Invasive Plants of Massachusetts

Native vs. Introduced Plants

Native plants are species or subspecies that are indigenous to a particular geographic area. They have existed within that area, in natural communities, for thousands of years. During that time they evolved in response to other indigenous species with which they interacted (e.g. animals, fungi, microorganisms and other plants) and in response to local climate, soils, hydrology, and other environmental conditions.

Plants that are native to a given area in North America may be defined as those species (or subspecies) that have existed in that area since pre-colonial times. (In other words, they were present before Europeans first arrived on the shores of North America and began spreading new plant species from other lands.)

A species' native range may shrink or expand over time as a result of changes in environmental conditions (e.g. due to climate change; land development or other changes in land use; the addition or loss of a predator, pathogen or competitor; the movement of continental plates; or cataclysmic events such as meteorite impacts). On rare occasions, a plant may spread naturally to an area far from its native range (e.g., after being carried by strong winds, ocean currents, or by a bird or other [nonhuman] animal).

When people—either intentionally or unintentionally—move a species to an area outside of its native range, that species is described as being "introduced" to the new area. Whereas the natural spread of species to areas far outside of their native range happens only rarely, human introductions of species occur quite frequently. Humans have been moving species to new areas on a regular basis since the onset of European imperialism (~15th century), and the frequency of these introductions has accelerated in recent times with increasing globalization (Meyerson and Mooney 2007).

When a species is introduced to a new area one of the following scenarios may occur:

1. The species fails to persist in its new environment. It may grow for a while but then disappear—it never becomes established in the flora. Botanists refer to introduced species of this type as "waifs."
2. The species persists at its planted location (i.e., typically gardens, farms, or other intensely managed or manicured sites) but does not usually spread into natural communities. This category includes many cultivated garden plants and plants grown for food.
3. The species persists in its new environment and becomes established in natural communities (it is said to be "naturalized"). Note: cultivated garden or agricultural plants are included in this category when they continue to persist after the managed site in which they were planted reverts to a natural environment.

In Massachusetts, botanists have identified 3,293 plant taxa that occur in natural areas of the Commonwealth (Cullina et al. 2011). Of these, 55% are native, 27% are established introduced taxa, and 18% are waifs. If the waifs are excluded (since they are not persistent in the flora), then the percentages are 67% native taxa and 33% introduced taxa (out of a total of 2,712 established taxa).

What are Invasive Plants?

A small percentage of the established introduced taxa have the potential to wreck havoc on local ecosystems and cause environmental and/or economic harm or harm to human health. These taxa are considered to be "invasive." Invasive plants are capable of aggressively invading natural areas and displacing native vegetation. Plants that qualify as invasive typically have high rates of growth, reproduction, and dispersal. They generally have fewer predators, pathogens and parasites, giving them a competitive edge over native species.

In Massachusetts, invasive plants are defined as "non-native species that have spread into native or minimally managed plant systems in Massachusetts. These plants cause economic or environmental harm by developing self-sustaining populations and becoming dominant and/or disruptive to those systems" (MIPAG 2005).

The U.S. government similarly defines invasive species as "a species that is non-native to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health" (Beck et al. 2008).

Why are Invasive Plants a Problem?

When one or more species of invasive plants spreads into a natural community some potentially devastating ecological impacts may occur. In an ecosystem, organisms making up a natural community (or communities) interact with one another and with their physical environment. These biotic and abiotic interactions ultimately result in a stable environment conducive to life on earth. Naturally functioning ecosystems provide us with a number of essential benefits, including food, timber, medicines, clean air and water, climate regulation, and protection from erosion and flooding. Ecosystems are incredibly complex, and when invasive species appear in ecosystems where they don't belong/didn't evolve, they can disrupt the functioning of these systems. We can think of the impacts caused by invasive plants in terms of biotic effects (i.e., their effects on other living things) and abiotic effects (their effects on the physical environment). Because of the complexity of ecosystems, and our limited knowledge of all of the biological interactions and physical processes involved, we can't predict all the ecological impacts that would occur in any given invasion scenario. However, we can generally describe some effects known to occur.

