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Invasive plants are a pervasive issue when stewardship natural areas in Pennsylvania. Management of these species should be a top stewardship priority.
A invasive plant species is an introduced species that rapidly spreads and outcompetes multiple native species, chiefly because of the absence of the predators, pathogens, and herbivores that keep it in check in its native range. Humans have caused the spread of invasive plant species throughout Pennsylvania and much of the world. Even though the occasional immigration of new species into plant communities is a normal process, the current high rate of introduction—fueled by the planting of introduced (nonnative) species for horticulture, wildlife management, and erosion control, as well as accidental importation via global commerce—is threatening the integrity of native plant communities, the survival of native plant species, and the resilience of the entire food web, including countless native animal species. While native plants can become aggressive, particularly where many other native plant species have declined due to deer and disturbance, they are not considered invasive in this handbook. Controlling or eradicating invasive plants is a difficult but important task for stewards of natural lands.
A note about terminology: The goal of this Stewardship Handbook section on management of introduced species that negatively impact local ecosystems is to promote biodiversity and resilience. However, it is important to recognize that the historically used language around invasive plants is being reconsidered due to its xenophobic and militaristic connotations. For instance, the terms "invasive," "exotic," and "native" can all have connotations beyond plants and link to xenophobic biases. Additionally, language such as "attack" for managing invasive plants can be unnecessarily aggressive and militaristic. The authors of this chapter have done their best to use mindful language where possible and are watching as language changes around this topic.
Not all introduced species are invasive. Nearly 1,000 introduced species have escaped to the wild in Pennsylvania, where they compete with the 2,000 native species. However, less than 100 introduced species have become invasive (around 50 of the invasive plant species currently associated with the greatest harm to native biodiversity in Pennsylvania are listed in the table, below). An invasive species displays one or more of the following characteristics:
Most invasive plants are particularly well adapted to take over disturbed areas. In Pennsylvania, the division and clearing of land associated with agriculture and sprawl development has created countless miles of edge between natural and nonnatural areas that is highly suitable to the proliferation of invasive species. The misguided promotion of several introduced species for erosion and livestock control as well as the state’s rich horticultural legacy, which has traditionally centered on introduced species, have provided plentiful seed sources for regional dispersal of numerous invasive species.
The presence of invasive plant species complicates the goal of maintaining healthy native plant communities as invasive plants compete vigorously with preferred native species for space, light, water, nutrients, and other factors that support plant growth. Invasive species can displace native vegetation, subvert the natural process of succession from field to forest, and homogenize the structural and wildlife food resources of a landscape or an entire region. They can also alter nutrient cycling, hydrology, and fire regimes.
These modifications to native plant communities reduce their habitat value for native wildlife, particularly migratory songbirds, which nest within different vegetation layers, and insects, which are vital links in many of the food chains that make up the food web in ecosystems. Many native insect species, both terrestrial and aquatic, are specialist feeders relying on just one native plant species or a narrow range of species. Introduced plants rarely serve as a food source for native insect species. Where a community features abundant and diverse native plants, it will also contain greater total insect biomass. Conversely, the more introduced plants are present in a community, the scarcer insects will be. Insects are the richest source of fats and protein for birds, fish, and many small animals that, in turn, are food for larger animals. Where introduced plants are abundant, far less of the total plant biomass is converted, via the food chains that make up the food web, into animal biomass. Invasive plants have adverse impacts on virtually all native wildlife populations, both by degrading habitat directly and by reducing the total food supply. Nationally, the destructive impact of invasive species on native biodiversity is exceeded only by direct habitat destruction.
To protect and improve biodiversity, invasive species should be managed to protect critical areas, limit further degradation, reduce the possibility of reestablishment or invasion by new introduced species, and best utilize staff and volunteer hours and resources. A permanent invasive plant control program should be established to create a systematic and efficient approach to long-term invasive plant management. First, the goals of the invasive plants management program should be identified. Goals may include reducing the impact of invasive species, protecting a critical area or particular native species, preventing the spread of new invasive plants into a site, and educating visitors about the threat of invasive plants. Other goals may be determined based on the specific needs and resources of a site. Once goals are identified, land managers can choose which strategies best meet the goals. The following strategies are recommended for an effective invasive species control program.
