Integrated Management of Invasive Plants in Natural Areas of Florida
Florida's natural areas encompass an incredible diversity of native plants and animals and provide a wide array of ecosystem services that benefit Florida greatly. Within the state, there are almost ten million acres of local, state, and federal public lands currently managed as natural areas for conservation. While natural areas are conservation lands that have been set aside for the purpose of preserving (or restoring) native plant and animal communities, they do require active management. One of the greatest management issues in natural areas is invasive plants. Invasive plants are species that are not native to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health. Invasive plants represent a subset of non-native plants that have been introduced into Florida. Of the approximately 4,875 plant species growing without cultivation in Florida, almost 32% are non-native (Wunderlin et al. 2022). Many of these non-native plants were originally introduced as garden ornamentals, agricultural crops, forages, or soil stabilizers. Others were accidentally introduced as contaminants of seed or as hitchhikers on animals or materials. Regardless of how they arrived, these 1,500+ non-native plants grew so well in Florida that they established and reproduced on their own and spread into natural areas. Out of these 1,500+ species, approximately 168 (11%) are considered invasive and may disrupt ecosystem services vital to the integrity of Florida natural areas (FLEPPC 2017). Among these, approximately 100 species require aggressive management.
Management of invasive vegetation in natural areas requires control methods that will minimize damage to non-target vegetation and other biotic and abiotic components of ecosystems. This need for caution often necessitates more time and effort than weed management in agricultural, industrial, or right-of-way settings does. Certain types of vegetation, such as woody or sprawling vegetation, may require removal of standing plant material even after they have been killed if the presence of standing material increases fire hazard, reduces aesthetic appeal, or has the potential to cause harm as it decays and falls. Control methods include cultural, preventative, manual, and mechanical removal, biological control, physical controls, and herbicides. Generally, these tools are utilized as part of a comprehensive management plan that considers a wide range of issues to control invasive plants and protect the native plants of a given natural area. These tools are not silver bullets and cannot be expected to solve invasive plant problems with a one-time shot. Eradication, which is the complete elimination of all living propagules of a species, is extremely difficult and often infeasible for well-established invasive plants. Without a clear management plan and long-term commitment, most single invasive plant control efforts result in short-lived success.
This publication provides land managers in Florida with current methods used to manage non-native plants. It does not address identification of plant species. For information on identification and recognition of many invasive plant species, consult Identification and Biology of Nonnative Plants in Florida's Natural Areas, 2nd Ed. by K. A. Langeland, H. M. Cherry, C. M. McCormick, and K. A. Craddock Burks (2008), which is available from the UF/IFAS Extension Bookstore (https://ifasbooks.ifas.ufl.edu).
Regulatory Agencies, Permits, Plant Lists, and the UF/IFAS Assessment
There has been much confusion over the legal ramifications of listing certain plants as invasive. The best example of this is the Florida Invasive Species Council (FISC) invasive plant list. FISC was formerly known as the Florida Exotic Pest Plant Council (FLEPPC) and maintains an Invasive Plant List. FISC is a nonprofit professional organization founded in 1984 to increase public awareness of the significant threat that non-native invasive plant species pose to native species, communities, and ecosystems, and to develop integrated management and control strategies to halt the spread of exotic species in natural areas. FISC maintains a list of plant species considered by a committee of botanists, ecologists, and land managers to be invasive in Florida. This list is available on the FISC website (https://floridainvasivespecies.org/plantlist.cfm). The purpose of the FISC list is to alert land managers to plant species that have demonstrated invasiveness in Florida. However, the list does not have statutory authority.
Plants that are legally regulated by statute are listed on the Florida Noxious Weed List, which can be found on the Florida Department of Agriculture and Consumer Services website (https://www.fdacs.gov/Agriculture-Industry/Pests-and-Diseases/Plant-Pests-and-Diseases/Noxious-Weeds). There may also be local, county, or city ordinances regarding certain plants, but they are beyond the scope of this publication.
Plant species included in this publication are not limited to either of these lists. They are included because they have warranted control measures in at least one natural area in the state and should be viewed as potentially invasive in other natural areas. Some of the plants are still used in landscaping and sold in the nursery industry.
Mention of species in this publication does not necessarily mean UF/IFAS recommends limitation of their use. The UF/IFAS Assessment of Non-Native Plants in Florida's Natural Areas is used by UF/IFAS to evaluate the invasiveness of non-native plants in Florida relative to UF/IFAS recommendations. The results of this assessment can be viewed on the UF/IFAS Assessment website: https://assessment.ifas.ufl.edu/. The UF/IFAS assessment is not regulatory. However, all UF/IFAS employees are required to follow the results of the assessment when making plant species recommendations.
