A Manager's Definition of Aquatic Plant Control*

Michael D. Netherland
US Army Engineer Research and Development Center
Environmental Laboratory
Editor, Journal of Aquatic Plant Management
Jeffrey D. Schardt
Florida Fish and Wildlife Conservation Commission
Invasive Plant Management Section


Aquatic plant management is a complex discipline that blends the predictable sciences of water chemistry and hydrology with the highly variable parameters of biology and meteorology for application in venues with boundaries defined by human values and economics.

Defining Aquatic Plant Control

During the past few decades demand for access and use of U.S. surface waters has increased. This is evident in freshwater systems where human activities have expanded. These uses include real estate, recreation, irrigation, hydropower, potable water, navigation, and efforts to conserve environmental attributes such as fish and wildlife habitat. Aquatic plants are a natural and important component of many freshwater systems, and resource managers consider a diverse assemblage and a moderate level of aquatic vegetation to be beneficial for numerous ecosystem functions. Nonetheless, an overabundance of aquatic plants, particularly invasive non-native plants, can impair freshwater systems, requiring some level of aquatic plant management to conserve water body uses and functions. These aquatic plant management activities routinely take place on water bodies ranging in size from small private ponds to large public multi-purpose lakes and reservoirs.

With increasing demands and values associated with surface waters has come a greater need for aquatic plant control. Nonetheless, the term “control” can take on many meanings depending upon the type and amount of use of each water body, the species of plants present, the responsibilities of resource managers, and the objectives of various stakeholder groups associated with the water body. A quick review of reference materials provides the reader with dozens of descriptions and synonyms for “control”, and yet for various reasons none of these efforts would provide a meaningful definition for aquatic plant management. The Aquatic Plant Management Society (APMS) looks to address this deficiency by providing an aquatic plant manager’s working definition of aquatic plant control.

While the terms aquatic plant control and aquatic plant management are often considered synonymous, many resource managers consider control efforts as being operational in nature, and management as a process more aligned with program goals and objectives.

The APMS defines aquatic plant control as techniques used alone or in combination that result in a timely, consistent, and substantial reduction of a target plant population to levels that alleviate an existing or potential impairment to the uses or functions of the water body.

The above definition best applies to management techniques that directly target a reduction in plant biomass. It is recognized that some management strategies seek to impact factors such as plant reproductive capacity (e.g., production of flowers, seeds, tubers, etc.) or nutrient availability, and while these techniques are often recognized as a valuable component of an integrated management program, physical reduction of plant biomass may not result for many years. Moreover, in our definition, the use of the term “substantial” may seem ambiguous; however, we feel there is an inherent problem with using quantitative guidelines (e.g., a 70 percent biomass reduction results in acceptable control) to define what is in most cases a series of qualitative field observations by the aquatic resource manager and stakeholders to determine the success of the management activity. Aquatic resource managers should always consider if the proposed management technique has a successful track record, and know the limitations of the potential strategy. Claims that a product or technique can provide control should be supported by peer-reviewed literature, experiences from other resource managers with similar management objectives, or current research and demonstration efforts.

No single definition of aquatic plant control can cover each specific contingency; therefore, good communication on the front end is a key. The resource manager and stakeholders must first establish expectations for the amount and duration of plant control prior to the initiation of a control activity, and then implement a management strategy to meet these expectations. This definition and the attached paper are intended to address factors that relate directly and indirectly to aquatic plant control. Numerous variables influence aquatic plant control operations and many of these parameters, including water body uses, environmental conditions, and available management tools are presented in Appendix A, along with the influences they may have on the planning or outcomes of aquatic plant control operations. The white paper and Appendix may be useful to managers responsible for conserving identified uses and functions of public waterways, and who must explain to stakeholders the reasoning behind management plan selection and the ultimate results.


Linking Management Decisions to Aquatic Plant Control Expectations:  Factors that Influence Decisions and Outcomes

Aquatic plants have been controlled in U.S. surface freshwaters under organized programs for more than a century, so it is natural to ask why it is necessary to provide a definition of aquatic plant control at this point in time. In questioning a number of managers, researchers, and other stakeholders, it became obvious that opinions on what constituted acceptable control of an aquatic plant population were widely varied. While agricultural managers have been using terms such as “weed free periods” and “crop yield reductions” to define the economic benefits of weed control in cropping systems, aquatic plant managers have a different focus than their terrestrial counterparts. Agricultural weed managers usually attempt to control a broad-spectrum of weeds in order to enhance one or more crop species in a fairly controlled environment with a specific function. Aquatic plant managers usually try to control one or two weeds (usually invasive exotic species) to conserve or enhance perhaps dozens of desirable plants as well as multiple uses of aquatic systems. In essence, an agricultural definition of “weed control” does not encompass many of the issues associated with aquatic plant management.

In developing a manager’s definition for control, it was initially tempting to utilize the language of research to provide a quantitative definition. Both the amount and duration of plant control can be readily quantified within the framework of an experimental study or demonstration project. Nonetheless, many experimental studies result in destructive sampling of the target plants at a given point in time (e.g. 90  percent reduction at 8 weeks after treatment), and they often don’t allow us to determine if even better control or subsequent recovery would result at a later point in time. While this efficacy information can be very useful to managers regarding the expected performance of a specific management technique, the uses, functions, and environmental conditions can vary widely among water bodies and within water bodies through time. This will influence not only the level of management that may be attempted, but also the outcomes of each control operation. While research projects utilize methods that allow for quantification of control, the vast majority of aquatic plant control operations are ultimately judged by fairly subjective visual observations and qualitative means (e.g. the target plants are near the bottom, difficult to find, and the current level of control is rated as good). Therefore, plant control or lack thereof is largely based on whether or not the resource manager and stakeholder expectations have been met.

As noted above, there are numerous issues that either directly or indirectly influence aquatic plant control and management strategies. Before selecting control tools or developing management strategies, three key elements should be addressed that will ultimately influence the manager’s decision making process.

Native vs. Non-native, vs. Invasive Aquatic Plant Control

The National Invasive Species Council defines an invasive species as:

“an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health”.