The site should be surveyed to identify, locate, and quantify invasive species. The site should be assessed to determine the level of degradation and impact caused by invasive species. Mapping the location and extent of a species can bolster the effectiveness of control strategies over time.
Management units (or areas) should be prioritized based on the level of invasive species prevalence. When deciding what areas to address first, a few factors should be considered.
1. Areas with the lowest density of invasive species should be given priority over heavily degraded areas.
Areas that are heavily degraded already have limited biodiversity and low-quality habitats, whereas areas with mostly native species still have high-quality habitats with biodiversity to protect.
2. Areas that are least likely to be recolonized should be prioritized.
Seeds are more likely to spread downhill, downwind, and downstream, so these locations should be addressed last. Additionally, minimally disturbed areas such as the interior of forests or meadows are less likely to be repopulated and should be prioritized over heavily trafficked areas such as trails.
3. Areas with high priority resources, such as rare, threatened, or endangered species, should be prioritized over areas without such resources.
4. Within each management unit, species should be managed with a top-down approach.
Vines impacting trees should be managed first, followed by trees, shrubs, and then herbaceous plants. This prioritizes plants that affect the most growing space and have the greatest effect on habitat. While this is a general rule, other factors may make a species a priority. For example, poison hemlock (Conium maculatum) and giant hogweed (Heracleum mantegazzianum) are highly toxic to people and spread rapidly. The risk to human safety and ability to spread prolifically make such species a top priority for management.
Based on these guidelines, the focus of initial restoration efforts should be to halt the degradation of the canopy layer in the healthiest areas, then moving down through the forest layers. Land managers would then move to the moderately invaded areas, and so on to the most degraded areas. Those areas that are severely invaded should, at first, be left for “dead.” Since they essentially cannot degrade any further, their restoration (which will usually require significant resources, including heavy equipment and years of high maintenance) is best left until the healthier, less affected sites are stabilized. This approach is also healthier, psychologically, for the people involved in restoration. Spending the initial phase of a project stabilizing the majority of a site is more rewarding than struggling through a small, highly degraded section.
There are of course exceptions to these guidelines. Invasive plant management and subsequent restoration priorities may need to be modified for best short-term efficiency of labor and long-term results, according to the time of year or availability of labor. For example, the cutting and herbiciding of understory invasive trees is best done during fall and early winter when sap is flowing into the roots, whereas the planting of seedlings is best done in the late winter and early spring. Priorities may also change based on funding available. If a large amount of funding is available to clear a dense patch of invasives and then immediately replant the area, that section may become a higher priority.
Species management can be carried out by staff members, contractors, or volunteers, depending on the type of work required and available resources. Volunteers can pull plants or cut them with hand-held tools like pruners and loppers, while properly trained staff or contractors can use larger equipment like chainsaws and mowers. Herbicide must be applied by licensed personnel.
If an invasive species makes up most of a structural layer, full removal at one time is not generally recommended, as this will abruptly remove cover for wildlife and leave a vacuum that the same or other invasives may rapidly colonize. Removing understory shrubs such as introduced shrub honeysuckles, privets, burningbush, or linden viburnum can transform a forest stand that provided cover for migratory and resident birds and other animals to one devoid of understory cover and thus no longer a viable refuge from predators. Removal without replacement has numerous subtle effects but some effects can be dramatic, such as a striking decline in birds that were once common.
In general, the restoration of a degraded community, particularly forest, should be done in a manner that removes less than 50% of the total biomass of any vegetation layer (canopy, subcanopy, shrub layer, ground layer), leaving wildlife plenty of space to find refuge and time to adjust to changing cover and food conditions. If the amount of invasive material is light and widely scattered throughout a forested area, the entire forest can be treated at the same time. However, if the shrub layer, for example, is heavily dominated by invasives it is best to treat the area incrementally in patches over several years, waiting for existing native shrubs to fill in the available growing space or planting new ones. Invasive vines are the exception to this rule, because they grow on and not in place of native species and can weaken, kill, or topple trees. All invasive vines should be treated as soon as possible.
