Managing Invasive Plants: Concepts, Principles, and Practices link

U.S. Fish & Wildlife Service
America's National Wildlife Refuge System

MANAGING INVASIVE PLANTS: Concepts, Principles, and Practices

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Management Methods: Chemical Methods

Chemical Methods in Practice

Federal and state laws regulate the registration, sale, and use of pesticides, including herbicides. In addition, the Department of the Interior and the USFWS have policies and review processes that further direct the use of herbicides on USFWS lands. Taken together, these authorities focus on the legal, safe, and effective use of herbicides. Complying with these and other applicable authorities ensures that the refuge staff understand basic principles of herbicide use and target plant biology, evaluate all plant management options and strategies, prepare proper documentation, and monitor the effects of herbicide use. Using herbicides within an integrated pest management framework can maximize herbicide effectiveness and minimize adverse effects to human health and the environment.

Regulation and Policy

Federal Regulation of Herbicides

The Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) established the basic system of pesticide regulation in the United States and is administered by the EPA. FIFRA governs the registration, distribution, sale, and use of pesticides nationwide and has several functions including:

  • requires all pesticides sold or distributed in the United States to be registered with the EPA
  • creates a classification system for pesticides based on toxicity
  • allows states to regulate pesticide use
  • requires certification for pesticide applicators

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Herbicide Use on National Wildlife Refuges

Herbicide use on National Wildlife Refuges must be in compliance with FIFRA and other federal laws and authorities including the National Environmental Policy Act, the Endangered Species Act, Migratory Bird Treaty Act, and the National Historic Preservation Act, state pesticide laws, and label instructions. The use of herbicides and other pesticides on refuges is governed by the US Department of Interior Integrated Pest Management Policy (517 DM 1), the USFWS Pest Management Policy and Responsibilities (30 AM 12), and the USFWS Refuge Manual (7 RM 14).

The USFWS policies and refuge manual state that we will use herbicides only after full consideration of management alternatives including chemical, biological, physical, and no action. If after considering all of these factors managers determine that we must use herbicides to meet invasive plant management objectives, then we will use the least hazardous, most effective herbicides to meet hose objectives.

Refuge staff must complete a Pesticide Use Proposal (PUP) whenever we use an herbicide on a refuge, including applications by staff, volunteers, contractors or in association with a right-of-way easement or Special Use Permit. Individuals with duties related to plant management and knowledge and experience with herbicides typically complete and submit the PUP. An online Pesticide Use Proposal Systems (PUPS) database enables staff to complete and submit PUPS electronically at The database was launched in 2007 and maintains all of the pesticide proposals, actual pesticide usage and efficacy data. This information can be queried, providing valuable information when generating enw PUP requests or viewing pest treatments across Refuges as a reference. Additionally, users can duplicate PUPS, further streamlining the submittal process. The PUP process ensures that refuge staff have properly read the herbicide label, assessed the need for the herbicide, evaluated potential effects to human health and the environment, and completed environmental compliance requirements.

Depending on the pesticide and other conditions listed in the PUP, the PUP may need Regional Office review and approval, and under some circumstances the Regional Office may need to submit the PUP for Washington Office (WO) review and approval. Criteria for PUPS requiring WO approval can be found at

PUPS that are part of an approved integrated pest management plan may receive five-year approvals. The Director periodically issues specific guidance that includes details about PUP approval authority and which herbicides and application scenarios require review beyond the field station.

A PUP database enables staff to complete and submit PUPS electronically. The database includes the following information:

Screenshot of Pesticide Use Proposal database home page.
Screenshot of the online PUPS database, where we enter pesticide use proposals.
  • person submitting the PUP and the herbicide applicator
  • description of the target species and how the pest species interferes with the site’s management objectives
  • description of proposed integrated pest management methods
  • proposed herbicide(s) to be used, including trade name(s) and application rate(s)
  • location of the pest management activities
  • potential effects of herbicide application(s) on nontarget and federally listed species
  • actual pesticide usage
  • efficacy of the treatment

Refuge managers or project leaders ensure that:

  • Pest management decisions are consistent with all applicable policies, laws and regulations.
  • Anyone applying pesticides, releasing biological control agents, and conducting other IPM activities has the appropriate training and equipment necessary to protect their safety and health.
  • We apply pesticides only after the appropriate reviewer approves the PUP.
  • We establish threshold levels of damage or pest populations according to Service or field station goals and objectives and applicable laws.
  • Staff store, handle, and dispose of pesticides and pesticide containers in accordance with the label and in a manner that safeguards human, fish, and wildlife health and prevents soil and water contamination.
  • Submit annual reports documenting pesticide use and efficacy into the online PUPS database.

Utilizing the PUPS Database as a Reference

  • The PUPS database contains a search engine for viewing proposals and usage data from across the Service. For this reason, accurate site information and effective use of the "comment box" will enable other users to utilize your PUP and usage data as resources. For example, comments on unexpected climate conditions or biotic interactions before, during, or after the application may affect pesticide efficacy (i.e., a large infestation of cattail caterpillars were noted at the time of application or a tropical weather event within 72 hours of application caused saltwater inundation across 1/3 of the acreage sprayed). This information helps you and others understand efficacy rates. Using the query functions in PUPS, enables a user to identify other refuges applying a variety of pesticides and the efficacy data reported for a particular species and habitat. This becomes an invaluable resource for planning future pesticide operations.

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When deciding to use herbicides to manage invasive plants, refuge staff should ensure that herbicide use is consistent with overall land use goals and objectives for invasive plant management. The objectives should focus not only on removal or control of the target species, but also on establishing and maintaining desired vegetation conditions and protecing uninfested areas from invasion.

The most basic principles regarding the use of herbicides to manage invasive plants are:

  • Refuge staff should use herbicides only in situations where the benefits of controlling invasive populations outweigh the overall risks of using herbicides, and/or other methods are prohibitively expensive, not effective, impractical, or likely to cause more damage than the herbicide.
  • Refuge staff should ensure that they are following all requirements on the herbicide label. Failure to do so is a violation of the Federal Insecticide, Fungicide, and Rodenticide Act. Also, we must follow any label posting requirements in addition to any other local or state posting requirements before, during, or after a herbicide application. Refuge personnel must comply with all other applicable federal, state, and local authorities, including the Endangered Species Act and National Environmental Policy Act.
  • The applicator must apply herbicides correctly and in a manner that enhances the benefits of herbicides while minimizing adverse effects. Safe and effective herbicide applications require: 1) an understanding of herbicide properties and interactions with target species and the environment, 2) a working knowledge of how to select appropriate herbicides for a particular situation and 3) how to safely handle herbicides.
Guidelines for Safe and Effective Herbicide Application
  1. Assess site conditions and infestation characteristics to determine if herbicide use is necessary and what precautions the applicator should take to minimize risk to nontarget resources. This includes the safety of humans, fish and wildlife, nontarget plants, and abiotic resources such as groundwater.

  2. Select effective herbicides and application methods that are appropriate for the target species and site conditions. Use only EPA registered herbicides and use them in accordance with FIFRA, EPA, state laws and regulations, and all other applicable authorities.

  3. Follow label requirements for mixing, loading, transporting, storing, and disposing of herbicides and containers to safeguard human health, fish and wildlife, and prevent soil and water contamination.

  4. Ensure that staff using herbicides have training in all aspects of herbcide applications and appropriate procedures for first aid and spill cleanup.

  5. Use personal protective equipment as required by the herbicide label. Ensure that staff who are mixing, loading, or applying herbicides have appropriate medical monitoring, as specified in USFWS policy.

  6. Use herbicides that are registered as Restricted Use Products (RUP) with caution and only by or under supervision of certified applicators. Consult the herbicide label and the state agency that regulates herbicides for other specific requirements.

  7. Follow all federal, state, and local posting requirements for every herbicide application.

  8. Monitor before, during, and after herbicide application to asses effects on target species, nontarget organisms, and the environment.