While there are major distinctions between invasive exotic and native species, the main objective of this white paper is to clarify the term “control” and as such will not make significant distinctions between managing invasive exotic species and nuisance growths of native plants. Whether a plant is a native or exotic, it can cause problems for given water uses (e.g., water conveyance, access). Nevertheless, two key distinctions between nuisance native and invasive plants deserve further discussion. First, problems associated with nuisance native vegetation are typically site specific while invasive plants can impair uses and functions of waters across a broad spectrum of conditions and on a regional scale. The vast majority of large-scale aquatic plant control efforts in the U.S. target invasive species. These plants have the potential to spread and dominate new ecosystems and they also have demonstrated the ability to become established in relatively stable aquatic systems. The philosophy behind invasive plant management programs often is to reduce the potential for spread within and among water bodies by reducing the plant biomass to the greatest extent practicable. The second distinction involves early detection and rapid response (EDRR) programs. These efforts are typically unique to invasive exotic species. A significant and costly multi-agency effort may be initiated to control a very small infestation; however, given the potential negative properties of many invasive exotic plant species, these front-end efforts are viewed as necessary and cost-effective.

Efficacy vs. Control

It is tempting to define aquatic plant control in terms of an expected percent reduction in coverage or biomass of a target plant population. Some regulatory agencies (e.g., California EPA, Canada Pest Management Regulatory Agency) require that herbicide manufacturers prove the efficacy of their products prior to registration. In this regulatory scenario, a product must reduce a target pest population by greater than 70 or 80 percent to provide efficacy. Within the discipline of aquatic plant management, numerous techniques can provide both a rapid and significant reduction in a target plant population (>70 percent), but these results may only be sustained for a few weeks or months. Therefore, depending upon when the efficacy of a management technique is measured, one assessment may suggest that control was achieved, while a subsequent assessment conducted weeks, months, or a season later may lead to the conclusion that the management effort failed to provide any level of control.

If resource managers and stakeholders have agreed to implement a strategy to provide an entire season of biomass reduction and the target plants recover within one or two months, then by our definition, control has not been achieved. In contrast, some methods may result in slow initial impact on a target plant population, but may ultimately provide one or more seasons of control. To complicate matters, many stakeholders fail to grasp that an aquatic plant problem may require more than one treatment or strategy. It is incumbent upon resource managers to understand the strengths and weaknesses of the various management techniques and then convey this information to the stakeholders. If expectations are not defined properly, the stakeholder may lose confidence in the management program. When managers do not establish clear expectations, they are often questioned if control was achieved. Attempting to assess aquatic plant control when clear expectations were not established on the front end is one of the biggest challenges in coming up with a meaningful definition or even assessment of control.

Environmental Controls

Managers must be careful not to confuse slow-acting control methods with natural variations in plant populations. While it is often tempting to link a prior control effort with the large-scale decline of a target plant population, environmental events (e.g. droughts, floods, hurricanes, seasonal senescence, etc.) often are largely responsible for these declines. If empirical data do not exist to support a cause and effect relationship between a control effort and plant biomass decline, managers should avoid making claims that can not be supported by evidence. Some managers rely on environmental events (e.g. flooding events that scour submersed plants or move floating vegetation, and prolonged periods of high dark water that prevent light penetration for submersed plants) to provide control. While this can be effective, there should be some level of predictability associated with the environmental event in order for it to be considered an aquatic plant management technique. From a management perspective there is a big difference in relying on routine seasonal flooding events to control a given plant population versus relying on 100-year floods or droughts to provide plant control.


Levels of Aquatic Plant Control

At the most basic level there are three possible aquatic plant control approaches: 1) no attempt to control, 2) control efforts to eradicate a plant species, or 3) some level of intermediate control that is either incomplete or temporary.

No Attempt to Control

Despite its connotation, the “no control” option is a valid management decision whose potential outcomes must be considered by managers and explained to stakeholders. Factors that influence a manager not taking active control measures may include:

  • plant species – Is the plant invasive?  Is it a native plant impairing water body uses or is it just unwanted by stakeholders?
  • size of infestation – Is this a pioneer infestation consisting of a few plants?  Is it an established, but stable, population?  Is it an established population or starting to approach problematic thresholds?
  • plant location – Is the infestation in an isolated location?  Is the location conducive to spreading the pest plant by fragmentation, flow, etc. Are there important nearby water bodies that are prone to becoming infested?
  • plant biology – Is there a likelihood of a rapid population expansion?  Would “no control” permit the plant to produce viable seed or vegetative propagules that could make later control efforts more difficult and expensive?  
  • exploitation – Is the plant species providing an ecological service (e.g. nutrient uptake, food source for waterfowl, habitat for fisheries, etc.)
  • managerial will – Managers may be under pressure to not control a plant because it provides benefits (perceived or real) to a user group. Stakeholders may oppose control because they are not familiar with proposed methods.
  • managerial experience – Inexperienced resource managers are often uncomfortable with making aquatic plant management decisions (especially on a large-scale). Until a manager understands the issues and situation, the “no control” option may be viewed as the safest and least controversial.

The consideration of these factors and others may justify a “no control” decision. There are consequences associated with all management decisions and “no control” is not exempt. As previously addressed, plant reductions related to environmental factors could be included within the realm of the “no control” option. While environmental events such as floods, droughts, freezes, or severe algae blooms can be quite effective in controlling aquatic plants, these events are not typically predictable and they are not initiated by managers. Nonetheless, the fact that some managers tend to rely on seasonal or weather events to provide effective control suggests the term “no control” may be a misnomer in these situations.


Much like defining control, eradication has proven to have numerous meanings to various managers, researchers, and stakeholders. In a strict sense, eradication means the complete and permanent removal of all viable propagules of a plant population. This is confounded when a population is removed and then reintroduced at a later time. Some plants may be eradicated following single management efforts (e.g. removal of water hyacinth (Eichhornia crassipes) plants prior to seed set) while others such as hydrilla (Hydrilla verticillata) may require years of intense surveillance and management. Eradication efforts are typically employed when a region, state, or watershed is threatened with a new introduction of an invasive species that has potential for significant economic or environmental impact. Based on efforts by various resource management agencies to date, aquatic plant eradication programs are characterized by:

  • sustained and multi-year efforts to insure elimination of the plant population;
  • small-scale efforts to control relatively few plants:
  • control costs on a per acre basis can be quite high;
  • the overall impact of repeated control efforts on the infested water body is continually weighed against the regional threat posed by the invasive plant;
  • control efforts may eventually be reduced; however, vigilant monitoring remains a key to success.

