7.3 Vegetation Treatment Options

When developing a vegetation treatment plan, it is important that the target vegetation and the objectives outlined in the IRVM plan are understood. In addition, it is important to understand the resources of value that may be affected with a vegetation treatment. These include water quality, wildlife habitat, visual resources, and pollinators. This section describes common vegetation treatments and how they affect these resources, with an emphasis on enhancing pollinators and pollinator habitat.

Because pollinator species can be strongly affected by weed control treatments, it is important to know the important pollinator species that inhabit the area during maintenance activities and develop weed treatments that minimize the negative effects on their populations and habitat. This section outlines vegetation management practices that can control target vegetation while supporting pollinators. Additional details are provided in two recent publications from the U.S. Department of Transportation (Hopwood and others 2015, Hopwood and others 2016), which review the best management practices (BMPs) for controlling roadside vegetation while optimizing pollinator habitat and reducing pollinator mortality. In addition, a review of Section 3.11.6, which covers control of unwanted vegetation before and during construction, may be useful.

Given the complexity of developing treatments that control target vegetation while maintaining or improving pollinator habitat, some general pollinator strategies may aid in designing vegetation treatments specific to a project environment. These are outlined in Figure 7-2.

7.3.1 No Action

While not taking an action to control vegetation is not a treatment, it is included in this section as a reminder that there are times when vegetation does not need to be treated. Reviewing vegetation management objectives in the IRVM plan, especially how treatments may affect resource objectives such as water quality and pollinator health, may provide justification for the "no action" treatment. On roadsides where rare plant species or at-risk pollinator species are present, taking no-action may be the most prudent approach to protecting species.

7.3.2 Mowing

Mowing is frequently used to maintain roadside vegetation by reducing invasive weeds and woody plants, improving driver sight lines, allowing vehicle pull off, and reducing the risk of wildfires. Typically, vegetation in the clear zone, a band of vegetation directly adjacent to the pavement or shoulder, is mowed regularly to keep the vegetation short for drivers who need to regain control of their vehicle and to lower the fuel level for spread of fires. Periodic mowing within the clear zone creates a habitat that is not typically used by pollinators. However, roadside vegetation beyond the clear zone can support pollinators.

Mowing at certain times can directly kill pollinator eggs or larvae present on the vegetation. It can also indirectly affect adult pollinators by temporarily removing host and flowering plants (e.g., food sources). For these reasons, it is important to carefully time or limit mowing during the growing season. This is especially true if the habitat supports endangered or rare and sensitive pollinator species. Higher mowing frequencies reduce native plant growth, plant diversity (Parr and Way 1988), and the ability of forbs to compete with grasses (Williams et al. 2007), and decrease the amount of nectar and pollen present on the roadside. More frequent mowing can also increase the roadkill of pollinators, especially butterflies (Skórka and others 2013). Therefore, minimizing the number of times a roadside is mowed benefits many pollinator species.

Reducing routine mowing of the entire right-of-way (fence to fence) benefits pollinators by allowing wildflowers to bloom and thereby supply nectar and pollen as food. It is best to restrict routine mowing to the clear zone as much as possible, and mow beyond the clear zone only when there are well-defined objectives, such as reducing brush or maintaining lines of sight. Reducing mowing beyond the clear zone to two or fewer times a growing season is best for wildflowers and pollinators. In some regions, mowing can be reduced to once a year per site, every other year, or even every two to three years, depending on the regional intervals of mowing needed to control woody plant encroachment or to reinvigorate populations of wildflowers.

When timed appropriately, mowing can reduce the effects on pollinators and promote plant diversity. It is optimal to delay mowing until autumn or after the first frost if regional constraints allow. When mowing is delayed, butterflies and other pollinators with larvae that reside on vegetation are able to complete their full life cycles, and flowering plants are able to bloom and provide pollen and nectar to pollinators uninterrupted throughout the growing season. If mowing must occur during the growing season, consider selecting a time to mow that balances vegetation management needs (e.g., noxious weed control) with the resource needs of pollinators (e.g., presence of flowers and host plants). For example, mowing at a time that promotes the growth of wildflowers benefits pollinators in the long term. Timing will vary with region. Wildflower growth is promoted by a mid-summer mow in some regions, while in others mowing after spring bloom might be optimal. It is also worth considering varying the season when mowing occurs every few years to increase plant diversity. Mowing consistently at the same time every year will select some plants over others. Plant diversity can be maintained by occasionally varying the timing of mowing, which will favor different plants and prevent certain plants from dominating.