  9. Discontinue use of herbicides that are ineffective or are causing significant undesirable effects.

  10. Develop and maintain accurate and comprehensive application records.

(adapted from Integrated Pest Management Policy 517 DM 1 and Tu et al. 2001)

Refuges should develop a set of project-specific Best Management Practices (BMP) for herbicide application such as

  • apply at wind speeds less than 10 mph (or as specified on the pesticide label – some of them state "no more than 4 mph).
  • calibrate application equipment
  • conduct field scouting and monitoring before application
  • create herbicide-free buffers around sensitive areas
  • use the lowest legal effective application rate
  • establish vegetative buffers to minimize offsite movement of herbicides
Photo of Nelson’s checkermallow plants.
BMP to protect the federally listed Nelson’s checkermallow (Sidalcea nelsoniana) from herbicide drift include shielding plants with suitable material such as a five-gallon bucket. Photo credit: USFWS

The staff at Willamette Valley NWRC in Oregon is working to restore native prairie on refuge and private lands. They implement BMPs when applying herbicides near federally listed species and sensitive habitats. BMPs may include establishing buffers near ephemeral or perennial watercourses, using procedures that control drift, or timing applications to occur when native plant species have senesced. The staff selects herbicides based upon the best available scientific information regarding efficacy and risk assessments (EPA 1998) that consider potential effects to federally listed aquatic and terrestrial species.



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Tactics for Chemical Methods

Specific herbicide application tactics can vary according to several factors including target species, site conditions and characteristics, and herbicide properties. Herbicides must be applied in strict accordance with the herbicide label, which describes safe and effective application timing, rate, and techniques for various environmental conditions (i.e., soil properties, depth to water table, weather conditions).

Herbicide effectiveness is a function of several biotic, abiotic, and procedural factors. Understanding these factors can help applicators

  • select safe and effective herbicides for target species and conditions
  • properly handle herbicides
  • minimize impacts to non target resources
  • determine the most effective time, rate, and technique for herbicide application
Factors Affecting Herbicide Effectiveness


  • plant growth stage
    seedling vs. mature
  • plant life cycle
    annual, biennial, perennial
  • growth activity
    actively growing vs. dormant
  • plant morphology
    grass vs. broadleaved


  • weather conditions
    precipitation, drought, wind, temperature
  • water chemistry in aquatic systems
    pH, turbidity, hardness
  • soil characteristics
    texture, pH, organic matter



  • application technique
    foliar, basal bark, hack and squirt, injection, cut stump
  • application timing
    pre emergence, post emergence
  • application rate
    range of recommended rates on label, proper calibration of application equipment
  • application accuracy
    equipment, personnel training and experience, technique

Herbicide effectiveness and characteristics are also affected by the quality of the water used to mix herbicides. For example, the minerals in water can react with some herbicides; water containing suspended solids (e.g., dirt and other organic matter) can deactivate certain herbicides; and water with a high pH can affect the solubility of certain herbicides, potentially damaging application equipment. Water pH, turbidity, and hardness should be tested prior to mixing.

Personal Protective Equipment

Individuals who are mixing, loading, transporting, and applying herbicides can minimize their exposure to herbicides by wearing the personal protective equipment (PPE) specified on the herbicide label. Applicators are legally required to wear the PPE listed on the label.

Individuals who are handling or applying herbicides may wish to use protective gear in addition to that listed on the label. In such cases, they should consult with their Regional Safety Officer to determine which equipment to use and if additional medical monitoring and clearance may be required.

Standard PPE used for most types of herbicide applications includes the following:

  • rubber boots
  • protective aprons or suits
  • rubber gloves
  • safety glasses or goggles

Herbicide Selection

An active ingredient’s effect on plants can vary with the product formulation and manufacturer. Therefore, one must understand how a given product will interact with plants, soil, and water. Refuge staff should base decisions about which herbicides to use in a particular situation on the product labeling and in consultation with experienced land managers, herbicide dealers, extension agents, and other experts. USFWS Pest Management Policy and Responsibilities (FWS 30 AM 12) states that staff should select herbicides based on factors including human safety and environmental integrity, effectiveness, and cost.