Temporary Control

Outside the realm of eradication, all other control efforts are temporary. Temporary control is essentially an acknowledgement that one hundred percent control is either not an economically viable management objective or is not physically achievable. Temporary control is a continuum that can be represented by the short-term reduction of target plants following mechanical harvesting or spot treatments with contact herbicides, to many years of control that may result from grass carp (Ctenopharyngodon idella) stocking for submersed plants, or decades of suppression of alligatorweed (Alternanthera philoxeroides) by the alligatorweed flea beetle (Agasicles hygrophila). Thus, temporary control results when the aquatic plant manager has made the decision that eradication is not a viable endpoint and some level of target plant persistence is acceptable in the management strategy for a given water body.

Temporary control is achievable using a variety of methods. Managers should evaluate each proposed method and the integration of various methods in terms of meeting specific control objectives.

Maintenance Control

Maintenance control is applied on a lake-wide or regional scale over time, usually to reduce and contain invasive species. Once established, invasive aquatic plants can be extremely difficult, if not impossible, to eradicate. However, managing invasive plants at some prescribed level that does not impair the uses and functions of the water body can reduce environmental and economic impacts. As the term implies, maintenance control indicates that a conscious decision has been made to actively control an aquatic plant problem with the added understanding that a long-term commitment to management rather than eradication is the goal. Simply stated, maintenance control involves routine, recurring control efforts to suppress a problem aquatic plant population at an acceptable level.

Maintenance control encompasses a continuum of control objectives. On one extreme, the goal of maintenance control may be to reduce and sustain a plant population at the lowest feasible level that technology, finances, and conditions will allow. This strategy has proven effective in managing established populations of highly invasive aquatic plants. By managing water hyacinth at low levels through frequent small-scale control operations, there is a corresponding reduction in the overall management effort, especially herbicide use and management costs. There also are environmental gains, such as reductions in sedimentation, and dissolved oxygen depressions. At the other end of the spectrum, maintenance control operations can be applied just prior to plant populations impairing the uses or functions of the water body. This strategy entails allowing plants to grow to the brink of problem levels, and therefore may be best employed in controlling slow growing or otherwise non-invasive plants.

Paradoxically, there is often more stakeholder support for crisis management (allowing plants to reach some problem or impairment level) than maintaining invasive species at low levels. This may be related to stakeholders being unaware of invasive plant growth potential. It also may be related to the public’s perceptions of control methods – for example, not understanding that less herbicide may be needed to maintain plants at low levels rather than waiting for an obvious problem to develop.

While the examples of grass carp and alligatorweed flea beetle describe multi-season impacts, it must be recognized that the basis for this extended control is the continued presence of adequate populations of the management tool (i.e. the carp or the beetle). If the carp numbers are reduced below a certain threshold (by predation, sportfishing, flooding, escape from the system), the target plant will generally re-colonize the aquatic system. Likewise, a severe winter can have adverse impacts on biological control organisms, and this may allow the target plant population to grow back to nuisance levels. The principle of maintaining a continuous pressure on the target plant is an important concept that is often not discussed when describing maintenance control provided by grass carp or biocontrol organisms. Maintenance control is often used to describe only ongoing herbicide programs, yet it is the integrated use and continuous pressure provided by grass carp  biocontrol organisms, and chemical control tools that best describe a maintenance control approach.

Adaptive management –
Since maintenance control represents a long-term commitment, it must also encompass a strategy known as adaptive management. Uses and functions of water bodies change through time, as do conditions within water bodies and among plant populations. Examples include target and non-target plant growth stages, water temperature, depth, clarity, and flow. All factors may change several times during the year and could require different control strategies or different expectations for control outcomes. Therefore, integrated management plans for each aquatic plant control operation must account for and adapt to these changes.


Communicating Control Expectations to User Groups

Many stakeholders view aquatic plant management endeavors as a one-time control effort with no further need for additional management. This does not reflect the reality of the discipline of aquatic plant management. The vast majority of management programs require a sustained effort over multiple years to keep unwanted vegetation under control. For example, while grass carp can provide long-term control of hydrilla, this result is due to their continuous presence and feeding on existing biomass and propagules. Carp can sustain control for many years, yet removal of the carp due to natural losses or on purpose will typically result in the recovery of the target plant. Likewise, a single treatment with fluridone herbicide may remove or reduce a target invasive plant such as Eurasian watermilfoil (Myriophyllum spicatum) within a system for one to several years. Upon discovery of new plants, many stakeholders are dismayed that the treatment did not eradicate the problem. In some cases these plants may have recovered from dormant seed or they may have been introduced from a nearby system that was not managed. Aside from the use of an effective classical biological control organism (highly selective) or high stocking rates of grass carp (non-selective), user groups must be informed about the importance of maintaining continuity in an aquatic plant management program. Single small-scale efforts that don’t address the problem at an adequate scale often lead to claims that “we tried that and it didn’t work.”  A lake full of hydrilla or Eurasian watermilfoil may require whole-lake management efforts. The control may last one or two seasons or even longer, but experience suggests that these invasive plants will ultimately return at some level.

One of the bigger challenges facing aquatic resource managers relates to the promotion of unproven and often costly technologies that are packaged as environmentally friendly approaches to aquatic plant management. As noted earlier, claims of a product or device providing “control” should be supported by published or ongoing research, or by another reputable resource manager who has successfully applied that technique or strategy and met similar control objectives.



Parameters that Influence Aquatic Plant Control Decisions and Outcomes

Aquatic plant management is a complex discipline that blends predictable sciences of chemistry and hydrology with variable parameters of biology and meteorology for application in venues with boundaries defined by human values and economics. Before aquatic plant control activities are initiated, one of the first and most important steps is to identify the various uses and functions of the water body. Identifying uses clarifies environmental and economic values of the water body that may be at risk. It also helps in selecting management tools and strategies that are compatible with, and will help to conserve, the various uses and functions of the water body.

After the uses and functions are identified, a management objective must be developed for the water body that considers these uses as well as concerns of the various stakeholders with interests in the water body. Management objectives are fairly straight forward for waters with relatively few uses or an emergency plant problem. Conflicts in developing objectives arise more frequently when there are many shared uses, multiple stakeholder groups, and an unclear vision if plants, that currently may be enhancing an identified use, may in time impair this or other uses. After management objectives are developed, managers must list all of the potential control tools and select the best tool or combinations that will achieve the stated objective.