Finally, the removal of vegetation in certain areas such as public waters and wetlands is regulated by state and local agencies. A permit may be required. For questions regarding permits to control vegetation in public waters, please visit the Florida Fish and Wildlife Conservation Commission Invasive Plant Management Section website: https://myfwc.com/wildlifehabitats/habitat/invasive-plants/. The FWC Invasive Plant Management Section is the lead agency in Florida responsible for coordinating and funding two statewide programs that control invasive upland and aquatic plants on public conservation lands and waterways throughout the state. The agency also ensures that beneficial native aquatic plants in Florida's ponds, lakes, and rivers are protected through permitting programs and funding of research to find more cost-effective management techniques.
For regulatory questions regarding vegetation control in wetlands, contact the Water Management District (WMD) in which you are located. These can be found at the Florida Department of Environmental Protection website: https://floridadep.gov/water-policy/water-policy/content/water-management-districts.
The following individuals have provided information for this or previous editions of Integrated Management of Nonnative Plants in Natural Areas of Florida: Mike Bodle, Jim Burney, Mathew Cole, Rodell Collins, Jim Cuda, Scott Ditmarsen, Jim Duquesnel, Roger Hammer, Dallas Hazelton, Jeff Hutchinson, Greg Jubinsky, Chris Key, Bill Kline, Mike Link, Mark Ludlow, Joe Maguire, Chris Marble, Michael Meisenburg, Vince Miller, Patrick Minogue, Shawn Moore, Romeo Morua, Brian Nelson, Jose Prieto, Jerry Renny, Adolfo Santiago, and Elroy Timmer.
Education and Prevention
Public education can significantly reduce the introduction and spread of invasive plants. Many in the general public are still unaware of problems caused by invasive plants and unable to recognize invasive species. There are numerous educational resources available online. A great starting point is the UF/IFAS Center for Aquatic and Invasive Plants (https://plants.ifas.ufl.edu). Its webpage provides information on the identification, history, biology, ecology, and control of hundreds of species. Other educational sites tailored specifically to the general public include PlayCleanGo.org and invasive.org.
Classical weed biological control is the introduction of reproducing populations of foreign insects or diseases that are host-specific and provide suppression of the target weed. Biological control efforts for weeds have a long history in Florida, but they have primarily focused on aquatic weeds, including alligatorweed, waterhyacinth, water lettuce, salvinia, and hydrilla. These efforts have yielded mixed outcomes, with the most effective results on alligatorweed and the least effective on hydrilla and water lettuce. In more recent years, efforts to develop biological controls for natural area weeds have focused on air potato (Dioscorea bulbifera), melaleuca (Melaleuca quinquenervia), tropical soda apple (Solanum viarum), Brazilian pepper (Schinus terebinthifolia), and Old World climbing fern (Lygodium microphyllum) (Table 1). Air potato biocontrol is quickly becoming a success story in many areas of Florida. The air potato leaf beetle has provided a high level of suppression, greatly reducing the number of aerial bulbils produced in many natural areas. Additionally, the suite of biocontrols released for melaleuca has been very effective in suppressing melaleuca growth and reproduction, especially after initial control efforts with herbicides and prescribed fire have been implemented. Tropical soda apple has declined in many areas following the release of Gratiana boliviana in 2003. While these are promising, the majority of invasive plants in natural areas in Florida still do not have biocontrol options. Classical biological control is a slow process requiring intensive vetting of prospective candidates to ensure both effectiveness and host specificity. In many cases, biological control may not be possible due to a lack of host-specific agents. However, research on Brazilian peppertree and Old World climbing fern is ongoing, and new efforts are underway on cogongrass, earleaf acacia, and downy rose myrtle. Biological control programs are typically implemented by state and federal agencies, and the potential role of individual resource managers and the public will depend on the particular action being implemented. Current information on the status of these and other biological control programs can be found at https://edis.ifas.ufl.edu/entity/topic/biological_control_of_weeds.
In addition to insects and pathogens, the use of herbivores such as cattle, sheep, goats, and sterile grass carp is possible for suppression or control of certain invasive plants. For example, kudzu has been reported to be effectively controlled with all classes of livestock (Loewenstein and Enloe 2014). Goats are very effective for controlling Chinese privet and many other invasive shrubs. However, use of herbivores for weed control on public lands in Florida is limited primarily due to higher costs of intensive grazing management. Grazing animals are often used on private lands across Florida to manage invasive plants.
Manual removal is a labor-intensive but often major component of effective invasive plant control. Seedlings and small saplings can sometimes be pulled from the ground, but even small seedlings of some plants have tenacious roots that will prevent extraction or cause them to break at the root collar. Plants that break off at the ground will often resprout, and small root fragments left in the ground may produce new plants. Therefore, repeated hand pulling or follow-up with herbicide applications is often necessary. Hand pulling is easiest in sandy soils and for immature shallow-rooted plants and most difficult in pine rocklandsand for well-established plants with extensive root systems.
