Physical Methods in Practice
Three basic principles can guide selecting and applying physical methods that achieve management outcomes while promoting and maintaining desired vegetation:
- Physical methods vary in the type of physical injury and stress they inflict.
- Plant species vary in their response to physical injury and stress.
- Ecosystems (biological communities and their environment) vary in their response to physical methods.
Setting realistic goals for applying physical methods successfully requires a thorough understanding of how invasive and desirable plant populations within a particular ecosystem will respond to the type of injury and stress a particular method inflicts.
From hand pulling, to soil solarization, to benthic barriers, each physical method requires different application tactics and has advantages and disadvantages. In general, controlling invasive plant populations successfully with physical methods depends on selecting a physical method that is compatible with the target species and the management area, and applying the method at the optimal time and intensity to achieve desired outcomes.
The physical methods that may be applied in terrestrial and aquatic environments vary considerably in the type of injury they inflict, their selctivity and potential for nontarget effects, and the procedures, skills, equipment, and cost they require. Understanding the limitations of each method is important in deciding which one to use.
Considerations for Selecting Physical Methods
Nature of the method
Nature of the infestation
Nature of the site
Nature of the Method
- Some physical methods may be applied selectively with a relatively low impact on other species and the environment. Other methods, however, may affect all plants in the treated area and even impact surrounding biotic and abiotic ecosystem components. Selective methods have the advantage of minimal impacts on nontarget species. However, they are labor intensive and time consuming, and often need to be repeated, so they are most appropriate for small, localized infestations and/or ecologically sensitive situations. These methods also might be appropriate following more broadscale treatments.
Nonselective methods will impact all plants-undesirable and desirable-within their range. However, their advantage is that they treat larger areas more quickly (and sometimes less expensively) than selective methods. Revegetation may be necessary following these more disruptive treatments if desired vegetation is inadequate.
Nature of the Infestation
- The biological characteristics-particularly the reproductive strategies-of the targeted species will help determine which physical methods are most likely to be effective. For example, hand pulling a rhizomatous species is futile because it is unlikely that all rhizomes will be removed from the soil and pulling may stimulate shoot production from remaining root buds. The location, distribution, and density of target plant populations can also influence method selection. For example, selective and precise manual methods such as stabbing are probably more feasible to use on sparse, localized infestations than on dense, widespread stands or populations. The age of the population is another important consideration for method selection because older stands of invasive plants are likely to have more substantial carbohydrate reserves and seedbanks than newly invading populations.
Nature of the Site
- The size and topographic characteristics of a site influence the choice of management method. Sites must be accessible, relatively large, and fairly level for methods such as chaining and tilling that require heavy equipment. Water bodies must provide enough depth and width to accommodate mechanical aquatic harvesting equipment. Terrestrial sites that are steep, forested, or have heavy brush may require a more precise method such as cutting or girdling that requires only hand-held tools. Such practical considerations will also determine whether a site is appropriate for flooding or water drawdowns. The ecological sensitivity of a site must also be considered. If threatened and endangered species are present, very selective physical methods may be chosen despite the additional time and expense of treatments.
Applying physical methods at an optimal time is important because the susceptibility of a plant varies according to its growth stage and seasonal status. Timing can therefore improve target plant control and minimize impacts to desirable biological communities and the environment.
- Though timing strategies may vary widely depending on physical methods and target plant species, generally the best time for treatment is when the target plant is most susceptible to injury and before it reproduces or spreads. A second consideration is timing treatment for periods when the desired plant community and other biota are least vulnerable. For example, studies have shown that the most effective time to mow invasive plants is when the desired plants are dormant and the target plants have reached the flowering stage (Sheley et al. 2001). Ideally, desired plants should be allowed to produce seed. Other studies have documented the importance of inundating saltcedar (Tamarix spp.) stands at an optimal time for control, and drawing down water to coincide with native seed dispersal (Sprenger et al. 2001, Taylor and McDaniel 1998).
- Seasonal fluctuations in site conditions may influence the feasibility and efficacy of some physical methods. For example, using heavy equipment for chaining invasive tree or shrub species during spring may be effective in removing target plants, but introducing heavy equipment under wet spring soil conditions may disturb and damage desirable plant communities.
The intensity of an invasive plant management method refers to the extent of its impact on target plants. Tactics for controlling the intensity of physical methods vary. For example, prescribed grazing intensity can be controlled by manipulating the number of grazing animals and the duration and frequency of grazing. Understanding how a method impacts the target species and environment is key to manipulating its intensity.
- For methods such as flooding, soil solarization, mulching, benthic barriers, and water drawdown, the duration of treatments may influence the intensity of the treatment. Experiments conducted in Nevada determined that perennial pepperweed (Lepidium latifolium) was able to survive a flooding duration of 50 days before critical physiological functions were affected (Chen et al. 2005).