There are direct and indirect environmental and economic costs associated with aquatic plant management activities. Responsible resource managers must understand these consequences and choose options that are proven effective and compatible with the current conditions at the site of interest. This information can be obtained through peer-reviewed literature, from direct experience, or through consulting with reliable sources with successful experiences controlling similar plant problems under similar conditions.

Table 1 lists various parameters to consider in developing an aquatic plant control program. Many of these considerations or constraints may influence both the scope of the program and the level of control achieved. While immediate and complete removal of a plant problem may be a desired goal or outcome, in practice, the control process may take months and may be temporary in nature and consequently will need to be repeated on a routine basis. Water body and plant conditions are constantly changing as are tools available to manage plants. Rarely can one person keep track of all of these changes or become an expert in each control tool; therefore, except for the most basic control situations, aquatic plant management experts should be consulted and stakeholders informed about impending aquatic plant control operations. Paramount in this communication is conveying to the non-technical stakeholder why particular methods were chosen and what are the anticipated or expected outcomes of selected (and perhaps rejected) control options, and a receptiveness of stakeholders to respect the multiple uses and functions that may be associated with each water body and to review control tools and options based on their potential for achieving management objectives rather than from a personal preference or bias.

TABLE 1 - Parameters to Consider

ParameterConsideration/ConstraintInfluencePlant type
Water uses and functionsidentify uses, values or functions of each water body to determine which if any may be at risk from invasive aquatic plants or nuisance growths of native and non-native plants - control tools and management strategies must be compatible with water body uses - water uses and conditions change and must be considered during the planning for each control operationthe uses of each water body must be identified and prioritized in order to develop management objectives - management objectives and water uses influence the tools and strategies best suited for aquatic plant control which in turn influence the spatial extent and duration of controlE = emergent
S = submersed
F = floating
Plant types are listed if their control is a primary consideration or influenced by this control consideration
Navigation and accessriver channels or boat ramps blocked, areas of lakes inaccessiblefrequent inspections and rapid response are necessary to sustain commercial navigation in rivers and canals - frequent inspections and control as necessary to conserve recreational access and navigationE, S, F
Transportationfloating plant masses jam against bridges and may cause structural damage or erosion around pilingsfrequent inspections and rapid response are necessary to prevent damage associated with aquatic vegetation, especially tussocks and floating islandsE, S, F
Flood controlplant masses can block or impede water flow in river channels, canals, lake outfalls, or flood control structuresfrequent inspections and control of invasive plants that may impact flood control to the lowest feasible level - control native and non-invasive plants as necessary to conserve flood controlE, S, F
Potable waterplants clog water intakesfrequent inspections and control of plants as necessary to prevent disruption of water supply - herbicides must have potable water tolerance, set-back distance, or concentration limitS, F
Irrigationplants clog water intakes, impede water flow in ditches, canals, and riversensure herbicides are compatible with irrigated crops, may need to treat when crops not in field, find alternate irrigation supplyE, S, F
Livestock wateringplants do not usually impact ability for watering livestock from water bodiesif herbicides used, may need to remove livestock from water body shoreline, find alternate watering sourceE, S, F
Downstream uses and needsplant masses prevent water releases for downstream uses like drinking, irrigation, wetland restoration, estuariescontrol plants to provide downstream water - herbicides must be compatible with downstream uses - coordinate control with water releases - frequent releases may dilute or draw off herbicide concentrationsE, S, F
Recreationidentify and assess recreational uses within the systemaquatic plants may enhance or hinder recreational activities within a water body that may be seasonal or year-round 
Boatingplants can restrict access and boating activitiesselect control methods and frequency to accommodate types and amounts of boating - inboard/outboard motor, sailing, canoe/kayak, rowing shell, etc.E, S, F
Fishingplants can block access to fishing areas - plants provide habitat to support fisheries and at high densities and cover can impair fish and wildlife habitatmanage invasive plants to conserve or enhance native plants - select herbicides that are compatible with fishery – try to time control to minimize impacts with bedding and increased activities like tournaments, weekends, holidays, etc.E, S, F
Huntingplants can block access to hunting areas - plants provide habitat and food source, especially for some waterfowlmanage invasive plants to conserve or enhance native plant habitat - plan control to minimize impacts with huntingE, S, F
Swimmingplants can cover swimming areas, increase danger of entanglement and drowningselect control method compatible with swimming or control during low or no swimming periodsE, S, F
Skiingplants can impede boat operation and increase danger of entanglement and drowningkeep designated ski / boating areas free of aquatic plantsE, S, F
Wildlife viewingplants can block access to wildlife viewing areas and view of wildlifework with wildlife management agencies to ensure access to wildlife areas is acceptable - keep designated areas open for boat accessE, S, F
Fish and wildlife managementidentify and assess wildlife uses and needs within the system - while moderate levels of plants may provide essential habitat or forage, too many plants may cover nesting, bedding and forage areasaquatic plants and control operations may enhance or hinder wildlife management activities within a water body that may be seasonal or year round 
Endangered species, including habitat and forage/preyplants may provide essential habitat for endangered species - conversely, plants can cover nesting, bedding and forage sites as well as impair habitat for forage animals - ex: in Florida, waterhyacinth may outcompete native plants essential for Everglades Kite nesting as well as cover their prey (apple snails) causing them to abandon nestsunderstand types and seasonality of endangered species as well as forage/prey habitat requirements, select control tools and timing compatible with endangered speciesE, S, F
Fisherymoderate levels of diverse plant communities are generally viewed as favorable for many sport fish populations - monocultures of nuisance or invasive plants can crowd out beneficial native plants, cover bedding sites, stunt or eliminate some fish populations, reduce dissolved oxygen leading to fish killsselect control methods compatible with fish management objectives for water body - ex: do not drawdown during spawn; repeated harvesting may reduce young of year sport fish, ensure herbicide is compatible with primary fish management objective, avoid formation of extensive surface mats of submersed or floating plants and large submersed plant treatments with contact-type herbicides during hot water/low oxygen periodsE, S, F
Waterfowl huntingplant monocultures can crowd out or cover beneficial native plantsif possible, control plants well in advance of or after hunting seasonE, S, F
Non-game wildlifeplant monocultures can crowd out or cover beneficial native plants or cover nesting and foraging sitesidentify areas or species of concern with wildlife management agency and select control tools and timing compatible with non-game species managed in the water bodyE, S, F