Manual removal may require a wide variety of implements, including shovels, saws, pick axes, Pulaski tools, and woody plant extraction tools such as the Uprooter® or the Extractigator®.
Regardless of the method, one major consideration in manual removal is disposal of the biomass after removal. Leaving it on the ground is often risky because stems of many species left on moist soils will root and establish new plants. Melaleuca, Tradescantia, Ligustrum, Phyllostachys, Selenicereus (Hylocereus), Kalanchoe, and Epipremnum are species that have spread in this manner. If removed biomass cannot be destroyed by methods such as burning, it should be piled in a delineated area that can be monitored where new plants can be controlled as they appear.
A second major consideration in manual removal is the extensive soil disturbance that often occurs in the process. Plant roots are often well-anchored in the soil; pulling plants may disrupt soil structure. This type of soil disturbance may result in further invasion by other non-native plant species and require additional follow-up control measures.
Mechanical removal involves the use of bulldozers or specialized logging equipment (skid steer mulchers) to remove woody plants. Intense follow-up with other control methods is essential after the use of heavy equipment because disturbance of the soil creates favorable conditions for regrowth from seeds and root fragments as well as recolonization by invasive non-native plants. Plans for management and replanting of sites with native vegetation following mechanical removal should be carefully developed prior to implementation. Mechanical removal may not be appropriate in many natural areas because of the disturbance to soils and nontarget vegetation.
Prescribed burning and water level manipulation are cultural practices that are used in management of pastures, rangeland, and commercial forests. In certain situations, they may be appropriate for vegetation management in natural areas. One important consideration is the degree of degradation of the area in question. Cultural practices may affect all parts of the habitat, including native species. If the habitat is so badly degraded that the need to reduce invasive plants strongly outweighs consideration of remaining native species, more aggressive control strategies can be considered. In less degraded areas, more careful use of integrated methods may be more appropriate.
The land use history of an area is critical in understanding the effects of fire and flooding on the resulting plant species composition. Past practices affect soil structure, organic matter content, species seed bank (both native and invasive non-native species), and species composition. While past farming and timber management practices may influence the outcome of cultural management, very little is known about the effects of specific historical practices. Similar management practices conducted in areas with different histories may achieve very different results. Even less is known about the effects of invasive plants in these communities, and the subsequent management effects of fire on the altered communities.
Understanding the reproductive biology of the target and nontarget plant species is critical to effective use of any control methods, especially fire management, that require significant preparation time. Important opportunities exist if management tools can be applied to habitats where non-native invasive species flower or set seed at different times than the native species.
Fire is a normal part of most of Florida's ecosystems. Native species have developed varying degrees of fire tolerance. Throughout much of Florida, suppression of fire during this century has altered historical plant communities, such as flatwood and oak scrub communities, enhancing fire-intolerant species and reducing the coverage of species that possess fire adaptations. Within these communities, the fire-tolerant woody species have lingered in smaller numbers, and less fire-tolerant species have replaced ephemeral forbs. Fire ecologists are developing an understanding of the amount, frequency, timing, and intensity of fire that would best enhance the historically fire-tolerant plant species, but little is known about how such a fire management regime could best be used to suppress invasive species.
In general, fire can be used to suppress plant growth and kill certain plants that are not fire-tolerant. Woody species are often reduced, but effects are less noticeable on herbaceous species. Some information has been published on responses of individual Florida plant species, but very little is known about the vast majority of native plant species. Even less is known about invasive exotic species. Tolerance to fire can sometimes be predicted in species that have thick bark or seeds in the soil or in the canopy; are adapted to fire (either tolerant of high temperature or requiring fire for seed release or germination); and have seeds that are disbursed over a wide area.
Added biomass by invasive plants can result in hotter fires and greatly increase the risk of fires spreading to inhabited areas. In these situations, use of fire to reduce standing biomass of invasive species may protect the remaining plant populations more effectively than inaction, but impacts to nontarget native species will occur. Under these conditions, the expense of reducing standing biomass of invasive plant species might be justified by the savings on subsequent fire suppression. Additionally, in certain cases, invasion of tree stands by exotic vines and other climbing plants has greatly increased the risk of canopy (crown) fires and the resulting death of mature trees.
Effects of a single burn are hard to predict. Under certain conditions, a single fire effect can persist for several growing seasons. The intensity of the fire, the timing (fire during the growing season can be more destructive than during dormant seasons), and the plant species involved determine the duration of the effect. Smoke is now recognized as a triggering mechanism for germination in fire-dependent and some non-fire-dependent species, so plant species composition following a burn is due in part to the type of fire and the distribution of its smoke. A single burn may or may not start a replacement sequence (succession) with its own effects on species composition.