- Treatment intensity may be controlled by manipulating the frequency of applications over time. The frequency of treatments such as mowing, cutting, girdling, and harvesting is determined by growing conditions that facilitate recovery (such as precipitation) and the tolerance of a species to treatment.
- Where an injury, such as a cut or stab, is located relative to vulnerable plant structures may affect treatment intensity. For example, a high mower blade can injure a plant with vulnerable aerial growing points, such as an erect perennial forb, but it will not damage plants with growing points at the base of the plant, such as perennial grasses. An effective strategy for mowing minimizes the removal of growing points of desired plants and maximizes removal of growing points of targeted species (Sheley et al. 2001).
- Water depth may be manipulated to control the intensity of flooding and water drawdown treatments. Depth and density of mulches are important factors in determining the success of physically obstructing plant emergence and limiting light (Teasdale and Mohler 2000).
Since physical methods can promote as well as suppress invasive plant species and can also affect nontarget plant populations and the environment, tactics for physical control of invasive plants may require
- controlling regeneration from remaining perennating tissues (viable growing points)
- controlling seed germination and dispersal
- mitigating conditions that promote invasive plant populations (e.g., disturbed soils, propagule dispersal)
- mitigating negative impacts on desirable vegetation and other ecosystem components
For example, properly disposing of vegetative debris that can resprout or produce viable propagules is an important step in preventing reestablishment or spread of invasive plants to a new area. Vegetation that has been pulled, cut, or mowed can be left onsite as long as it cannot reroot or resprout, it will not go to seed, and it decomposes quickly. Piling cut, harvested, or dredged plant material is another option, but piles and adjacent areas should be monitored for resprouts or new seedlings. For plants that spread by seed, cutting, bagging, and hauling flowerheads/seedheads or entire plants offsite is a good approach; however, the destination of the debris (such as a landfill) must not become a source of further infestations (Holloran et al. 2004).
Monitoring plays an essential role in managing invasive plants by providing data to support management decisions. Through systematic data collection before and after implementation of physical methods, monitoring can be used to demonstrate where they are effective in meeting invasive plant management objectives, or to detect where methods are ineffective or having unintended nontarget effects.
Specific monitoring objectives and procedures will vary across the range of physical methods as they are applied in different ecological settings. When using physical methods, it is especially important to monitor for regrowth, reestablishment, and dispersal of invasive plants and propagules. As previously discussed, some physical methods can promote invasive plants by stimulating vegetative growth from established individuals, encouraging germination or sprouting from propagules already present at the site, providing open niches for establishment of invasive plant propagules from offsite sources, and facilitating dispersal of invasive plant propagules to other sites.
Physical methods can be used to implement a number of invasive plant management options.
In some cases, physical management may help prevent invasive plant establishment, as when soils are frequently hoed or tilled to disturb the soil seed bank and uproot seedlings (Radosevich et al. 1997). Physical methods can be used to alter growing conditions to make them less favorable to establishment of unwanted species. After any physical treatment, it is important to inspect tools and equipment to prevent moving plant propagules to new sites (Holloran et al. 2004).
Physical methods can suppress invasive plants by reducing a population’s competitive capacity in the plant community. For example, carefully timed mowing treatments can favor desired plants that compete with invasive plants (Sheley et al. 2001).
Physical methods that reduce or prevent seed production, such as mowing and cutting, can effectively contain an infestation by eliminating the capacity for plants to reproduce.
The probability of successfully eradicating invasive plant populations generally increases as infestation size decreases. Physical removal or destruction of invasive plants can often be the first, simplest, and most cost effective response to a small new population (Venner 2006). Eradication projects may require continual monitoring and follow-up treatments.
Used alone, physical methods are sometimes too labor intensive and expensive to justify the level of invasive plant control achieved, particularly when applied to well-established or large-scale infestations. Often, repeated treatments are needed to achieve acceptable levels of control, further compounding the fiscal and labor demands required to implement physical methods. Therefore, physical methods are typically applied to complement other methods as part of an integrated strategy to manage invasive plants. Many published studies have examined the interactions and efficacy of combining methods. Below are several examples.
Physical Methods and Chemical Methods
Combining physical methods with chemical methods is common and each method can improve the efficacy of the other. For example, comparisons of different control methods for Japanese barberry (Berberis thunbergii) indicated that pulling and cutting alone were not as effective as cutting and applying an herbicide to stumps (Myers and Bazely 2003). This integrated method has also been used effectively on saltcedar (Tamarix spp.) when the species grows in relatively small infestations or is interspersed with desirable vegetation (Shafroth et al. 2005). Evidence suggests that combining mowing with herbicides can enhance perennial invasive plant control, probably by improving herbicide contact with leaf surfaces. One study showed that mowing two or three times a year enhanced chemical control of Canada thistle (Cirsium arvense) (Sheley et al. 2001). Fall tillage followed by fertilization or herbicide treatments reduced populations of leafy spurge (Euphorbia esula) in North Dakota (Lym and Messersmith 1993).