Habitatplant monocultures can crowd out or cover beneficial native plantscontrol invasive or nuisance plant populations to conserve or enhance diverse beneficial native plant assemblagesE, S, F
Nesting / foragingplant monocultures can cover fish bedding sites, interfere with rookeries, cover or exclude prey or forage animals and plantscontrol invasive or nuisance plant populations to conserve nesting and foraging sites, ensure control tools are compatible with important forage plants and animalsE, S, F
Vegetation planting projectinvasive and nuisance plant growth can cover or crowd out newly planted vegetationprevent invasive or nuisance plants from covering revegetation projects, select control tools and timing that are compatible with planted speciesE, S, F
Mosquito controlinvasive floating plants and surface mats of submersed plants are ideal mosquito breeding sitescontrol invasive and nuisance plant mats, especially in quiescent waters in urban areas to reduce mosquito habitatS, F
ParameterConsideration/ConstraintInfluencePlant type
Control feasibilityvarious parameters influence whether or not a plant can be effectively controlled including; available tools, water body physical and chemical conditions, and plant susceptibility and growth stagelist and consider all control tools that have been proven successful in the water body in question or in similar waters and conditions - integrate the best tool or tools compatible with water body uses, functions, and conditions, that meet management objectives into the control programE = emergent
S = submersed
F = floating
Available methodslist all plant control tools that have been demonstrated effective in controlling plant(s) in question - demonstrated through documentation, contact with experienced managers that have effectively applied that control strategyintegrate tools into control plan that have been demonstrated to be effective - if tool is new, unproven, experimental, etc., approach implementation as operational research and convey to stakeholders the level of control anticipated and level of confidence in achieving controlE, S, F
Biologicalusually refers to releasing an animal species including fish, arthropods, or pathogens to suppress or control target aquatic plants to some extenteffectiveness may vary from suppression to complete control so target plant susceptibility and management objectives must be clearly evaluated and conveyed to stakeholdersE, S, F
Fish – grass carpgeneralist feeder that may control target and non-target plants - prefer some plant species over others - sterile, triploid chromosome variety available - mobile river fish that may need to be contained with physical or electric barrier - may control plants for up to a decade - may require permit from fish and game agency - extremely difficult to remove and determine population size in system after stocked(easier to add more if needed than to remove after stocking)test to ensure that only sterile triploid grass carp are released - ensure target plant is susceptible to grass carp, stock at the lowest feasible level - consider controlling target plants with other methods first to reduce biomass - install containment strategy - identify non-target susceptible plants - develop integrated strategy to augment control - stock 10”-12” fish in cooler months to reduce losses from predation, heat stress, and low dissolved oxygen - stocking rate can change significantly, ex: if water levels increase or decrease after stocking or sudden natural declines in vegetation (shading, etc.) can cause “overstocked” situationS, F
Arthropodsmost classical biological control is conducted with insects - agents must be approved by the USDA as well as state regulatory agencies prior to release to ensure host specificity - agents may reproduce in self-sustaining populations or may need additional releases to sustain sufficient levels to suppress or control plantsimpacts from insects may range from no observable control to decimation of target plant depending on insect species, plant type and climate at release site - predation from native animals (birds, fish, wasps, etc.) may influence the biocontrol population size and therefore the level of stress, suppression, or control achievedE, S, F
Pathogenssome plant pathogens, especially fungi can stress aquatic plants - commercially available pathogens(bioherbicides) are under research evaluationnaturally occurring outbreaks may increase efficacy of herbicide treatments, ex: water hyacinth control in some Florida watersE, S, F
Chemical Herbicideschemical herbicides must be registered for aquatic use by the USEPA and state regulatory agency - permits may be required from state or local governments before using registered herbicidessites and maximum rates are regulated by the federal and state label - susceptible plant species and lower than maximum use rates are determined through laboratory and operational researchE, S, F
Contact/systemicherbicides fall into two general categories, faster acting contact type herbicides that the kill the portion of the plant to which they are applied, and slower acting systemic type herbicides that translocate within the plant killing the entire plant including the rootsfaster acting or contact type herbicides may be more conducive to controlling submersed plants in flowing waters - slower systemic herbicides may be more suited to large-scale treatments to minimize oxygen consumption during plant decompositionE, S, F
Liquid/pellet formulationherbicide formulations fall into two basic formulations; liquid or aqueous, and solid pellets, flakes, wettable powders, or granulesliquid formulations are usually less expensive and are a better choice in waters with thick soft sediments where pellets can sink, diminishing effectiveness - pellets applied in slow flowing waters with firm substrates sustain prescribed concentrations for longer periodsE, S, F
Plant growth regulatorsPGRs do not kill, but rather suppress growth of target aquatic plantherbicides at low rates may provide some plant growth regulation - may lead to increased resistance in plants if not killed - application of this control strategy not well developedS
Harvesterremoval of plant mass from water body - may control non-target plants and animals - various designs, sizes, and hauling capacity available – may provide immediate control of small scale plant problemsmay fragment and spread target plant - must find disposal sites - removes target and non-target plants and animals - more efficient harvesters may harvest larger fish and wildlife that cannot escape path - efficiency may be increased with barges to shuttle plants to disposal site – may create turbidity in shallow watersE, S, F
Barge mounted hoe/draglineremoval of dense mats of plants and floating islandsremoves dense masses of vegetation and other material from canals and river channels as well as bridges and flood control structures - may fragment and spread target plant - must find disposal sites - may remove target and non-target plants and animalsE, S, F
Shreddervarious designs are available to shred floating masses of herbaceous and woody plants and floating masses or islands of sedimentsused for emergency restoration of access, navigation, or flood control attributes as well as around bridges - generates fragments that may spread invasive plants - controls all plants and animals in control area - may require additional shredding or harvesting of materials that float back to the surface - may generate extensive turbidity - drops mater on bottom - not advisable for repeated use at boat ramps, navigation channels, residential shorelines, etc.E, S, F
Rotovatorunderwater apparatus or arm extending from barge with rotating tines to tear plants from sedimentsgenerates fragments and may spread invasive plant infestation - may need to harvest uprooted plants - disturbs sediments and may generate extensive turbidityE, S
Barrierspassive devises to cover target plants, or to contain plant fragments, turbidity, herbicide-treated water - may be highly labor intensive to install/removemay be used in small areas where other options are less practicalE, S