However, intensive management projects can be undertaken where a large number of invasive plants are removed at one time if the land manager is prepared to immediately replant the entire area with a substantial quantity of native plants to reestablish habitat. The land manager will also have to be prepared to regularly monitor the area and control any invasive plants that begin to reestablish. While this can impact wildlife habitat, it may be the most efficient way to restore an area if sufficient funds are available.
To prevent new colonies from forming and current populations from expanding, care should be taken to disturb the soil as little as possible during invasive plant removal, because soil disturbance can trigger germination of dormant seeds. Once invasive species have been removed, the treated areas should be replanted with native species to restore habitat and discourage invasive plants from reestablishing. Additionally, areas where plants are removed should be monitored and any reestablishing invasive plants should be controlled.
General techniques for invasive plant management are given in the Management Methods section below.
Invasive species must be monitored yearly to determine whether treatment has been effective and what further treatments are necessary. Skipping even one year can result in previous efforts being undone by fast-growing invasives. Initial treatment, especially of large infestations, often needs to be repeated before it is successful. Treatment efforts may also need to be reassessed and different treatments pursued if initial efforts are unsuccessful. Ongoing monitoring is necessary to identify any regrowth from the seed bank and to prevent reestablishment of the population. Most importantly, monitoring is necessary to identify newly established invasive species so they can be treated before they spread further. It is easier to control and eliminate a species if it is found early, when the population is small and requires fewer resources to manage. This should include monitoring just outside the site’s boundaries to note invasive plants on adjacent properties that are likely to spread.
Several actions can be taken to prevent the further spread of invasive species onto or within a property. First, soil and vegetation disturbance should be minimized for all projects on a property, regardless of whether they relate to invasive plant control. Soil disturbance stimulates the seed bank to germinate, and many of the invasive plants have a competitive edge when colonizing disturbed areas. Second, forest and meadow edge should be minimized, as invasive plants thrive in edge environments. To avoid fragmentation, areas of the same cover type (forest or meadow) should be connected and edges should be rounded off. In most cases, “peninsulas” of forest and fencerows of trees fragmenting a meadow should be converted to meadow and small meadows fragmenting a forest stand should be converted to forest. Third, landowners should not plant nonnative invasive species on the property. This includes landscape and recreation areas.
Finally, to further reduce the spread of invasive plants, visitors to a property should be educated on cleaning shoes and clothing to remove invasive plant seeds and other material before entering. A short explanation posted on a kiosk could help make the public more aware of the risks of being unintentional carriers of invasive seeds. Installing a boot brush station next to such a kiosk reinforces the point and has been shown to reduce invasive seed input in natural areas. To be effective, such stations need to be cleaned regularly of the seed-filled dirt mound that accumulates underneath and any weeds growing from the mound; the worn bristles need to be replaced at least once a year; and any broken parts should be repaired promptly. Interpretive signage at a park or open space can inform visitors about the threat of invasive plants and call out ones that are visible in the landscape. Additionally, interpretive signage can be posted to explain invasive species control projects, especially ones that create a significant visual change that could be concerning to visitors.
Replacement planting should be undertaken in the same year as invasive species removal. This will provide the native species with an edge in recapturing the growing space made available by weeding out invasive species. It should be emphasized that successfully establishing native species after treating invasives will hinge on effective deer management—either restricting access to the plantings or establishing and maintaining the appropriate deer density. Any site where plants to be removed comprise more than 25% of the cover within their forest layer (canopy, subcanopy, shrub and vine, herbaceous) will probably require planting to augment any natural regeneration. Removal should be undertaken at times of year when direct disturbance of wildlife is minimal, preferably late fall or winter. Replacement plantings should precede the onset of the spring breeding season because many birds return to the same sites year after year to reestablish territories and renest. To ensure their survival and to maintain ecosystem integrity, replacement plants must be native tree, shrub, or herbaceous species carefully selected to be appropriate to soil conditions and the community type at each individual restoration site within the natural area (see Native Plants and Their Care chapter).