Fire's potential to play a logical role in suppression or elimination of invasive exotic plant species depends on many factors. In addition to the aforementioned factors, the resource manager must consider potential fire effects on soil loss and water quality, historical and economic impacts to buildings, possible harm to human life, and the potential for a fire's escape to nontarget areas.
Fire has been successfully used to manage plant species in grasslands, to maintain open savannahs (scattered trees in habitats dominated by herbaceous species), and to promote seral (fire-induced or fire-tolerant) stages of forest succession. However, very little is known about the use of fire to enhance native species while reducing invasive exotic plant species. As a final word of caution regarding the use of fire, overly frequent burning has been shown to reduce plant diversity under many conditions. It is also possible that increased fire frequency could provide opportunities for invasive plants to enter new areas.
Water Level Manipulation and Hydrologic Restoration
Some success has been achieved by regulating water levels to reduce invasive plant species in aquatic and wetland habitats. Dewatering aquatic sites reduces standing biomass. However, little else is usually achieved unless the site is rendered less susceptible to repeated invasion when rewatered. Planting native species may reduce the susceptibility of aquatic and wetland sites in some cases.
In most situations, water level manipulation in reservoirs and hydrologic restoration (recreation of natural hydrologic conditions) have not provided the level of invasive plant control that was once thought achievable. Ponds and reservoirs can be constructed with steep sides to reduce a habitat's susceptibility to invasion, and levels that promote invasive species can be avoided, but rarely are these management options adaptable to natural areas.
Carefully timed water level increases after mechanical removal, prescribed fire, or herbicide treatment of invasive species has been one of the most effective methods of water level manipulation. This integrated approach can provide effective control of subsequent germination and, with some species, resprouting. This is often most easily accomplished during the transition from the dry to the wet season in early summer when water levels typically begin to increase. Specific methods applicable to natural areas have not been clearly described.
Reestablishment of Native Plant Species
Planting native species can be an effective but expensive way to reduce the likelihood of exotic species reinvasion following removal of non-native species. Commercial plant nurseries currently offer seeds and plants of several wetland and upland species. Because some species cover a wide range of habitats and latitudes, care should be taken to obtain plant material suitable to the habitat under consideration. Seed collected from plants growing in northern latitudes may do poorly in Florida. Introduction of seeds, plant parts, or whole plants should include thorough screening for any unwanted plant or animal pests.
It often takes several years for plantings to become thoroughly established. Extra water, nutrients, and protection from fire and pests may be necessary for a while. During this establishment phase, past management practices may also have to be altered to avoid injury to the plantings. For example, if periodic burning or flooding is part of the current management practice, it may be necessary to reduce the intensity or duration until the plantings are able to exhibit their typical resistance to injury. Unfortunately, little is known about requirements for successful establishment of many native species, and less is known about their tolerances to cultural invasive plant management techniques. Even when tolerances are better understood, responses may be affected by historical site effects, traits of particular genetic strains, site-specific nutrition, soil mycorrhizae, light conditions, and interactions of soil type, hydroperiod, and microclimate.
Chemical Control (Herbicides)
Training and Certification
Anyone who applies herbicides in natural areas should have basic training in herbicide application. This publication only addresses topics pertinent to herbicide use in natural areas. Therefore, the reader is expected to have prior knowledge of basic herbicide application techniques.
A pesticide or its use is classified as restricted if it could cause harm to humans or to the environment unless applied by certified applicators who have the knowledge to use pesticides safely and effectively. The basic knowledge of herbicide technology and application techniques needed for safe handling and effective use of any herbicides can be obtained through restricted use pesticide certification training. This training is available through UF/IFAS. Certified applicators can be licensed as either public applicators or commercial applicators. Persons must successfully complete a general standards core examination and a category examination before they can apply to the Florida Department of Agriculture and Consumer Services (FDACS) for a license. Categories applicable to target species in the publication include the Natural Areas category and/or Aquatics category. Additional information about pesticide applicator licensing can be found on https://edis.ifas.ufl.edu/entity/topic/pesticide_applicator_certification.
Herbicide Active Ingredients and Formulations
An herbicide formulation, or product, consists of the herbicide active ingredient dissolved in a solvent (e.g., oil, water, or alcohol) or adsorbed to a solid such as clay. Formulations often include an adjuvant that facilitates spreading, sticking, and wetting of the spray solution. Special ingredients may also improve the safety, handling, measuring, and application of the herbicide. Products mentioned in this publication contain the active ingredients 2,4-D, aminocyclopyrachlor, aminopyralid, fluazifop, glyphosate, hexazinone, imazamox, imazapic, imazapyr, metsulfuron, sethoxydim, and triclopyr (Table 2).
