each year. Millions of dollars of costs may be incurred, in management expenses and in lost or impaired values (recreation, property value, flood control) associated with the lake.
Aquatic plant control programs are developed with spatial and time considerations: evaluating whether the area and length of control justify the effort applied.
Spatial: Spatial control is the area in which target plants are impacted. Floating and emergent plant control usually has a one-dimensional component described as spot control, or acres managed. Similarly, submersed plant control has a lateral component usually described as acres of plants to which contact herbicides are applied, or acres anticipated to be controlled with systemic herbicide applications. Control areas can range from fractions of an acre to hundreds or thousands of acres. Submersed plant control, especially when applying plant growth regulators, can also have a vertical component measured in percent of the water column occupied by the plants.
Duration: The amount of time, or duration, between management efforts is also a measure of control effectiveness. Example: Hydrilla control duration is affected by a variety of factors including water depth, transparency, temperature, and thoroughness of the initial control effort. Regrowth is faster from injured stems vs. tuber germination if the entire root crown is controlled.
Knowing the approximate time between herbicide application and plant death provides important information for planning control programs and establishing management expectations. Generally, managers prefer to control plants before they reach critical levels so slower-acting systemic herbicides can be applied. Slower control generally translates to lower oxygen demand for microbial plant decomposition and slower release of nutrients from decomposing plants. When using systemic herbicides that need several months of exposure for large-scale hydrilla control, applications may need to begin in December or January to ensure control before the rainy season can flush herbicides from the waterbody, and before native plants begin active growth.
The FWC contracts with private-sector companies and local government agencies for all aquatic plant control funded through FWC. Most contractors have the basic equipment to apply herbicides to aquatic sites, including airboats and equipment to apply liquid and pellet formulations. Additional contracts are in place to provide specialty equipment or additional crews for larger or more complex control operations.
Herbicides are applied in a variety of ways depending on target plant types, extent of control, and formulation of the herbicide. Liquid-formulation herbicides (applied by handgun from backpack sprayers or from ATVs) treat small areas of floating or emergent plants along shorelines. Airboats are used most often for offshore plant control. Herbicides are pre-mixed with water in storage tanks (usually from 50- 100 gallon capacity) onboard the boat, or drawn directly from the herbicide concentrate container and metered with water in the pump system. Drop hoses suspended from the front or back of the boat are most often used to apply liquid formulations to submersed plants on a small or moderate scale. The length of the hoses depends on whether the application is a subsurface injection to allow highly soluble herbicides to mix within the water column; or a deep-water / bottom placement to apply the herbicide below a thermocline or nearer the root zone. For large-scale applications, usually 200 or more acres, helicopters are used for floating, emergent, and submersed plant control. Herbicide pellets or granules are applied by broadcast spreaders mounted on the bow of boats for small-scale control; or from helicopters for larger operations.