Replanting after removing invasive plants accomplishes several objectives. Where invasive species have eliminated entire forest layers (e.g., Norway maple and English ivy, which eradicate native shrub and herbaceous layers in forests), replanting after removal restores long-lost vertical forest structure and bird cover. Where invasive plants are removed from streambanks or floodplains (especially Japanese knotweed) or from steep slopes, replanting renews protection against soil erosion. In all cases, the planted native species restore lost components of the indigenous food web; invasive species’ leaves and stems are little utilized as food by native wildlife, which is one of the reasons they succeed so well here.
It must be emphasized, however, that planting should be viewed as only one component of forest restoration where invasive species are removed. The goal of maintaining natural lands as a set of natural communities dominated by native species will be met only by reducing the deer population to a level that allows natural regeneration from seed produced by native species already growing on the natural lands. Once natural regeneration is restored, a healthy crop of seedlings and saplings of native species will be poised to assume the growing space vacated by the natural decline and mortality of native species and the deliberate removal of invasive species.
The following are techniques and procedures for addressing different types of invasive plants. Ongoing monitoring and repeated application will likely be necessary for successful control. Where trade names are used no endorsement is implied. The authors of this document are not liable for problems associated with the use of herbicides described therein. For more specific information by plant species, see the Related Items of Note.
Equipment: Mower, herbicides, backpack sprayer, wick applicator
Herbaceous invasives (e.g., garlic mustard, Japanese stiltgrass, mile-a-minute, Japanese hops, Chinese silvergrass, Japanese knotweed, giant knotweed) are probably the most difficult to control because they are mostly available for treatment during the growing season when native plants are growing nearby. They also quickly colonize disturbed areas, including areas where invasive shrubs and trees have been removed. Small areas of many herbaceous invasives can be pulled, dug, or mowed until they stop resprouting. Large areas of invasive herbaceous plants can be sprayed with glyphosate during the growing season although care must be taken to avoid collateral damage to native species. In some cases, herbaceous invasive plants can be treated in late winter or early spring before native herbs appear. For example, the basal leaves of garlic mustard (an herbaceous biennial) can be sprayed with glyphosate on warm (above 40˚ F) days; early sprouts of mile-a-minute can be treated similarly. Also, some evidence suggests that applying a pre-emergence herbicide (a chemical that kills seeds sprouting just under the soil) can be helpful in heavily affected areas of mile-a-minute, Japanese hops, and Japanese stiltgrass, which are annuals.
Japanese knotweed, giant knotweed, and the hybrid between the two, Bohemian knotweed, are especially hard to control. They cannot be pulled up by the roots or eliminated by repeated cutting. They often form large, monospecific stands on streambanks, where they accelerate soil erosion by having deep roots that are poor at stabilizing soil, by dying back to the ground over winter, and by having dense canopies that exclude any species with strong, fibrous, soil-holding roots like native groundcovers, shrubs, and newly established trees. A formula for treatment that has shown success is:
Equipment: Pruners, pruning saws, loppers, blade weedwhips, chainsaws, herbicides, backpack sprayer, wick applicator
Invasive groundcovers can be pulled on a regular basis, or herbicides can be used to control or eliminate patches. Care must be given not to spray nontarget species. For evergreen groundcovers (e.g., English ivy, Japanese pachysandra, common periwinkle, wintercreeper), a mixture of triclopyr and diesel fuel has been used successfully as a foliar spray on warm (above 45˚F) winter days.
The priority in invasive species control is to remove invasive vines affecting canopy trees, mainly Oriental bittersweet, English ivy, Japanese honeysuckle, porcelainberry, five-leaf akebia, and wintercreeper. Cut woody vines both at ground level and at least 5 feet above ground level and remove from trees if removal won’t cause damage. Immediately following cutting, large stumps should be painted with a systemic herbicide such as glyphosate or triclopyr.
It should be noted that even though invasive vines pose a significant threat to the forest, there may be native vine species within a natural area that have high food value for wildlife. Poison-ivy, Virginia creeper, American bittersweet, trumpet honeysuckle, trumpet-creeper, and the five grape species native to our region (summer, fox, New England, riverbank, and sand grapes) should not be cut from trees unless they begin to seriously compromise the health of the tree. Among native vines, this is likely to happen only with grapes, which can eventually overtop the canopy of a tree. If overtopping or threatening to overtop a valued tree, a grape vine should be cut but not treated with herbicide so that it can resprout. (The non-native wine grape and its hybrids occasionally escape from cultivation in our region, mainly in highly disturbed areas, but it is rarely seen in the wild and is not invasive.)