The active ingredients 2,4-D amine, aminopyralid, aminocyclopyrachlor, triclopyr amine, triclopyr acid, triclopyr choline, imazamox, imazapyr, and hexazinone are formulated as water-soluble liquids (L). They are generally not compatible with oil-based diluents and are instead diluted in water for foliar applications. For cut-stump applications, they are diluted in water or applied in their concentrated form. These ingredients are not normally used for basal bark applications. Triclopyr acid is the exception, as it can also be mixed with oil and applied as a basal bark treatment.
Triclopyr ester, imazapyr, fluazifop, and sethoxydim are formulated as emulsifiable concentrates (EC). Emulsifiable concentrates are compatible with oil-based diluents and also contain emulsifiers that allow the formulation to mix with water. Agitation is used to mix the EC formulated herbicides in water. They may be diluted in water for foliar applications or certain ones may also be mixed with oil-based diluents for basal bark applications.
Where Herbicides Can Be Used
No pesticide may be sold in the United States until the United States Environmental Protection Agency (EPA) has reviewed the manufacturer's application for registration and determined that use of the product will not present unreasonable risk to humans or the environment. Pesticide users are required by law to comply with all instructions and directions for use on pesticide labels.
The EPA approves use of pesticides on specific sites, i.e., for use on individual crops, terrestrial non-crop sites, or aquatic areas. Only those herbicides registered by the EPA specifically for use in aquatic sites can be applied to plants growing in lakes, rivers, canals, etc. For terrestrial uses, the EPA requires herbicide labels to have the following statement: "Do not apply directly to water, to areas where surface water is present, or to intertidal areas below the mean high mark." (Dow AgroSciences 2018). Several active ingredients in this publication have separate products that are registered for direct application to water for control of aquatic weeds (Table 2). Other products mentioned can be used in non-cropland areas and certain low-lying areas, including wetlands, but cannot be applied directly to water (Table 2).
Herbicides recommended in this publication for invasive plant control are systemic. They move within the plant to the site where they are active after absorption by foliage, roots, or bark. The following herbicides can be absorbed by plant leaves and are effective for foliar applications: 2,4-D, aminocyclopyrachlor, aminopyralid, glyphosate, imazamox, imazapyr, metsulfuron, and triclopyr. Addition of an appropriate surfactant, as recommended on the herbicide label, is essential. Glyphosate and 2,4-D are adsorbed by soils or broken down quickly in soil and are not absorbed effectively by plant roots. However, aminopyralid, aminocyclopyrachlor, imazamox, imazapyr, hexazinone, and metsulfuron are readily absorbed by plant roots (Table 3). Triclopyr generally exhibits a short half-life in soils, which limits root uptake and subsequent control of target species. However, high concentrations applied directly around the base of trees has led to non-target injury of some species growing in close proximity. Only oil-soluble herbicide formulations (i.e., emulsifiable concentrates) are absorbed readily through tree bark at a level that provides effective control. However, young saplings of many woody species possess very thin green bark. Injury can occur if foliar sprays directly contact the thin green bark and this has occurred with glyphosate and triclopyr.
Behavior in Soils
Herbicides used for invasive plant control vary in their persistence and sorption to soils (Table 3). The most important factor is the ability of various soil types to chemically bind herbicides. Soil-applied herbicides, such as hexazinone, have label recommendations that vary the application rate for different types of soils. In general, soils with more organic matter and/or clay have greater capacities for binding herbicides than coarse, sandy soils and require higher application rates. Because woody plants are a problem on a range of Florida soils, including highly organic muck, sand, and very thin soil layers over limestone, a broad range of herbicide behavior in different soils can be expected. Generally, soil activity can be a double-edged sword: it can improve herbicide activity (i.e., imazapyr for cogongrass control) but potentially result in nontarget damage (i.e., imazapyr damage on oaks). Applicators should use herbicides with soil residual activity carefully to prevent nontarget issues.
Herbicide selectivity is highly desirable in natural areas as protection of native plant species is critical. The ability to selectively control target vegetation with herbicides without harming nontarget plants is related to the sensitivities of target and nontarget plants, absorption and chemical characteristics of the herbicides, and placement.
Herbicides vary in their potential to damage nontarget vegetation. Unwanted results can be prevented or minimized by making the best choice of herbicides in conjunction with careful application. Fluazifop and sethoxydim, which kill many grasses, can be used to selectively manage invasive grass species among nontarget broadleaf plant species. Formulations that contain the active ingredients 2,4-D, metsulfuron, and triclopyr can often be used selectively because many broadleaf species are more sensitive to them than perennial grasses are. Because 2,4-D, triclopyr, and glyphosate have little root activity and break down quickly (Table 2), they have little potential for causing nontarget damage due to root absorption when carefully applied to target vegetation. However, caution must be used with root-active herbicides (i.e., aminocyclopyrachlor, aminopyralid, hexazinone, and imazapyr) to minimize damage to nontarget vegetation by root absorption. In shallow, porous soils, extra care should be taken to avoid root absorption of all herbicides by nontarget plants.