It is important to time mowing to avoid vulnerable stages of the life cycle of any rare or declining species that are present. For example, in Texas, to reduce harm to monarch butterflies, it is best to avoid mowing before March and between May and August, time periods during which monarchs are breeding in the region. Finally, it is helpful to adjust the height of the mower. By mowing vegetation at a height of 12 to 16 inches, vegetation recovers more quickly and plant stress is reduced, particularly during dry periods or drought. Mowing also leaves a greater depth of vegetation for pollinators to use.

There may be a public perception that by reducing mowing, road departments are not taking care of roadsides, which can be an obstacle to implementing mowing strategies that benefit pollinators. Public education may change this perception with time as the public becomes more aware of the benefits of roadsides to pollinator populations and health.

7.3.3 Manual Removal

Hand-pulling weeds can be the most exhaustive and complete method of controlling specific weeds because it is highly targeted. Hand-weeding is well suited for removal of weeds that occur in low numbers or that are scattered throughout a site. Hand-weeding is often the least intrusive method of removing weedy species as it has the least effect on pollinators.

Tools that help with hand-weeding include hoes, picks or pulaski axes, trowels, and shovels. The Extractigator® or the Pullerbear™ are designed to provide grip and leverage for removing deep-rooted species such as scotch broom. It is important to target weeds during their active growth stages before the weeds have flowered and set seed. For perennial and rhizomatous species, it is best to remove as much of the root material as possible because many plants can re-sprout from root fragments.

One strategy for hand-pulling is the Bradley Method (Brock 2016) which prioritizes the areas to be weeded, beginning in undisturbed areas first, and then working out toward more heavily infested areas. When pulling weeds by hand or with tools, it is useful to minimize soil disturbance so that weeds do not become established. It is also important to dispose of weeds in designated areas. If weed removal results in large patches of bare soil, re-colonization by unwanted species can be reduced by seeding with desirable species.

7.3.4 Herbicides

Herbicides are used to control woody vegetation as well as target weed species on roadsides. The use of herbicides can benefit pollinators by suppressing undesirable plants and encouraging the valuable native plants that provide them with food or shelter. However, used indiscriminately, herbicides can reduce the quality of roadside habitat by removing floral resources and host plants, and may be directly toxic to some pollinators (Mader and others 2010; Russell and Schultz 2010; LaBar and Schultz 2012). Overuse of herbicides can also weaken stands of vegetation, making them more vulnerable to weed invasions, which also indirectly affect pollinators (Inset 7-1). By using herbicides as efficiently as possible, maintenance staff can reduce both the amount applied and the effect on plants that benefit pollinators. Using products selectively, timing applications carefully, and following label directions can increase the effectiveness of herbicide use and decrease impacts to pollinators and other resources.

It is important to use the appropriate products and application rates that are effective in controlling the target weed species yet have minimal effects on non-target plant species. Whenever possible, the use of selective herbicides—those formulated to control specific weeds or groups of weeds—can reduce damage to nontarget plants. Nonselective herbicides—those that are broad-spectrum and kill or damage all plants—can also be used selectively to reduce effects on nontarget plants. For example, nonselective herbicides can be used selectively by applying them on weeds when desirable native plants are dormant and by using directed or targeted applications (e.g., spot spraying). In addition, understanding how quickly the herbicide being applied degrades on the site ensures that seed germination and restoration planting are not negatively affected. Herbicide labels include information about selectivity and persistence.

Applications can be timed to be most effective based on the herbicide's mode of action and the application technique. For example, when using a systemic herbicide (absorbed by the plant and transported throughout the plant by the vascular system), perennial weeds can be treated in late summer and fall. During this period, perennials begin to move sugars down to their roots, and the herbicide is translocated to vegetative reproductive structures where it is most effective at controlling the plant. Applications of herbicides at the stage of growth when the weed is most vulnerable can make applications most successful. For many weeds, this is the seedling or rosette stage.

Reducing off-site movement of herbicides and the use of nonselective broadcast applications can help avoid damage to non-target plants that provide pollinators with food or shelter. Referring to road inventories of unwanted vegetation (Section 7.2.1), conducting training in weed identification, and using plant identification reference materials to recognize noxious and invasive weeds will help distinguish these species from similar non-target species. In order to avoid weakening non-target species, weeds can be targeted using spot treatment applications made with a backpack sprayer, weed wiper, or similarly appropriate technology. Using highly targeted applications on cut stems, stumps, or under bark can also reduce unnecessary effects to desirable plants. Broadcast treatments or pellet dispersal are recommended only for dense infestations of weeds or for clear zone or guardrail treatments.