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An important first step when selecting an herbicide is to determine which products are labeled for use on the target invasive plant species. FIFRA Section 2(ee) allows the use of herbicides on target species that are not listed on the label as long as the application site conditions are consistent with those permitted on the label. However, certain states prohibit the use of herbicides on plant species that are not listed on the product label. Other factors to consider include proximity of invasive plants to non target plants, soil composition, and likelihood of the chemical to contaminate water.

Generally, herbicides that are less likely to move in the environment, degrade quickly, and have low levels of non target toxicity are desirable, particularly for sensitive sites. In some circumstances, however, a single application of a more toxic or persistent chemical that completely controls the target invasive plant may be preferable to repeated applications of a less toxic, less effective herbicide.

Application Technique

Select an application technique that minimizes risks of exposure to the applicator and others who may be in the area during and after herbicide application and that also minimizes the offsite movement of the herbicide. The labor and equipment required to implement different application techniques vary and can influence the overall cost of herbicide application.

Typical Herbicide Application Techniques


  • foliar
    herbicide can be applied using a spot application with a spray bottle or backpack applicator; a wick application in which the herbicide is wiped on; or a boom application that sprays herbicide from a vehicle, airplane, or helicopter
  • basal bark
    herbicide is applied to bark in a band encircling the base of the trunk
  • hack and squirt
    the bark is cut and herbicide is immediately applied
  • injection
    herbicide is injected into woody trunks
  • cut stump
    herbicide is applied to a cut stump immediately after the tree or shrub has been cut


  • foliar
    herbicide is applied directly to the leaf surface to control floating and emergent species
  • water column
    herbicides are applied directly to the water column to control submerged species; the plants take up the herbicide from the water



Herbicide application timing refers to the time of year, time of day, and sequence of herbicide applications relative to other management activities (e.g., livestock grazing) or events (e.g., rainfall). The timing of application can depend upon the target species, the herbicide mode of action, the growth stage of the target plant, or environmental conditions such as drought.

Some herbicides may be more effective than others at a certain time of year. For instance, herbicides that inhibit growth are most effective when plants are actively growing, and herbicides move more readily through the plant. Timing herbicide application to the most active growth stage of the target species can reduce the amount of herbicide needed as well as potential impacts on non target vegetation. For example, control of Hydrilla (Hydrilla verticillata) is usually done during the winter and early spring when it is actively growing but most native plants are dormant (Cervone and Schardt 2003).

Herbicide applications should be timed to maximize favorable weather conditions. To avoid herbicide drift and potential non target impacts, spray herbicides should be applied when wind speeds are less than 10 mph. Other weather conditions that influence how an herbicide works include temperature, moisture, and humidity. Warm conditions (65° to 85° F) are usually favorable for herbicide application, but hot, dry conditions can slow plant metabolism, making plants less susceptible to herbicides. The length of time required between herbicide application and rainfall, referred to as the rainfast period, varies for different herbicides.

Application Rate

The herbicide label provides a range of acceptable application rates based on soil conditions and characteristics, time of year, and target species. Herbicide application rates on the label are given in terms of amount of formulated product per acre (e.g., pints per acre). The applicator must often determine the pounds of active ingredient per acre to be applied at a given application rate. For example, if a glyphosate formulation has three pounds of active ingredient per gallon of formulated product, and the label application rate is two quarts (0.5 gallon) of formulated product per acre, then the amount of active ingredient per acre would be 1.5 pounds per acre.

Diagram of equation to determine pounds of active ingredient applied per acre.

Although use of the lowest effective rate is desired for environmental and economic reasons, certain factors such as humidity, soil type and pH, and plant age may require higher herbicide application rates. For example, some herbicide labels recommend higher application rates for highly acidic soils. Misuse of herbicides can increase the risk of adverse effects to non target plants and may decrease effectiveness of some herbicides.


