Problematic Invasive Plants of the United States: An Illustrated Key

Introduction

Horticulture and nonnative plants

Impacts of Invasive Species

Impacts of Climate Change on Plant Invasions

Problematic Invasive Plants of the United States

Japanese Honeysuckle (Lonicera japonica)

Japanese Barberry (Berberis thunbergii)

Kudzu (Pueraria montana)

Callery Pear (Pyrus calleryana)

Purple Loosestrife (Lythrum salicaria)

Tree-of-heaven (Ailanthus altissima)

Poison hemlock (Conium maculatum)

Japanese knotweed (Polygonum cuspidatum)

Water Hyacinth (Eichhornia crassipes)

Eastern Red Cedar (Juniperus virginiana)

Literature Cited

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Introduction

When examining the impacts of invasive species, plants are often overlooked, despite causing significant ecological, agricultural, economic, and even human health harm. Invasive plants are plants that have been introduced by humans into a novel environment in which they did not evolve and where very few factors which limit their growth and reproduction exist (Westbrooks 1998). Once established, invasive plants cause serious ecological harm and incur economic costs (Westbrooks 1998; Ranney & Ranney 2004). Some invasive plants even impact human health, either directly, such as by poisoning, or indirectly, by providing habitat for or attracting hosts for organisms that carry diseases afflicting humans, such as ticks. Most of the invasive plant species in the United States were introduced for horticulture purposes (Harris et al. 2009). The impact of invasive plants on ecosystems, natural resources, and managed lands is expected to continue to increase with climate change (Bradley et al. 2010; Turbelin & Catford 2021). Climate change will create more favorable conditions for invasive plants to expand into new ranges that were previously inhospitable (Bradley et al. 2010).

Horticulture and nonnative plants

Horticultural practices are a significant contributor to the spread of invasive plant species worldwide historically and today. Most invasive plants were initially introduced to the United States for horticultural purposes. Horticulture continues to be a source of plant invasions with nurseries playing a major role in the introduction of non-native plants (Ranney & Ranney 2004; Harris et al. 2009). The vast majority of the invasive plants in the United States were introduced intentionally as ornamental plants with only a few being accidentally introduced through commerce activities. Most invasive plants have been introduced repeatedly, colonizing new areas with each introduction and increasing the likelihood that the plant will become invasive.

Many of the exotic non-native plants sold in nurseries can easily become invasive in new territories without the limiting factors present in their native range. Once established, invasive plants reproduce rapidly, displacing native plants, and disrupting ecosystems (Ranney & Ranney 2004). The nursery industry is a significant contributor to invasive plants in the United States. Over 85% of invasive woody plants were initially introduced for ornamental use in landscaping (Ranney & Ranney 2004). Repeated introductions of nonnative plant species in lawns, gardens, and landscaping have sped up invasions (Harris et al. 2009).

Family membership and native origin are common indicators of the potential of a plant to become an invasive species in the future (Harris et al. 2009). Species that belong to certain families and come from certain parts of the world have a higher probability of being introduced to new areas through horticultural use. A study of 462 horticultural plants carried by nurseries concluded that native origin and family membership were effective indicators to identify invasive species (Harris et al. 2009; Boyce 2010). Species of plants from Asia tend to be most popular, most likely to be carried by nurseries, and also have the highest potential invasiveness (Harris et al. 2009).

Urban and suburban ecosystems tend to be “hotspots” of invasions due to horticultural practices, the prevalence of areas of disturbance, and transportation networks (Gaertner et al. 2017). Human population density often correlates with species richness in a variety of ways. For example, the species richness of invasive woody plants increases with human population, with areas heavily populated with humans serving as sources for invasive plants that have escaped cultivation (Gaertner et al. 2017; Boyce 2010). Most of the ornamental plants sold in nurseries and used in urban and suburban landscaping are non-native, with plants from Asia and those from families with tendencies to become invasive being more heavily represented (Harris et al. 2009; Reichard & White 2001). The vast majority of the plants we use in horticulture, agriculture, and forestry are not native to the areas in which they are planted and some have escaped cultivation and become invasive (Reichard & White 2001). These invasions come with significant consequences. Invasive plants, animals, and fungi are the leading cause of endangerment to native plant species second only to habitat loss and degradation (Reichard & White 2001).