The off-site movement of herbicides can be reduced by selecting appropriate spray equipment, periodically calibrating equipment, and adhering to the pesticide label. Nozzles that produce larger droplets are less likely to cause herbicides to drift off target. Equipment that is calibrated regularly limits over and under applications. It is best to avoid applications when wind speeds are greater than 15 mph and during a temperature inversion (when warmer air above traps cooler air near the ground) when herbicides and other pesticides can linger in the air and move long distances offsite with air movement.

To reduce direct contact exposure to pollinators, herbicides can be applied during a time of day when pollinators are less active. Many pollinators (but not all) are less active before the sun rises or after the sun sets, and are also less active at cooler temperatures (below 50 ° F). Additionally, avoiding broadcast applications of systemic herbicides and herbicides with long residuals reduces exposure to butterfly and moth caterpillars that can be poisoned by consuming contaminated vegetation.

7.3.5 Grazing

Grazing is used to limit tree and shrub invasion, provide structural diversity, and encourage the growth of nectar-rich plants. However, livestock grazing is only beneficial to plant diversity, and in turn, pollinators, at low to moderate levels during short periods of time separated by long recovery periods (Hopwood and others 2016). Grazing can negatively affect insect communities by changing the plant community structure and diversity (Kruess and Tscharntke 2002). Insufficient forage from grazing can decrease bumble bee populations (Carell 2002; Hatfield and LeBuhn 2007) and destroy potential nest sites through trampling (Sugden 1985). Intensive grazing can also affect butterfly populations through trampling (Warren 1993) and altering plant community composition (Stoner and Joern 2004).

Development of a grazing plan that includes careful consideration of the type of grazer, its food preference, and how well the grazer can be managed is important for managing invasive species and compatibility with pollinator health and other resources. For example, goats and sheep can be controlled through herding when they are brought onto a site and when they are removed, bracketing periods of time when pollinators and pollinator habitat are least affected. Goats and sheep preferentially eat broadleaf plants and are therefore the preferred grazers for sites where broadleaf weeds are an issue. At specific densities and duration, goats and sheep can control large infestations of invasive weeds. In addition, they can be controlled in areas near water, thereby reducing effects on water quality, and can be effective in inaccessible spots, such as steep slopes.

It is best to introduce grazers at a time when they preferentially feed on the target weed species. Grazing is most effective, for example, when target weeds are palatable; however, this period may be detrimental to pollinators. If rare or imperiled pollinators are present, timing grazing so as to avoid breeding and foraging periods is best. Avoiding grazing during the adult flight period or when imperiled butterfly or moth larvae are feeding on host plants reduces the effect on pollinators. It is also important to avoid grazing during periods when floral resources are already scarce, as grazing during such times can eliminate pollinators from sites over time. Lastly, the stocking density of grazers can help to determine the duration of grazing. If stocking density is high, it is best to keep the duration relatively short so that desirable vegetation is not affected.

7.3.6 Fire

Prescribed fire is used to manage roadside vegetation and rejuvenate plant diversity in some regions of the United States that have a history of natural fires. Prescribed fire can benefit pollinators through restoration or maintenance of suitable habitat (e.g., Huntzinger 2003), but it can also be harmful when not applied appropriately and have long-term effects on the populations of some species (e.g., Ne'eman and others 2000). For example, burns during the growing season destroy eggs, caterpillars, and above-ground nests and remove vegetation at a time when pollinators need floral resources, host plants, and nesting materials, while winter burns destroy species that overwinter in leaf litter or stems. The scale of the prescribed fire can also affect pollinators. For example, an extremely large and expansive fire may kill pollinators overwintering in above-ground biomass, and such fires during the growing season may eliminate all floral resources in a given area. Smaller, dispersed fires, on the other hand, conserve floral resources and support pollinators in the area by providing refuges.