Benthicfabric laid over plants on substrate - must anchor to bottom - place over live plants or control plants to substrate and place barrier to control re-growthevaluate potential impacts to target and non-target plants and animals - may need to clean barrier to prevent plant growth on topE, S
Curtainsvertical barrier in the water column to minimize water exchange from one site to another - can either be manufactured curtain to prevent water exchange to contain herbicides, or a strip of plants left on the edge of harvest or shredding sites to contain fragments or turbidityprevent or reduce herbicide dilution and turbidity in flowing or open watersE, S
Benthic rollersdevise usually anchored to a piling or dock to roll over plants and sedimentsmay be effective on small scale - needs power source and frequent monitoringE, S
Drawdownwater control structure must be available - reducing water levels to accommodate aquatic plant control must be compatible with other uses and functions of the water body - consider ability to refill water body after drawdowndrawdowns need to last for several months - must be complete to desiccate plants - best applied in winter to include impacts from freezing - compatible with prescribed fire for emergent plant control - try to avoid during fish spawn, waterfowl hunting, endangered species nesting foraging - partial drawdowns during growing season may allow invasive or nuisance submersed plants to colonize into deeper waters expanding the problem - incomplete drawdowns may allow wetland plants like cattail or willow to reach nuisance levelsE, S, F
Desiccationextreme drawdown must be of sufficient duration to dry target plants and preferably sediments - not appropriate during wet or growing seasonplants that produce underground tubers (hydrilla) or extensive seed bank (water hyacinth) are not well suited to control by drawdown - in some areas floating islands may develop upon re-flooding and may need to be controlledE, S, F
Freezingfreezing enhances desiccation and amount of controldrawdown needs to expose sediment to reducing insulating effect from water - conversely, summer drawdowns can increase spread of invasive (torpedograss) or native plants (willow) can expand to nuisance levelsE, S, F
Prescribed fireplanned burning of emergent vegetation to reduce standing crop - burning must be compatible with surrounding land usereduces standing crop and stimulates re-growth in some species - be prepared to follow up with other methods including herbicides upon re-flooding – may not be practical in urban areas or near high traffic highwaysE
Floodingflush floating plants or mats of plants out of system or into uplands, - increase water level to shade and stress submersed plantsraising the water level to flush and strand floating plants or mats of plants into uplands is an option in waters with flood control structure and few to no houses or structures along shoreline - other flooding methods include lowering water levels to treat submersed plants, then re-flooding to reduce light and further stress plants - some emergent plants (torpedograss) can be controlled by dewatering, burning, and re-flooding to suppress re-growthE, S, F
Dredge – barge mountedlarge-scale dredging operation that removes rooted plants and sediments - sediments returned to water column or pumped to settling basinmay miss plants - may fragment and spread plants - may increase turbidityS
Dredge – diver assistedhand-held suction devise controlled by underwater diver using snorkel or SCUBA - dislodge plants by hand and place into suction lift to screen plants onshore or on attending bargelabor intensive - effective in small areas where other methods are not practical - may cause or may be impeded by siltation / turbidityS
Dyesartificial dyes like natural tannins color water, reducing light penetration to control or suppress submersed plant growthmay provide submersed plant and algae suppression in small areas where water flow, volume, and exchange are lowS
Hand pullingremoving plants by hand - includes tossing rakes or hand-held cutting blades to sheer plantsimmediate control - labor intensive - may be suitable for new infestations around boat ramps, docks, trash rakes at water intakes, pumps, etc. - may use rakes and cutting blades to clear small areas of plant material - creates fragments that may spread plants to other areasE, S, F
Shearing - chains, etc.includes any of a number of devises that are dragged through rooted stands of plants including chains pulled by hand or steel bars towed by boat or bargelabor intensive - disturbs sediments - creates fragments and turbidity - may need to clear obstructions - used in some canal systems where most plants may be considered undesirable and substrate habitat is a low concernE, S
Waterbody parameters   
Water depthwater depth can influence the cost and duration of control - water control structures can give the flexibility of reducing and increasing water depths to accommodate controlre-growth of submersed plants to the surface is faster in shallow waters - do control costs, methods, etc. warrant short term control? - control of submersed plants with herbicides requires treating much or all of the water column - shallow water should be less costly to treat than deep water - increasing the water depth after a submersed plant herbicide treatment reduces light penetration enhancing the amount and duration of controlE, S
Water volumeimportant for herbicide control since effectiveness of many herbicides is dependent upon sustaining a prescribed concentrationreducing water volumes before herbicide treatments for submersed plant control can save money and increase efficacy - increasing water volume before use of herbicides to control submersed plants can dilute concentration and reduce or negate control efficacyS
Water flowstatic vs. moving water can play an important role in selecting control methodsimportant in determining pelletized vs. liquid formulation herbicides - dilution from flow may be too great to apply herbicides, especially slow acting systemic compounds - flow may dictate urgency of control, ex: to keep floating plants from clogging flood control structures or jamming against bridges - keeping flow unimpeded may impact ability to contain grass carp with conventional physical barrierE, S, F
Springs / sinkholesrelated to flowgroundwater may dilute or dissipate herbicidesS
Tidal influencetides can raise or lower water levels and volumes, can flush herbicides, and regulate plant growthmay dilute herbicide concentrations by adding water volume at high tide or flush herbicides out of treatment area as tide recedes - depending on salt content, may preclude use of some herbicides not registered for use in brackish or marine waters - may restrict access for herbicide spray boats, harvesters, barges, etc. due to low (grounding) or high (bridge clearance) water level - invasive plants may not reach problem level if salt content sufficiently high - ex: hydrilla in brackish water - may favor invasive species tolerant to low salinities - ex: Eurasian watermilfoilS
Water chemistry   
Dissolved oxygenoxygen is needed to sustain aquatic life and decompose organic sediments and detritus - warmer water holds less dissolved oxygen than cooler watercheck oxygen level prior to herbicide use - slow acting or systemic herbicides or treating smaller areas with contact type herbicides can reduce amount of plant decomposition and demand on oxygen to avoid stressing or killing fish - try to conduct large-scale plant management in cooler months before plants reach peak biomass (more oxygen / less decomposition)S, F
pH, alkalinity, and hardnessthese parameters may be important in determining invasiveness of plants in certain waters - ex: water hyacinth and hydrilla do not grow as well in low pH waters - pH, alkalinity, and hardness modify performance of certain herbicideslow alkalinity and pH increase copper toxicity to fish - high pH decreases efficacy of flumioxazin herbicide for submersed plant control - hard water binds with glyphosate and reduces efficacyS