Equipment: Pruners, pruning saws, loppers, Weed Wrench, blade weedwhips, chainsaws, tractor-mounted brush hog, herbicides, backpack sprayer
Eliminate or control invasive shrubs and saplings by manually or mechanically pulling or by cutting. Stumps cut manually should be immediately painted with a systemic herbicide such as glyphosate or triclopyr; it is best done in late summer or fall when sap is flowing into plant roots. Thin-barked shrubs can also be treated with a 20% mix of triclopyr in basal oil applied in a band around the base of the trunk during the growing season.
Equipment: Pruners, pruning saws, loppers, Weed Wrench, chainsaws, front-end loader, herbicides, backpack sprayer, wick applicator
In areas adjacent to structures or high-use locations, drop invasive and hazardous trees without damage to surrounding trees and either let them lie or cut them into pieces to create brush piles for wildlife habitat. Severed trunks and limbs of Norway maple or black locust (considered invasive in some communities such as meadows and savannas) that are large (>6-inch diameter) and straight (>8-foot sections) may be useful for trail stabilization and restoration. Some other invasive tree species such as ailanthus will decay rapidly and are not useful for this purpose. Stumps of felled trees should be immediately treated with a systemic herbicide such as glyphosate or triclopyr.
Ailanthus and black locust will root-sprout vigorously following cutting, even with herbicide treatment. Do not cut, but apply basal herbicide as described above. Other thin-barked trees such as young Norway maple can also be treated this way. Smaller limbs and related debris should be left to rot or fly-chipped on-site. In appropriate areas, larger (>6-inch) trees can be girdled to create snags for cavity-nesting wildlife. All dead trees, snags, or branches that do not pose a safety hazard or a threat to the ecological health or stability of the forest should be left in place for their wildlife habitat benefits.
To create a brush pile, first build a base by placing four large logs, set 1 foot apart and parallel to each other, and then place four more logs of the same size, stacked perpendicular to the first logs. Add brush to the top and sides, starting with the larger limbs first, then adding smaller pieces until the pile is about 6 feet high and 6 feet wide. Gaps should be left withing the debris pile to allow wildlife to shelter within the debris pile. Do not compact the debris.
Prescribed fire and spot-burning can be used to control invasives and give an advantage to native, fire-tolerant species. This can be effective for herbaceous invasive plants and invasive shrubs and small saplings. However, the use of prescribed fire and spot-burning should be carefully considered prior to implementation due to multiple factors. First, there are some species that may aggressively regrow after being exposed to fire (e.g., autumn-olive, black locust, ailanthus, kudzu). Land managers should research the most appropriate control methods for species being targeted to determine if fire is effective or not. Secondly, issues can arise from the misuse or improper application of fire. Third, local governments and fire companies may not be receptive to the use of fire to restore and maintain native biodiversity and ecosystem function. If you plan to use fire to manage natural lands, it is crucial to acquire any necessary permits, notify neighbors, and coordinate with local and state authorities and, of course, the local fire company. You will need to prove to these authorities that you are properly trained and equipped (see the Prescribed Fire chapter) to undertake this activity. As with herbicides, only properly trained individuals should utilize fire as a management tool.
To be effective and safe, weather and fuel conditions must meet narrow parameters (the burn prescription). Historically, it has usually been best to burn in early spring—mid-March to mid-April for herbaceous invasives, late April to early May for woody invasives—a time when many natural fuels reach a peak of flammability but weather conditions typically make containment simpler. Furthermore, invasives usually sprout earlier than native species, making them vulnerable to fire at a time when many natives are highly fire-tolerant. Late summer-early fall fires are feasible in certain communities, depending on fuel characteristics. They can be useful, for example, to counter the tendency of warm-season grasses in meadows to outcompete other graminoids and forbs and form a monoculture. Climate change impacts, such as extended periods of rain or dry spells, particularly in the spring, are making it more difficult to reliably burn within these time windows. Qualified fire crews are often fully committed, and together with uncooperative weather, unable to carry out all of their scheduled prescribed fires. Getting fire crew commitments early pays off.