Biotic Effects

The most obvious effect of an invasion is that the invasive plants will displace the native plant species. At first, there will be an increase in the number of plant species within the invaded area, as invasive plants grow alongside the native plants. As the invasive plants continue to spread, there will be a decrease in the abundance of native plant species, and in some areas a loss of these species. Thus areas with incursions of invasive plants may ultimately experience a decrease in plant biodiversity as native species begin to disappear. This loss of native plants occurs due to the superior competitive ability of the invasive species— which are typically not subject to the same selection pressures that keep the populations of native species in check. In some cases the decline of native species may be hastened by hybridization with genetically related invasive species— with the native genotypes becoming scarcer or disappearing, as hybrid plants backcross with the more abundant invasive plants.

Because plants form interactions with many other types of organisms in their native ecosystems, the loss of native plant species will generally have cascading effects on other organisms in the ecosystem (e.g., through loss of food sources, cover, nesting sites, or loss of mutualistic or symbiotic partners). Among other ecological interactions, plants serve as host food plants to insects, which in turn are eaten by other organisms in the food web. Insects are believed to be the primary means by which energy from plants is transferred to higher trophic levels (Tallamy 2019). An estimated 90 percent of insect herbivores require specific plant hosts— they are capable of feeding on only one or a few plant lineages (Forister et al. 2015, Tallamy 2019). The different taxa of native plants found in ecosystems are not equally palatable to insects— a relatively small fraction of plant taxa support the majority of insect herbivores. In the United States, a mere 14% of local plant genera support more than 90% of Lepidoptera (butterfly and moth) species (Narango et al. 2020). Thus the loss or decline of one or a few key native plant species from an ecosystem could potentially lead to the loss or decline of a multitude of herbivorous insect species, with subsequent losses likely reverberating through the food web.

In addition to adversely affecting biodiversity, invasive plants may cause an ecosystem to be simpler (e.g., with fewer interacting species) (Richard et al. 2018, Tallamy 2019), less productive (e.g., as measured by the numbers of animals and/or amount of animal biomass supported by the ecosystem) (Richard et al. 2018, Heleno et al. 2009, Narango et al. 2018), and less stable (i.e., less able to return to its functioning state after a perturbation) (Zimmer 2010, Corbin and D'Antonio 2012, Tallamy 2019). Also, invasive plants may associate with or foster the growth of fungi and microbes that differ from those that predominate under native conditions, resulting in a shift in the microbial and fungal communities in the soil (Kourtev et al. 2002, Gibbons et al. 2017, Zhang et al. 2019). Some invasive plants may alter habitats so as to foster the growth of pest species and potentially increase transmission rates of pathogens. For example, invasive aquatic plants may create habitat for mosquitoes, which may lead to increased transmission rates of malaria (Stone et al. 2018). The invasive Japanese barberry (Berberis thunbergii) provides habitat that may promote the proliferation of Lyme disease-carrying ticks (Williams et al. 2017).

Abiotic Effects

When an ecosystem becomes dominated by invasive plants, a number of effects occur to the physical nature and functioning of the ecosystem. These effects may include the following:

• Changes in soil chemistry. Plants release chemicals into the soil from their roots; they also release leaf litter. As a result, they may affect chemical properties of soil, such as pH, salinity, concentrations and availabilities of nutrients and minerals, and organic content. For example, the invasive Japanese barberry (Berberis thunbergii) and Japanese stiltgrass (Microstegium vimineum) are known to increase soil pH and available nitrate, and these changes in soil conditions may potentially favor the growth of invasive plants over native flora (Kourtev et al. 1999). Plants may also release chemicals that provide them with a competitive advantage over other species. The invasive garlic mustard (Alliaria petiolata) releases an antifungal chemical that ultimately harms the growth of native tree seedlings. The seedlings are affected because the chemical acts on the mycorrhizal fungi with which the seedlings form mutualistic associations (Stinson et al. 2006).
• Changes in the physical structure, properties, or processes of soil, including alterations to biogeochemical cycles. Through the actions of their roots, the chemicals and organic matter that they produce, and their interactions with microbiota, plants can affect soil structure, retention of water and nutrients, rates of soil accumulation and erosion, and rates of decomposition of organic matter. Plants, in conjunction with the microbiota, can affect the cycling of carbon, nutrients, and other compounds and elements. Invasive plants tend to be fast-growing, with high nutrient demands, so they are typically associated with faster litter decomposition rates and increased rates of nutrient cycling (Eviner et al. 2012, Liao et al. 2008). Through their interactions with soil microbiota, they may increase CO₂ efflux from soils (Zhang et al. 2019, Waller et al. 2020).
• Changes in hydrology and effects on water quality, quantity, and flow. Depending on where and how densely they grow, their consumption and retention of water, and filtration capacities, plants can affect water levels, flooding regimes, and water chemistry. Aquatic invasive plants, such as Eurasian water-milfoil (Myriophyllum spicatum) and water-chestnut (Trapa natans) can choke waterways, affecting navigation and recreational uses of water bodies (Capers et al. 2005). When large amounts of aquatic vegetation decay (e.g., from seasonal dieback of invasive aquatic plants), the resultant decline in oxygen levels may harm or kill fish and other aquatic organisms (Catling et al. 2003).
• Alterations to fire regimes. Depending on their chemical composition, abundance, and life histories, plants may affect the frequency, intensity, and duration of fires. For example, the invasive cheatgrass (Bromus tectorum) is known to increase the frequency and size of fires in western North America (D'Antonio and Vitousek 1992). Common reed (Phragmites australis), which dominates many wetlands in New England, may increase the likelihood of marsh fires (Marks et al. 1994).
• Other effects. Some other abiotic impacts may include effects on air quality and microclimate. Plants may affect air quality by filtering out (absorbing or adsorbing) pollutants or by promoting the release of chemicals into the air. When kudzu (Pueraria montana) invades ecosystems, it may substantially increase emissions of nitric oxide from soils, resulting in an increase in ozone pollution (Hickman et al. 2010). Plants may affect microclimates (i.e., climate conditions within small/localized areas), which in turn may affect the types or abundances of organisms that live in that area. For example, Japanese barberry plants (Berberis thunbergii) create microhabitats with high humidity, which promote the growth and survival of ticks (Jones 2011, Williams et al. 2017).

In some instances, invasive plants may alter an ecosystem to such an extent that the ecosystem cannot be readily restored to its former condition. When this happens, the invasive species are said to exert a "legacy effect"— even after removing the invasive plants, the biotic and/or abiotic effects of the invader persist. Therefore, to restore ecosystem functions, some additional action needs to be taken, or perhaps a long period of time needs to pass before the system can return naturally to its desired condition (Corbin and D'Antonio 2012).

Other Effects

In addition to impacting natural/unmanaged ecosystems, invasive plants may adversely affect farmlands, pasturelands, and rangelands, resulting in decreased forage, reduced crop yields, a reduction in the quality of meat, milk, wool and hides, and large economic losses (DiTomaso et al. 2017, FICMNEW and Westbrooks 1998). They may degrade recreational areas (FICMNEW and Westbrooks 1998), clog water intake pipes (Capers et al. 2005), reduce property values (Zhang and Boyle 2010), and often require continual expenditures for management and control. A few invasive plant species pose direct threats to human health, such as giant hogweed (Heracleum mantegazzianum), which can cause burns and blindness (Marrison and Goerig 2007).

Invasive Plants of Massachusetts

The Massachusetts Invasive Plant Advisory Group (MIPAG) has compiled a list of invasive plant species specific to Massachusetts. MIPAG categorizes invasive plants as "invasive" (known invasives), "likely invasive" (species that have invasive potential but do not currently meet all of MIPAG's critera for invasiveness), and "potentially invasive" (Non-native plants that are not yet known to be naturalized in Massachusetts but have potential to be invasive here). Of the approximately 2,712 established plant taxa found in natural areas of Massachusetts, MIPAG classifies approximately 2.8 percent (75 taxa) as "invasive," "likely invasive" or "potentially invasive"; these invasives make up about 8.4 percent of the established introduced taxa (MIPAG 2024, Cullina et al. 2011).

The Massachusetts Department of Agricultural Resources, with input from MIPAG, developed a list of Massachusetts Prohibited Plants, which includes species identified as invasive and/or noxious. It is currently illegal to import, propagate, sell, trade or distribute any of the plants on this list within the Commonwealth of Massachusetts. The Department derives its authority to enact this ban under Massachusetts General Law, including Chapter 128 Section 2 and Sections 16 through 31A.