Nutrient contentnutrient content in aquatic macrophytes and in the sediments may be re-suspended in the water column after controlling aquatic plants - nutrients are released from decomposing plants and in shallow waters, sediments may be stirred by wave and water currentsnutrient content may be a concern when planning large-scale management - some nutrients are released by decomposing plants - removing plants from the system to remove nutrients may not be cost-effective since aquatic plants are mostly water - sediment nutrient re-suspension may be significant after the calming effects of plant cover is removedS, F
Water transparencywater transparency affects the amount of and depth to which light penetrates the water column to stimulate submersed plant growth and growth of new emergent plant shootsgenerally, submersed plants grow faster in waters with higher transparency with all other factors being equal - conversely, lower transparency can retard growth of submersed plant shootsS
Color / tannic contenthighly colored or tannic water limits light penetration and can suppress submersed plant growthsubmersed plant recovery after control can be retarded in highly colored or tannic waters - anticipate increased submersed plant control durationS
Turbidity / suspended particlesturbid water limits light and suppresses submersed plant growthsubmersed plant recovery after control may be retarded in highly turbid waters - suspended clays and organics can neutralize diquat and fluridone herbicidesS
Algal type and concentrationsome algal blooms can suppress submersed plant growth either through light attenuation or perhaps allelopathy with blue-green bloomstreating large areas of submersed plants during a planktonic algae bloom may perpetuate or enhance the bloomS
Sediment characteristics   
Composition - sand, clay, organicssediment type plays an important role in plant growth as well as control, especially chemical optionsclay sediments inactivate diquat herbicide, high levels of organic sediments can adsorb fluridone herbicideS
Sediment depth / locationcheck sediment type and thickness prior to herbicide treatmentsthick soft sediment layers can reduce or negate pelletized herbicide formulation efficacy - harvesting in shallow waters above flocculent sediments may result in turbidity problemsS
Potential for re-suspensionextensive plant cover, especially submersed plants, can retard organic sediment decomposition or allow suspended particles to settle out of flowing water forming thick flocculent layerdiquat herbicide is inactivated by suspended clay particles - high suspended organic particle content can reduce fluridone herbicide efficacy - removing calming effect of plants (after control) may allow water flow or waves to agitate sediments, especially in shallow waters, re-suspending sediments and associated nutrients - result may be increased turbidity or algae bloom - agitation from harvester paddle wheels can increase turbidity in shallow waters with flocculent sedimentsS
Plant physiology   
Plant origin/growth characteristicsproblem plants in a proposed control area should be characterized as native or exotic, and if exotic, they should be characterized as either a nuisance under the conditions present in the water body, or an invasive species in that regionthe invasiveness and extent of the plant in the region influences the intensity of control - ex: a newly discovered plant that may be invasive in waters across the region may trigger eradication efforts - a native plant that interferes with boat ramp access may be beneficial throughout the rest of the water body triggering only local controlE, S, F
Native planta plant species that evolved in the general region where it is now founda diverse assemblage of native plants is generally viewed as favorable - native plants do not generally impair natural waters, they may present problems to various uses and functions of the water body on a local scale - problems associated with native plants are often generated by watershed alterations including stabilized water levels and increased nutrient content - plants native to a region can cause problems in man-made waters like shallow canals or aqueducts where presence of any plant species may be considered undesirable or problematicE, S, F
Exotic / aliena plant that has been transported to a region in which it did not evolveexotic plants do not necessarily cause problems in the ecosystems in which they have been introduced - causes of problems may be similar to those associated with native plants and therefore may be localizedE, S, F
Invasivea plant that is non-native to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health – even if an invasive plant species does not cause problems in one waterbody, it may serve as a contamination source for adjacent waters that may be more conducive to invasionnewly discovered populations of invasive plants should be considered for eradication or containment - delays may allow spread within infested waters or to additional waters - invasive plants may not be invasive in all cases - ex: water milfoil may cause problems in clear, shallow, stabilized waters, but may not be problematic in deep or turbid lakes or reservoirs with widely fluctuating water levelsE, S, F
Plant growth stageplants are susceptible to various control methods based on current weather and growth conditionsmost herbicides need actively growing plants to be effective - new growth is generally easier to control with herbicides than mature plants with high starch reserves and larger rhizome / root massE, S, F
Target plant / non-targetit is important to understand the growth stage of target plants as well as commingled non-target plantsconsider controlling target plant while non-target plants are dormant or after they have produced seeds and are senescing - control target plant while infestation is still low to minimize effects on desirable comingled native plant speciesE, S, F
Plant susceptibilityplants must be susceptible to control tools to avoid wasting valuable time and moneyevaluate effectiveness of control tools through literature reviews or contact with managers with similar problems and conditions - plant susceptibility may change from one control event to the next related to such parameters as plant growth stage or water conditionsE, S, F
Target plant / non-targetprior to initiating aquatic plant control in systems where a diverse native plant community is desired, it is important to identify non-target plants to develop control programs that conserve or enhance these speciesimpacts to non-target plants can be reduced through selection of control methods, timing of control, using lowest feasible herbicide rates, and controlling target plants, especially invasive plants, before they become widespread and require large-scale control efforts - ex: stocking sterile grass carp early after an infestation of susceptible plants or reducing plant biomass prior to stocking allows the lowest number of fish to be released lessening non-target plant controlE, S ,F
Potential for re-growth  E, S, F
Target / non-targetcontrol operations may be expensive - evaluate the potential for re-growth for proposed control methods or strategiesconsider cost-effective control measures that selectively control target plants while conserving or enhancing non-target species - evaluate cost-effectiveness of proposed control - ex: controlling a new infestation of hydrilla or Eurasian watermilfoil in two feet of water in an attempt to eradicate may be cost-effective - controlling widely dispersed and established hydrilla or EWM in two feet of water where re-growth to the surface may take 1-2 months may not be cost-effective managementE, S, F
Weatherdaily weather conditions seasonal weather conditionsrain may wash off herbicides before they are effective - treat early in day during summer months in thunderstorm prone areas - check weather report prior to herbicide applications for wind and rain forecast - several cloudy or rainy days after a large submersed plant treatment with contact herbicides may result in substantial dissolved oxygen reductions use caution applying systemic herbicides requiring 2-3 months of contact in areas impacted by tropical or seasonal monsoonal weather - take advantage of winter dieback by controlling plants before they become a problem in spring or summerE, F, S 