Several refuges have combined physical and chemical methods to control a variety of species. At Ridgefield NWR in Washington, herbicide applications to control reed canarygrass (Phalaris arundinacea) were enhanced by disking treatments that removed dead vegetation and exposed mudflats for germination of desirable wetland species (Paveglio and Kilbride 2000). Mowing and herbicides were combined at Willapa Bay NWR in Washington to effectively control smooth cordgrass (Spartina alterniflora) (Major et al. 2003). Herbicide treatments followed by mechanical removal reduced saltcedar stands on the Bosque del Apache NWR in New Mexico (Taylor and McDaniel 1998).
Physical Methods and Biological Control
Depending upon which physical method and biological control agent are used, combining these two methods may either enhance or reduce overall invasive plant control. For example, physical methods may increase a plant's susceptibility to disease and pathogenic biological control, but regrowth foliage produced after cutting or mowing may be either more or less suitable for herbivorous insect biological control agents compared with the original foliage (Hatcher and Melander 2003). Properly timed, cutting and mowing may enhance the effects of biological control agents by providing new growth for agents to feed upon and by further stressing plants that expend energy to regrow. In studies by Wiggers et al. (2004), biological control agents significantly suppressed regrowth from pruned stumps of the invasive tree melaleuca (Melaleuca quinquenervia).
Physical methods such as mowing and cutting should be properly timed to avoid disturbing or destroying biological control agents in the aboveground portions of the plant. Methods such as tilling, pulling, and digging may disturb the soil and remove or destroy host plants for biological control agents. Methods that completely remove host plants may not be desirable because they remove food and habitat for biological control agents and may remove the agents themselves. Methods that disturb the soil are likely to have considerable impact on soil-dwelling biological control organisms (Hatcher and Melander 2003).
Physical Methods and Prescribed Grazing
Physical methods and prescribed grazing can be integrated to improve the effectiveness of treatment application. For example, prescribed grazing can be used to clear vegetation and facilitate access to a site, or remove excess canopy vegetation to aid in locating low-growing target species for treatment with physical methods. If targeted plants are too tall for grazing animals to reach, physical methods can be used to reduce the height of target plants and improve the effectiveness of prescribed grazing treatments (Campbell and Taylor 2006). Prescribed grazing may also be applied to control resprouting and regeneration of tree species that have been treated with physical methods (Sharrow 2006).
Physical Methods and Prescribed Burning
Combining physical methods and prescribed burning can enhance control. For example, surface fires may improve access to a site so that invasive plants may be located for mechanical or manual removal (Eckardt 1987). Prescribed burning can be applied after physical methods to remove dead biomass (Bell 1997, Dudley 2003) and prevent resprouting from cut and piled debris (Taylor and McDaniel 1998).
In an Australian study, bulldozing was used to compact dead branches and increase the burning temperature of lollipop mimosa (Mimosa pigra) that had been previously treated with herbicides (Paynter and Flanagan 2004). A similar effect was achieved by crushing gorse (Ulex europaeus) plants to enhance the use of prescribed burning (Rees and Hill 2001).
Several National Wildlife Refuges have combined prescribed burning with flooding treatments to control common reed (Phragmites australis) in the eastern United States. At Wertheim NWR in New York, a 20-30 acre freshwater impoundment was drained in the fall, burned the following winter, and then reflooded with several feet of water (Marks et al. 1993). Common reed was eliminated from the half of the marsh that was treated and the area remained free of the grass for at least three years.
Physical Methods and Revegetation/Restoration
After using a broadscale, nonselective physical method such as flooding or tilling, revegetation may be necessary to reestablish desirable plant communities. Revegetation with desirable and competitive plant species is the best long term, sustainable method of suppressing or preventing the establishment of invasive plants (DiTomaso 2000).
Periodic flooding combined with mechanical removal of saltcedar has been used to restore desirable plant communities in the western United States. Mechanical clearing provided light to the soil surface, beneficial soil disturbance, and minerals and nutrients for desirable seedlings (Shafroth et al. 2005).
Physical methods include some of the earliest and simplest tools used to manage invasive plants and other vegetation. Despite their labor intensiveness relative to other methods, physical methods may provide the most feasible management option for ecologically or socially sensitive sites commonly found on refuge lands. The following slide show and case study illustrate applications of physical methods on refuges, used alone and in combination with other invasive plant management methods.