Care must also be taken to avoid unwanted drift of herbicide spray to nontarget plants when applications are made. Particulate drift can be minimized by avoiding windy conditions while spraying and by using low pressures and large nozzle orifices. Volatile compounds such as ester formulations may cause nontarget damage due to vapor drift when applied on hot days. This damage, which may be observed as wilting or curling leaves, has been minimal and has not caused permanent harm to most woody nontarget plants.
Invasive plant management is often conducted in natural areas to maintain or restore wildlife habitat. Therefore, it is essential that the herbicides themselves are not toxic to wildlife. Assessment of risk to wildlife is conducted as part of the registration procedure for herbicides and is determined as the product of hazard and exposure. Hazard is measured as the toxicity of the herbicide to test animals, and exposure depends on the use and persistence of the compound. Herbicides recommended in this publication have shown very low toxicity to wildlife with the exception of the relatively low LC50 of triclopyr ester (0.87 ppm) and fluazifop (0.53 ppm) for fish. Neither of these can be applied directly to water (Table 3). Ester formulations are toxic to fish because they irritate gill surfaces. However, because triclopyr ester and fluazifop are not applied directly to water, are absorbed by soil particles, and have low persistence, exposure is low, which results in low risk when properly used.
Herbicide Application Methods
Herbicide in foliar applications is diluted in water and applied to the leaves with aerial or ground equipment. Dilution is usually about 20 parts water to 1 part herbicide concentrate (5%) for aerial applications, and 30 to 400 parts water to 1 part herbicide concentrate (3.0–0.25%) when making ground applications for woody plant control. Adjuvants, such as surfactants, drift control agents, or other spray modifiers, are often added to the spray mix as specified on the herbicide label. Ground equipment ranges from handheld spray bottles for applications to small individual plants to large high-pressure vehicle- or boat-mounted sprayers for larger areas. Foliar applications can either be directed, to minimize damage to nontarget vegetation, or broadcast. Broadcast applications are used where damage to nontarget vegetation is not a concern or where a selective herbicide is involved.
For directed spray or selective applications, backpack sprayers such as the Solo Model 475 with diaphragm pump or Hudson SP1 are commonly used. A spray tip such as a TP 2503 or TP 2504 produces large spray droplets to reduce spray drift. The 2503 spray tips may be installed in the spray wand that comes with the backpack sprayer, or a Model 30 GunJet with the 2503 or 2504 spray tip may be attached to any backpack spray unit. If an adjustable tip is used, a TeeJet 5500 or equivalent is recommended. All backpack sprayers and spray guns should have chemical-resistant seals for the herbicides being used.
Power-driven ground equipment is commonly used to spray large and/or tall plants or large areas. Properly adjusted equipment should deliver a uniform spray with nozzle pressures of about 30 to 80 psi and should generate large spray droplets to reduce potential for spray drift. Higher spray pressures produce many small spray particles that may drift onto sensitive desirable plants adjacent to the treated area. Application is performed by directing the spray onto the target foliage. Be sure to spray the growing tips and terminal leader. Techniques must be employed to prevent the spray from coming into contact with foliage of desirable plants.
Commonly used power equipment consists of portable, power-driven spray units mounted on a truck or all-terrain vehicle. A wide variety of pumps, tanks, and accessories is used. The most common and maintenance-free pump is a diaphragm pump driven by a gasoline engine, or a self-contained, 12-volt pump unit. Routinely used spray guns are Spraying Systems Model 2 and 2A GunJets. These are adjustable spray guns that produce patterns ranging from a solid stream to a wide cone spray. These spray guns may produce small spray particles at the cone spray setting, resulting in spray drift. A Model 30 GunJet with a TeeJet 5500-X10 adjustable tip is very effective for power sprayers. Dual spray GunJets that accommodate two flat spray tips with different volumes and patterns are available. The spray gun can immediately be switched from one spray tip to the other by rotating the spray head. The most commonly used spray tips for the spray gun are TP 0512, TP 4010, or TP 4020. These tips produce few fine-spray particles, so spray drift potential is reduced.
Basal Bark Applications
In basal bark applications, herbicide is applied, commonly with a backpack sprayer, directly to the bark around the circumference of each stem or tree up to 12–18 inches above the ground. The herbicide must be in an oil-soluble formulation (EC). If it is not in a ready-to-use form, it may be mixed with a specially formulated penetrating oil. There are also two modified methods using the basal bark approach. The first is a basal stem treatment. This is similar to basal bark, except it is used for non-woody to semi-woody species such as bowstring hemp, or large vines such as golden pothos. The lower section of stems is carefully treated in this method. The second is an apical bud treatment. This is often done for invasive palm trees and certain herbaceous plants such as oyster plant. In this treatment, the terminal growing point of each stem is sprayed until wet with the typical triclopyr ester herbicide at 10–20% in an oil carrier. For palms, this is useful for solitary trunked palms but much more difficult for multi-stemmed clusters. Care should be taken to avoid the sharp, spiny growth of certain palms, most notably the Senegal date palm.