Light intensityan important plant growth factor along with temperaturesome herbicides’ primary breakdown pathway is via photolysis; efficacy may be reduced in the summer or in shallow clear waters - consider with water transparency for predicting submersed plant growth along with herbicide selection and treatment timing - light intensity triggers tuber production in hydrillaS
Water temperaturetemperature influences plant growth and the amount of dissolved oxygen in the water column as well as microbial activity important for decomposing plant material and degrading some herbicide compoundswarming winter and spring temperatures can trigger plant growth, important for herbicide uptake especially in submersed plants - warmer water holds less dissolved oxygen than cooler; important for planning size of herbicide treatment and mode of action (fast acting contact vs. slower systemic)S, F
ParameterConsideration/ConstraintInfluencePlant type
Other considerationsin addition to physical parameters, there are human values to consider when deciding the level of aquatic plant control to attempt on a water bodythese influences do not necessarily reflect the level of control that may be achieved, but rather the will of stakeholders to commit to attempting some level of control effortE = emergent
S = submersed
F = floating
Costvalue judgment - does the anticipated outcome of controlling or not controlling plants justify expenditure?the benefits of control must justify control expenditures - control must meet reasonable management objectives, including duration of control, restore or conserve uses and functions of water body, protect public health and safety, etc.E, S, F
Anticipated amount of controlaquatic plant control is complex and many stakeholders have a rudimentary understanding of available tools and realistic control expectations - the public usually expects control to resolve impaired uses or functions of water bodies - responsible aquatic plant managers and researchers must clearly convey to stakeholders why they select or support control options as well as the anticipated amount and duration of controlmanagement objectives should address anticipated extent of control - control includes the level of impact to the standing crop as well as underground roots, rhizomes, tubers etc. that influence ability of the plant to recover; therefore, control also includes the degree of impact to the problem-causing plant, the time to alleviate impaired uses, and the expected amount of time control will last; i.e. time until water uses may again be impaired 
Spatial – acres, % of water columncontrol area includes the coverage of plants to be controlled, expressed in acreage, square meters, etc. - also includes the percent of the water column in which plants are controlled, expressed as percent volume infested - can also include the below ground portion controlled (runners, roots, corms, tubers, etc.)control using different tools or applied to different plant species provides variable results - managers must select tools that provide a level of control that satisfies management objectives and convey this reasoning or expectations to stakeholdersE, S, F
Time to achieve controldepending on the method(s), the amount of time to achieve control may be immediate or may take months or longer, if achieved at allcontrol methods may provide immediate relief of a problem (ex: harvesting adjacent to flood control structures or bridge pilings) or take months (ex: systemic herbicides, biological controls)E, S, F
Length of control in timethe applied control method(s) as well as environmental parameters impact the duration of control achieved - ex: control may be achieved in a matter of a few days to a few weeks, but plants may re-grow to problem levels within a monthcontrol may last a few days to several years depending on method and water body conditions - ex: a summer contact type herbicide treatment of hydrilla or torpedograss growing in 1-2ft of water may only last a few weeks before plants refill the water column while a winter fluridone treatment in 12-15 feet of water may prevent hydrilla from growing back to the water surface for 18-24 monthsE, S, F
Suppressionincludes reducing plant vigor as well as flowering, seed productionmany biological controls as well as plant growth regulators stress plants but by themselves may not provide a level of control that meets management objectives or stakeholder expectationsE, S, F
Water body values at riskassess various uses of water bodies and estimate economic and environmental costs as well as impacts to human health if plants are controlled or not controlledassists in establishing management objectives as well as level of control and choosing control optionsE, S, F
Alternative water bodyif plant control cannot be achieved in a water body, identify any alternative waters to serve the uses and functionsthis is a temporary solution while eradication or management efforts are being devised or applied in a water body - access to the infested water body may be closed during eradication efforts or control delayed in infested waters while higher priority waters are managed, especially if other nearby waters are available - efforts should be made to resume use of water body as soon as possibleE, S, F
Contractor / equipment availabilityensure availability of contractor and equipment to address all anticipated control possibilitieshave back-up labor and equipment contractors available - securing contracts can take time which may be critical for eradication or in emergency situations – large-scale control operations or operations in waters with multiple uses and functions may have very narrow windows of opportunity to implementE, S, F
Control history in similar watersapply control tools or management strategies with proven or demonstrated effectiveness and compatibility with uses and functions of systemmonitor efficacy of each control event - determine causes of poor or no control and avoid repeating - for new infestations look to successes or failures with various control options in waters as similar as possible to proposed control siteE, S, F
Coordinate with stakeholderscontrol operations should be developed with stakeholders that have expressed interest in understanding the intricacies of aquatic plant control - the public should be notified through some means of any use restriction of impending herbicide control operationsstakeholders may view aquatic plant control and control tools from a single or less than holistic perspective - education and outreach efforts are important in addressing public concernsE, S, F
Support – verbal, financial, in-kindimportant tiebreaker for waters of equal importance when factors such as funding, technology, contractor availability, or cost/benefit ratios are insufficient to implement control projects in all water bodies - especially for lower priority uses or waterswork with all stakeholders to clarify management objectives - in low priority management waters, if support is high, then elevate to higher priority than equal priority waters where support is low or stakeholders oppose controlE, S, F
Publiclevel of verbal support from homeowner or public or private stakeholders or associationsfor equally ranked control project priorities, public support may elevate control projects, especially above projects where there is no support or open stakeholder opposition to controlE, S, F
Agency – federal, state, locallevel of verbal, financial, or in-kind service support for controlling aquatic plantsexternal funding or services may elevate a control project to a higher priority above otherwise equally evaluated projects with no external assistanceE, S, F

*This paper was written by Michael D. Netherland and Jeffrey D. Schardt for the Aquatic Plant Management Society. It was first published in Aquatics magazine, Vol. 31(1):6, 9-19 (2009), and subsequently published by the Aquatic Ecosystem Restoration Foundation (AERF) in Biology and Control of Aquatic Plants – A Best Management Practices Handbook edited by Lyn A. Gettys, William T. Haller and Marc Bellaud (2009).


Last updated: 01 December 2011