In most cases the use of herbicides alone is not an effective long-term solution for controlling invasives. Difficulties in delivering adequate amounts to the target plants at the correct time in their growth cycle, the near-impossibility of avoiding collateral damage to native plants and other organisms, and the potential health risks to workers are all drawbacks to their use. In addition, inherent in the sole reliance on herbicides is a “once and done” attitude that is not conducive to the long-term control of invasives. Inappropriate use of herbicides can degrade soil and water resources and harm humans and wildlife, particularly amphibians and aquatic animals. Used appropriately, however, herbicides can be an important tool for land managers in certain situations. Herbicides should be applied in natural areas only by qualified applicators trained in both the safe use of each herbicide and the identification of native species versus invasive species. Training and licensing for herbicide application is provided by the Pennsylvania Department of Agriculture.
To safely administer herbicides to a target plant it is best to cut it back as much as possible and wait for it to resprout prior to herbicide application. To control small trees, shrubs, or vines, an herbicide with glyphosate may be applied either to the stumps immediately after cutting or to fresh sprouts two weeks later. Larger plants can be most effectively controlled by applying an appropriate formulation of the herbicide triclopyr or glyphosate directly to the freshly cut stump or to the uncut stems of shrubs and trees with smooth bark (ailanthus, young Norway maple). This second method works best in fall when sap flow is into the roots. It should be noted, however, that there is some risk to nearby, non-target trees from herbicide application. Research has shown that herbicides can be translocated through root grafts (a relatively common occurrence) or mycorrhizal fungi into other trees. Care should be exercised in treating invasive trees in close proximity to highly valued native trees.
It is particularly important to establish trees in forest gaps where invasives have been removed, to cast shade and inhibit recolonization by invasives. This can be done through natural regeneration or by planting trees. The former is the preferred method because new seedlings are more likely to be derived from a gene pool that has evolved under the environmental conditions of the region over thousands of years. However, the prevalence of overabundant deer throughout the region often necessitates planting to establish native species more quickly. Information on planting technique and native species is included in the Tree Selection and Planting chapter.
In general, late fall and winter are the most efficient and least arduous times to perform invasive species control. Problem areas are more easily traversed and cool-weather clothing gives added protection to the work crew. Systemic herbicides are also most effective in the fall when sap is flowing into the roots. The exception to this rule is for herbicide applications that target the foliage of invasives, such as spraying to control herbaceous plants (creeping thistle, mile-a-minute, common mugwort) in meadows or small shrubs (young autumn-olive, invasive honeysuckles, privets) in meadows or forests. The timing of such foliar applications depends on the specific species but they are often done between midsummer and fall. Any heavy equipment use should be conducted when the ground is dry or frozen.
Trees and shrubs should be planted in early spring before they leaf out or in early fall to allow for root growth before the ground freezes. Protect plantings with fencing or tree tubes until plants are large enough to withstand deer browsing and rubbing. This is typically 3.5 inches in diameter and 6 feet high for trees and after 5-10 years for shrubs. See the Tree Selection and Planting chapter for more information.
Following initial treatment, an annual inspection and control schedule should be adopted to prevent undesired conditions from recurring. After a thorough first treatment, regular but small-scale treatments are often sufficient to preserve the native diversity, ecosystem integrity, and aesthetic quality of a site.
Until natural regeneration becomes adequate in forest areas, the planting of trees and shrubs should continue on an as-needed basis to ensure that sufficient advance regeneration is available to replace canopy trees as they die. See the Native Plants and Their Care chapter for more information.
Invasive Plant Species (linked table)
If a link is broken, try searching on the keyword string preceding the link.
Invasive Plant Atlas of the United States (invasiveplantatlas.org, as of 2024)
Pennsylvania Department of Conservation and Natural Resources: Invasive Plants Fact Sheet (dcnr.pa.gov/Conservation/WildPlants/InvasivePlants/InvasivePlantFactSheets/Pages/default.aspx, as of 2024)
Penn State Extension: Invasive and Competing Plants (extension.psu.edu/forests-and-wildlife/forest-management/invasive-and-competing-plants, as of 2024)