Label recommendations can be further refined by reviewing pertinent scientific literature regarding treatment of the target species where specific chemical application techniques, timing, and rates are used under different environmental conditions. Data from documented field experiments can be extremely valuable in improving the ability to predict how a particular method will work under similar conditions.

Combining Herbicides

Certain herbicides can be combined for broader control of target species and to reduce the potential for herbicide resistance. Combining herbicides can also reduce the amount and frequency of application. The term tank mix refers to two or more formulated herbicides mixed in a spray tank at the time of application.

Herbicides can be both chemically and physically incompatible. Chemically incompatible herbicides can react and reduce the efficacy of one or all of the herbicides in the mixture, whereas physically incompatible herbicides can damage application equipment. Mixing instructions are included on some, but not all, herbicide labels. When in doubt, contact the herbicide manufacturer. For tank mixing, use the most restrictive label limitations for each of the products being used. Mix only the products the label lists as compatible with each other.

A jar test is performed to observe signs of incompatibility between herbicides. Generally, a small amount (e.g., 1 pint) of diluent (e.g., water) is measured into a jar. Formulated herbicides are added with compatibility agents, activators, and other adjuvants in a sequence based on the type of formulation (i.e., wettable powder, water soluble concentrate, emulsifiable concentrates) and label guidelines. After mixing, the solution is observed for signs of heat and formation of scum, clumps, or other solids. These signs may indicate incompatibility, and the herbicides probably should not be mixed together.

With respect to toxicity of herbicide mixtures, individual components may interact in a variety of ways: additively, antagonistically, and synergistically. Synergistic toxicity poses the greatest problem in the assessment of potential risk to human health and other organisms.

  • additive
    toxicity of the mixture is equivalent to the sum of the toxicities of the individual components
  • antagonistic
    toxicity of the mixture is less than the additive
  • synergistic
    toxicity of the mixture is greater than the additive

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Monitoring is an essential part of any management action. Through the repeated collection of data, the effectiveness of methods in meeting invasive plant management objectives may be determined. Monitoring involves not only evaluating the effectiveness of herbicides on the target plant species or population, but it may also involve detecting changes in desirable plant species, and evaluating potential impacts on non target organisms, and soil and water resources. This is particularly important in situations that require repeated herbicide applications. Monitoring also reveals any necessary adjustments to herbicide application rate and timing.

Many refuges are using Refuge Lands GIS (RLGIS) as a monitoring tool to survey and map invasive plant infestations. Using point, line, and polygon features on a GPS handheld unit, RLGIS can help managers prioritize treatment areas and track changes in infestation size over time. In this way, it can also be used to track the effectiveness of chemical and other treatments.

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Specific monitoring objectives and protocols will vary depending upon the methods, target species, and site conditions. The US Department of Interior Integrated Pest Management Policy (517 DM 1) requires monitoring before, during, and after any herbicide application.

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Invasive Plant Management Options

Herbicides can be used to implement a number of invasive plant management options.


Herbicides are used on existing infestations and should not be used as a preventative measure against establishment of invasive plants.


Herbicides can suppress invasive plant populations, allowing desirable vegetation to become established.


Herbicides may contain infestations that are too large for total eradication. Herbicides can be applied around the borders of infested areas, protecting neighboring uninfested communities. Containment programs require long term herbicide application because they do not eradicate or reduce the invasive plant population but simply limit its spread. Herbicides can also be used as a rapid control measure to treat satellite populations located within uninfested plant communities.


Newly invading plant species detected early are excellent candidates for eradication. Herbicides provide an effective eradication option because they are fast acting and can be applied directly to the target plants. Spot treating small plant populations can be very effective.

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Integrated Methods

When used alone, herbicides can provide rapid and effective control of target plant populations. Integrating herbicides with other management methods can enhance the effectiveness of each method and is more likely to achieve sustainable, long term solutions.