For all basal bark or modified basal bark approaches, the spray tip should be a narrow angle (15–25°), flat fan-tip nozzle such as a TP 1502, TP 1503, or TP 2502/TP 2503, a solid cone nozzle, or an adjustable cone jet such as a TeeJet 5500-X4 or 5500-X5 or equivalent. Any of these tips can be installed in the spray wand that comes with the spray unit. A good alternative is a brass tip shutoff wand, such as a Spraying Systems Model 31 with brass extension and tip shutoff or a Spraying Systems Model 30 GunJet. A TP-0001/TP-0002 tip or DE-1/DE-2 disc should be used with the Model 30 GunJet. The GunJet can be attached to most backpack spray units that produce pressures between 20 and 50 psi. All backpack sprayers and spray guns should have chemical-resistant seals for the herbicides and carriers in use.
Hack-and-Squirt, Frill, and Girdle Applications
These techniques involve making angled downward cuts through the bark and cambium into the shallow sapwood with a machete or hatchet around the circumference of the tree. A concentrated herbicide solution is then applied into the cuts, often with a squirt bottle. Cuts should be made in a continuous or overlapping pattern or in an evenly spaced pattern as directed by the herbicide label. Frill cuts are used for species with pliable bark that can almost be peeled down to make a larger cup for the herbicide application. Girdling is an extreme hack approach where a band of bark, often 4 to 6 inches wide, is removed. Girdling is used for control of melaleuca and some other large, difficult tree species. Chainsaw girdling is a variant that makes a single continuous cut around the entire circumference of the tree. It has been effective for large Casuarina trees. In general, girdling is very labor-intensive and greatly reduces crew productivity. When using the hack-and-squirt technique, do not make multiple cuts directly above or below each other. This will inhibit movement of the herbicide. Although water- or oil-soluble formulations can be used for frill or girdle applications, water-soluble formulations are often preferred. Backpack sprayers, 1- to 2-gallon pump-up sprayers, or smaller 32-ounce spray bottles are suitable for these applications. Spray bottles should contain chemically resistant seals such as Viton.
Incision Point Application
Incision point application (IPA) is a modified version of hack-and-squirt that is new for invasive plant control. IPA utilizes a reduced number of hacks per stem and concentrated herbicide (100%, no diluent) in very small amounts (0.5 mL/hack) compared to traditional hack-and-squirt, frill, or girdle treatments. Herbicides recently tested for IPA in Florida include Method (aminocyclopyrachlor) and Milestone (aminopyralid). For single-stemmed trees, Method has been shown to be effective on Chinese tallow (trees up to 6 inches DBH), tung (trees up to 8 inches DBH), and bishopwood (trees up to 8 inches DBH) with a single hack and 0.5 mL of 100% herbicide. Milestone has also been found to be effective on those species at similar sizes. Method has also given approximately 79% kill of melaleuca and Australian pine for trees ranging from 2–10 inches in diameter. In melaleuca, however, it is very difficult to get the small quantity of herbicide through the thick bark to the cambium without a needle injection system. More research is needed on both of these species. Limited observations suggest mimosa is also susceptible to IPA. Leadtree, a similar woody legume, was not consistently killed with this approach.
For the multi-stemmed Brazilian peppertree, both Method and Milestone are extremely effective when one hack is made on each main stem and treated with 0.5 mL of either herbicide. However, this can result in high application rates for individual trees with several stems, greatly limiting the number of individual multi-stemmed trees that can be treated on a per acre basis. The approach is already allowable for Method but is not yet labeled for Milestone. Research is ongoing to reduce this to fewer hacks per multi-stemmed shrub.
Cut Stump Treatments
Stump treatments are applied after cutting and removing large trees or brush. The concentrated or diluted herbicide is sprayed or painted onto the cut surface of the stump immediately after cutting. The cut surface should be as level as possible so that the herbicide solution does not run off. Sweep off dirt and sawdust that may prevent the stump from taking up the herbicide solution. The herbicide is usually applied in a band around the outer 2 inches of the entire circumference. This ensures good coverage of the phloem and cambium layer. Depending on label instructions, the entire stump is sometimes covered with dilute herbicide solution. Water- or oil-soluble formulations can be used. Spray equipment can be used as long as it contains chemical-resistant seals. Best results are obtained if the herbicide is applied no more than one hour after cutting, especially when using a water-soluble formulation. With less susceptible species, seconds can count. Oil-soluble formulations can be effective when applied after a few days; they should be applied to the sides of the stump as well. This is a hybrid cut stump plus basal bark application method.