Invasive plant species found in Massachusetts are listed below. Please note, this is not an all-inclusive list and it is likely to change over time. Not all authorities agree on which species should be considered invasive. In some cases we don't have enough data on a species' abundance and spread to determine whether it is potentially invasive. Also, new species are continually being introduced that may become invasive; and some introduced species that were not problematic in the past, may at a later time spread aggressively. In addition, some species currently considered invasive could lose their invasive tendencies due to biocontrol measures or other factors.

The list below includes (1) all species listed by MIPAG, (2) species that are listed as invasive or potentially invasive (by one or more agencies or organizations) in four or more northeastern states (excluding species not known to occur in New England), and (3) additional species currently found in Massachusetts that are potentially invasive here (see notes below).

A Few Notes about Some of the Invasive Plants

Two species included in the table above are not listed as invasive or potentially invasive in any of the northeastern states— feathertop reed grass (Calamagrostis epigejos) and lily-of-the-valley (Convallaria majalis). However, these introduced species exhibit aggressive growth tendencies in certain natural habitats in Massachusetts and should be considered potentially invasive. Feathertop reed grass tends to proliferate in open, dry, fire-adapted habitats. It is especially problematic in southeastern Massachusetts, where such habitats are very common.

Lily-of-the-valley is widely sold in garden centers and is commonly planted in gardens as a ground cover. It is capable of spreading by rhizomes or by seed (MISIN 2020). The plant is toxic to animals (Colorado State University 2019), and thus is resistant to deer and other grazers. However, the berries, which are "sweet tasting" although poisonous (Colorado State University 2019), are likely spread— at least to some extent— by birds or other wildlife— perhaps ingested by mistake. Lily-of-the-valley grows best in rich, well-drained, moist soil, in part shade (Swearingen and Bargeron 2018a). Once it becomes established in natural areas, it may spread aggressively via rhizomes, forming dense colonies that displace native herbs.

Lesser periwinkle (Vinca minor) is another herbivore-resistant ornamental ground cover that is commonly sold and planted in New England. It is listed as potentially invasive in only two northeastern states (New Hampshire and Rhode Island). Its threat is often underestimated because it is only known to spread vegetatively— via rhizomes. Thus its infestations are generally restricted to areas near landscaped or formerly landscaped properties. However, many of our conservation areas are located near or include currently or formerly landscaped areas. Conservation lands occurring near residential or other developed areas serve as important reservoirs for native species, which otherwise tend to be lost from these locations (due to the cultural mandate for lawns, meticulous removal of leaf litter, and widespread plantings of Eurasian ornamentals). Lesser periwinkle is tolerant of a variety of conditions, but grows best in moist, humus-containing soils in part shade (Missouri Botanic Garden 2020). It is capable of forming extensive mats in forested areas, smothering out native species (Swearingen and Bargeron 2018b).

Other ornamental species that pose threats to New England habitats are chocolate vine (Akebia quinata) and wisteria (Wisteria floribunda and W. sinensis). These woody vines are considered invasive in the eastern U.S., particularly in the mid Atlantic and southeastern states. They are known to form dense thickets, shading out and smothering native vegetation. The exotic wisterias can also kill trees by girdling. Chocolate vine and exotic wisterias are widely available for purchase from nurseries and garden centers. However, in Connecticut, it is now illegal to buy, sell, import, cultivate, move or distribute exotic wisterias (CT Invasive Plants Council 2024). Wisteria floribunda and W. sinensis are known to hybridize. Most or nearly all of the invasive wisterias may consist of hybrids of these two species— at least that appears to be the case in the southeastern U.S. (Trusty et al. 2007). It may be impossible to reliably distinguish between the hybrids and the pure forms of W. floribunda and W. sinensis using morphological characteristics (Trusty et al. 2007).