Chemical Methods and Physical Methods iconChemical Methods and Physical Methods

Physical methods such as mowing can be used either before or after herbicide application to enhance the effectiveness of either treatment. At Bosque del Apache NWR in New Mexico, saltcedar (Tamarix ramosissima) was considerably reduced after treatment with herbicide followed by mechanical removal (Taylor and McDaniel 1998). At Willapa Bay NWR in Washington, smooth cordgrass (Spartina alterniflora) was effectively controlled using mowing followed by herbicide application (Major et al. 2003). Removal of reed canarygrass (Phalaris arundinacea) was successful using a combination of disking and herbicide application at Ridgefield NWR in Washington (Paveglio and Kilbride 2000). Although spraying successfully killed reed canarygrass, removal of dead vegetation by disking was necessary to expose mudflats for germination of desirable wetland species.

Chemical Methods and Biological Control iconChemical Methods and Biological Control

Herbicides should be combined with biological control methods only when the use of herbicides does not interfere with the biological control agent. Herbicides can be toxic to insect biological control agents, particularly during certain insect life cycle stages, thus proper application timing is essential (Messersmith and Adkins 1995). By controlling target plant populations during critical feeding periods, herbicides essentially remove an exclusive source of food and habitat for biological control agents. Herbicides have been successfully combined with biological control agents in North Dakota to decrease leafy spurge (Euphorbia esula) densities. Both methods were more effective when used in combination than alone (Lym 1998).

Chemical Methods and Prescribed Grazing iconChemical Methods and Prescribed Grazing

Prescribed grazing can be used either before or after herbicide application to improve the effectiveness of each method. Prescribed grazing can be used to further suppress cheatgrass (Bromus tectorum) on sites that have been treated with herbicides (Mosley et al. 1999). In Montana, herbicide applied in the spring to rangelands infested with spotted knapweed followed by repeated sheep grazing controlled target species and allowed competitive desirable species to increase (Sheley et al. 2004). Grazing may be restricted during and following application of some herbicides.

Chemical Methods and Prescribed Burning iconChemical Methods and Prescribed Burning

Prescribed burning can improve the effectiveness of herbicides in several different ways, such as removing litter to expose new growth and increasing detection of the target species for follow-up treatments. In California, summer prescribed burning followed by herbicide application the following winter or spring provided the best control of yellow starthistle (Centaurea solstitialis) populations compared to the other methods studied (DiTomaso et al. 2006). The burn stimulated germination of yellow starthistle plants that were then controlled by herbicide treatment the following year. At Columbia NWR in Washington, a fall prescribed fire followed by herbicide application significantly reduced cover of cheatgrass (Link and Hill 2005). In Utah, rangelands infested with medusahead (Taeniatherum caput-medusae) were burned in the fall and immediately treated with herbicides (Monaco et al. 2005). This treatment successfully controlled medusahead, allowing a window of opportunity for revegetation with native perennial species.

Chemical Methods and Restoration/Revegetation iconChemical Methods and Restoration/Revegetation

The ability of desirable plants to compete with target species and reestablish on a site after their removal is critical to the success of any control program. Sometimes desirable species can reestablish from existing propagules, but on severely degraded sites revegetation is often needed. The selectivity, persistence, and timing of herbicide application are important considerations before any revegetation project can begin (Jacobs et al. 1999).

If there is no competition from other desirable plant species, target species can rapidly reinvade even after herbicide application due to seedbank recruitment. To prevent reinvasion, seeding with desirable species is recommended. In Nebraska, treating leafy spurge with fall-applied herbicide and seeding in the spring with native tallgrasses successfully reduced leafy spurge and increased productivity of native grasses (Masters and Nissen 1998). Similarly in Nebraska, fall-applied herbicide followed by spring seeding with native grass species suppressed leafy spurge and allowed for successful establishment of desirable species (Masters et al. 2001).

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Chemical Methods in Action

Chemical methods are the most commonly used invasive plant management techniques because they can provide rapid and effective control. Many refuges use herbicides within an integrated plant management framework to control undesirable plant species. Chemical herbicides combined with other methods can help achieve long term results while reducing the risk of non target impacts. The following slide show and case study illustrate applications of chemical methods on refuges.

Eradicating Orange Hawkweed
Screenshot of first page of “Eradicating Orange Hawkweed” case study.