A soil application of granular herbicide formulations can be applied by handheld spreaders, specially designed blowers, or air. Soil-applied water-soluble or water-dispersible formulations can be used with the same type of equipment described for foliar applications or spotguns that can accurately deliver a measured amount of herbicide. This is generally limited to hexazinone and imazapyr for certain invasive plants. Soil applications are often reserved for very specific situations. For example, hexazinone has been used for melaleuca control in south Florida in seasonally dry areas, and imazapyr has been used for bamboo control. Both treatments will injure or kill oaks and other nontarget vegetation and should only be used in very specific conditions.
Colorants: Marker Dyes and Spray Indicators
Marker dyes and spray indicators are useful for keeping track of vegetation that has been treated when making applications to large numbers of trees or stumps. They come in a few colors including blue, red, and purple. Color contrast is most important when selecting a product. Red spray indicators may not be visible for applicators with red/green color blindness. Cleanup is generally easier for spray indicators than for true dyes. These colorants are a useful indicator of the applicator's efficiency and precision. They are also useful for training new crew members who have not had previous experience using a backpack sprayer.
Herbicide Control Methods for Invasive Non-Native Plants
Herbicide control methods used by land managers in Florida for invasive non-native plants are listed in Table 5. All herbicides listed have been found to be effective under certain circumstances. However, many factors can affect the performance of an herbicide application; therefore, results may vary. Choice of application method, herbicide, and herbicide rate or concentration for individual species will depend on environmental conditions and personal experience. Treatment success may vary from site to site and sometimes even within the same site. These herbicide recommendations are not exhaustive. Imazapyr, which is highly effective for many species, is often not recommended due to significant concerns of nontarget damage.
Pesticide product labeling is the primary method of communication between an herbicide manufacturer and the herbicide users. It provides instructions on how to use the product safely and correctly. Changes in herbicide label directions may occur that are not concurrently updated in this publication. Because pesticide users are required by law to comply with all the instructions and directions for use on the pesticide label, no herbicide applications should be made based solely on information presented in this publication. Pesticide users must review and comply with all conditions set forth in the pesticide label.
NOTE: All dilutions of Garlon 4 basal bark and cut stump applications are made with an oil carrier. Original branded product names are used for convenience. Generic products that contain the same active ingredient may be available for many older active ingredients. Refer to Table 2 for active ingredients.
Cuda, J. P. and J. H. Frank. 2019. Florida's Established Arthropod Weed Biological Control Agents and Their Targets. ENY-853. Gainesville: University of Florida Institute of Food and Agricultural Sciences. https://edis.ifas.ufl.edu/publication/in779
Dow AgroSciences. 2018. "Specimen label: Garlon® 4 Ultra." Labels & Safety Data Sheets. Accessed on July 17, 2018. https://assets.greenbook.net/16-33-15-25-08-2017-ld7IN006.pdf
FISC. 2021. "Florida Invasive Species Council's 2021 List of Invasive Plant Species." Florida Exotic Pest Plant Council Invasive Plant Lists. Accessed on August 12, 2022. https://floridainvasivespecies.org/plantlist.cfm
Loewenstein, N. J. and S. F. Enloe. 2014. "The history and use of kudzu in the southeastern United States." Alabama Cooperative Extension System. Accessed on August 29, 2022. https://www.aces.edu/blog/topics/forestry-wildlife/the-history-and-use-of-kudzu-in-the-southeastern-united-states/
Shaner, D. L. (ed.) 2014. Herbicide Handbook. 10th edition. Champaign, IL: Weed Science Society of America.
Wunderlin, R. P., B. F. Hansen, A. R. Franck, and F. B. Essig. 2022. Atlas of Florida Plants: Institute for Systematic Botany. Accessed on July 16, 2018. https://florida.plantatlas.usf.edu/
Established arthropod biological control agents of invasive weeds of natural areas in Florida. Modified from Cuda and Frank (2019).
Herbicides used in natural areas of Florida.a
Soil behavior of herbicides commonly used in natural areas of Florida (Shaner 2014).
Toxicity of herbicides commonly used in natural areas of Florida (Shaner 2014).
Control methods in use for non-native plants by land managers in Florida. Techniques are categorized in the following order for each species: manual, cut stump (sometimes referred to as cut surface), basal bark (sometimes referred to as basal stem), foliar, and hack-and-squirt (referred to as girdle or incision point application). See the Control Methods section for additional details on these techniques.
Appendix A: Plants sorted by common name.
Appendix B: Plants sorted by scientific name (from Wunderlin et al. 2022).