Waterwheel plant (Aldrovanda vesiculosa) is an aquatic species of recent concern in New England. It is a carnivorous plant that is considered globally endangered (Cross and Adamec 2020). In the late 1980s to early 1990s carnivorous plant growers in Virginia introduced waterwheel plant to ponds on their properties in an attempt to cultivate it (Lamont et al. 2013). Cultivation proved successful, and by the late 1990s waterwheel plant was reportedly growing at several sites in multiple counties in Virginia (Lamont et al. 2013). Encouraged by the success of the Virginia cultivation efforts, a carnivorous plant enthusiast implemented an assisted colonization plan in 1999, in an attempt to help conserve this rare and dwindling species. The assisted colonization involved obtaining waterwheel plants from a location in Virginia and introducing the plants to about a dozen sites in New Jersey and to one site in New York (Lamont et al. 2013). Unfortunately the colonization effort had unintended consequences, with waterwheel plant dominating and threatening native ecosystems at some of the introduction sites. Waterwheel plant now occurs at a limited number of sites in New England, in addition to locations in Virginia, New Jersey and New York.

Of note, two species that are widely regarded as invasive in the northeast have native, similar-looking congeners (species belonging to the same genus). These species are common reed (Phragmites australis) and oriental bittersweet (Celastrus orbiculatus). The fact that there is a native and a non-native taxon of reed in New England was confirmed only recently, through genetic analyses (Saltonstall 2002, 2003a, b). The native species of reed and bittersweet are both considered to be rare in Massachusetts. The native American reed, Phragmites americanus, is on the state's Plant Watch List, and the native American bittersweet, Celastrus scandens, is listed as state-threatened. Hybridization between the native and non-native species of reed apparently occurs rarely, if at all (Saltonstall 2003b). However, the native and introduced bittersweet species have been shown to readily hybridize, potentially threatening the genetic identity of the native bittersweet species and contributing to its decline (Pooler et al. 2002).

As a final note, there are two species that are commonly considered to be invasive in New England whose origin (native vs. introduced) is somewhat questionable or unclear. These are reed canary grass (Phalaris arundinacea) and black locust (Robinia pseudoacacia). Reed canary grass has a circumboreal distribution (Gleason and Cronquist 1991). Its native range is believed to include temperate areas of North America, Europe, and Asia (Waggy 2010). The populations indigenous to different parts of the world are genetically distinct. (These genetically distinct forms of the same species are described using different terms—strains, types, genotypes, and ecotypes (Waggy 2010)— we will use the term "strains" here.) Although reed canary grass is considered to be native to New England, strains from Europe have been introduced to North America (including New England) on multiple occasions; consequently, there has been a large intermixing of genes between native and introduced populations (Lavergne and Molofsky 2007). Reed canary grass has also been widely cultivated. So genes from cultivars are probably also present, contributing to the vigor of invasive populations. Unfortunately the native reed canary grass can't be distinguished from introduced or hybrid strains using morphological characteristics (Merigliano and Lesica 1998). It's possible that pure native populations of reed canary grass may no longer occur in North America (Waggy 2010). Given the uncertainty concerning the origin of reed canary grass in New England, it is perhaps best not to perform control actions for this species unless the population in question appears to be negatively impacting the natural community or ecosystem in which it occurs (Waggy 2010).

Although black locust was introduced to New England and New York, where it was planted at various locations, its native range is believed to be located nearby— extending as far northeast as central Pennsylvania (Huntley 1990). The natural range of black locust is not accurately known; it is mapped as occurring roughly 100 km from the New York border, 270 km from the Connecticut border and 300 km from the Massachusetts border (Huntley 1990). The seeds of black locust often fall near the tree (Sunderland et. al. 2000); however, they may also be dispersed by wind (Robinson and Handel 1993) and possibly by birds (Lambers et al. 2005). Thus it is quite possible that some of the black locust trees found in New England and New York arrived on their own, through natural range expansion. Given the proximity of this species' native range, it's quite likely that many if not most of the species that it forms interactions with in its native range also occur in New England and New York. This appears to be true at least with regards to butterflies and moths that utilize black locust as a larval host plant. In central Pennsylvania—within its native range—black locust serves as a host plant to 64 species of caterpillars; it supports similar but slightly lower numbers of caterpillar species in New England and New York (e.g., 61 species in New York, 59 in Connecticut, and 57 in Massachusetts [National Wildlife Federation 2020]). (Note: as you would expect, as you move further away from its native range, black locust supports fewer species of caterpillars [and one would suspect that as you continue moving further away from its native range, it would have fewer interactions with native species in general and be more likely to behave in an invasive manner.])