Burns that are timed so as to have the least effect on pollinators and limiting the scale and frequency of each burn are important pollinator-friendly practices. It is best to use prescribed burning on sections of the roadside corridor rather than the entire corridor. By leaving unburned roadside habitat, enough pollinators remain to recolonize the burned areas. Rotational burning, such as burns conducted three to five or more years apart, allow time for pollinator populations to recover. Rotational burning can provide the benefits of prescribed fire without irreparably damaging the local pollinator community (Black and others 2011). Varying the timing of prescribed burns can also reduce harm to pollinators by avoiding continuously affecting certain pollinators and components of the roadside plant community. Burns affect pollinators no matter when the burn occurs, so altering the timing of burns can reduce negative effects to a particular group or suite of pollinators.

7.3.7 Biological Control

Biological control is the process of introducing natural enemies of the target weed that occur in the geographic region of the weed. Introductions are highly regulated by the U.S. Department of Agriculture and are monitored by government scientists, university researchers, and state agencies. Although biological weed control is not currently widely implemented by state departments of transportation, several have released natural enemies to such weeds as purple loosestrife (Lythrum salicaria), leafy spurge (Euphorbia esula) (Johnson 2000), yellow star thistle (Centaurea solstitialis), and Russian thistle (Salsola kali) (Harper-Lore and others 2013). Biological control can be an effective and focused approach to weed control. However, there are ecological and economic risks associated with introducing a species outside of its natural range, including unpredictable and irreversible consequences (Simberloff and Stiling 1996). The Eurasian weevil (Rhinocyllus conicus), for example, was introduced to control musk thistle (Carduus nutans), but expanded its host plants to include native thistles after introduction, including several rare thistle species (Louda et al. 1997). The loss of native thistles or other native species affects the wildlife that depend upon the plants, including pollinators that visit the plants for pollen and nectar or use the plants as hosts for their caterpillars.

It is best to avoid using natural enemies that have expanded their hosts to include native plants. Coordinating with state agencies, keeping records about locations of releases, and monitoring the target weed populations and potential non-target native species can be used to evaluate the potential impacts of biocontrol agents.

7.3.8 Mechanical Removal

Weed populations can be opportunistically removed during road construction. Removal of populations is accomplished by excavating soils within the populations at least a foot deep and transporting the weed-contaminated material offsite to an approved storage area.

Brush may also be removed from roadsides. Brush removal can benefit pollinator health when brush and trees that pose no risk to motorists are left on site and by opening up the canopy along forest edges. Mechanical trimming to remove problematic shrubs or trees and selective trimming to partially remove woody vegetation can benefit pollinators by creating opportunities for wildflowers to grow. However, complete removal of trees and shrubs is not always beneficial because many butterflies and moths use woody native plants as hosts and to roost during the flight period, and some tunnel-nesting bees use the stems of some shrubs as nesting sites.

Removal of any brush or trees that could be hazardous to motorists is important. This includes plants that could impede sight distance, become dangerous fixed objects, fall onto the highway, or shade the road in winter creating patches of ice. When possible, consider leaving snags or trees with cavities in areas where they are set back from the road and pose no safety risk. Snags can provide nesting habitat for some bees, as well as habitat for birds and bats.

Transitional areas between forest and grass can be created by using brush removal to feather or soften forest edges adjacent to clear zones. Edge feathering involves thinning portions of the forest canopy along the edge next to grassy areas and removing undesirable or unhealthy trees. Periodic cutting to maintain healthy growth and an open canopy benefits remnant patches of savanna, forest, or other habitat dominated by woody vegetation, improves the quality of the habitat for pollinators and many birds, and is aesthetically pleasing.

7.3.9 Haying

In some states, landowners are permitted to cut and remove roadside vegetation for animal fodder. States might grant emergency hay permits under drought conditions, for example, or allow annual haying by adjacent landowners on certain roads throughout the growing season. Haying is not a tool typically used by roadside maintenance staff, although it does affect roadside vegetation and thus pollinators by the abrupt removal of flowers and host plants from a site. In general, haying once in the middle of the growing season can favor wildflowers and cool-season grasses that are often suppressed by dominant warm-season grasses. However, too-frequent haying in a growing season can reduce roadside revegetation over time (Jacobsen et al. 1990), thereby reducing floral resources for pollinators. A poorly timed haying may have severe consequences for rare or endemic pollinator species. When possible, maintenance staff who communicate with landowners can suggest they hay a portion of the entire site at a time, leaving a refuge for pollinators. Additionally, setting the mower blades at 12 to 16 inches reduces the effect on vegetation structure that provides nesting and overwintering habitat, and allows vegetation to recover and bloom more quickly.