Impacts of Invasive Species

Invasive plants, along with climate change, are two of the most far-reaching and serious causes of ecosystem disturbance and loss of biodiversity worldwide (Bradley et al. 2010). Invasive plants are one of the largest challenges facing natural resource management. They can alter ecosystems, degrade ecosystem services, and impact water and soil quality (Pearson & Ortega 2009). Invasions by nonnative plants also result in the loss of habitat and the loss of forage for wild herbivores and livestock and impact ecosystem services for humans, including agricultural and forestry resources (Pearson & Ortega 2009). The economic cost of invasive plants on managed lands in the United States is billions of dollars per year (Bradley et al. 2010).

In addition to these impacts, invasive plants can decrease species richness, alter nutrient cycling, compete with native plants for pollinators, spread disease, impact fire regime, and interbreed with native plants (Pearson & Ortega 2009; Boyce 2010). Invasive plant species compete with native plants, including threatened and endangered species, for space, nutrients, sunlight, and other resources. This competition leads to changes in plant populations and a decrease in plant biodiversity (Boyce 2010; Bradley et al. 2010; Rockwell-Postel et al. 2020). Animals are impacted by invasive plants through feeding interactions and changes to the amount of available food, structural changes to ecosystems, such as the reduction of habitat, and poisoning via toxic plants (Rockwell-Postel et al. 2020).

Recent studies show that the southern United States is an area of high concern due to the spread of nonnative invasive plant species and their displacement of native species (Oswalt & Oswalt 2011). Southern forests are experiencing invasions of nonnative plants that reduce forest productivity, degrade biodiversity, and impact wildlife habitats (Miller et al. 2015). There are currently more than 50 non-native plants aggressively invading the 13 Southern states and more with the potential to become invasive (Miller et al. 2015; Oswalt & Oswalt 2011).

Impacts of Climate Change on Plant Invasions

Plant invasions and climate change are major environmental issues threatening ecosystems globally. The two issues may also have important interactions, and climate change is expected to increase both plant invasions and their impacts. Climate change facilitates plant invasion by changing environmental conditions, increasing environmental disturbances via extreme climatic events, and human responses to climate change impacts (Turbelin & Catford 2021). While all plants will presumably be affected by climate change, the environmental impacts of climate change will likely be favorable to invasive plants at the expense of native plants (Turbelin & Catford 2021). Studies indicate that higher atmospheric concentrations of CO2 increase the competitiveness of invasive plants over native species (Bradley et al. 2010). Further, the ways in which invasive plants impact the environment, such as through changing soil properties and chemistry and altering hydrology and fire regimes, can contribute to or intensify the effects of climate change (Turbelin & Catford 2021).

Plant populations are expected to migrate to higher latitudes and elevations as climate change intensifies (Hickman & Lerdau 2013). As winters continue to warm, more species will expand their ranges north (Hickman & Lerdau 2013; Rockwell-Postel et al. 2020). Warmer summers will make lower latitudes and elevations less hospitable and less habitable. In addition to native range shifters, invasive plants are projected to shift their ranges to higher latitudes and elevations as well (Hickman & Lerdau 2013).

This climate change-driven migration will create additional hotspots where invasive species establish and spread. The Southeast U.S. has been an area of high risk for invasions for decades and the Northeast is expected to become a hotspot as warming increases and the climate continues to change (Hickman & Lerdau 2013; Rockwell-Postel et al. 2020). The negative impacts of invasive species in the Southeastern states could expand into the Northeastern states as invasive species expand their ranges (Rockwell-Postel et al. 2020). It is expected that climate change will shift the risk of plant invasions several hundred kilometers north by the end of the century (Bradley et al. 2010).

Problematic Invasive Plants of the United States

Japanese Honeysuckle (Lonicera japonica)

Figure 1. Japanese Honeysuckle

Photo by: Chuck Bargeron, University of Georgia, Bugwood.org

Description

Japanese honeysuckle (Lonicera japonica) is a perennial woody vine that was first introduced for ornamental purposes because of its fragrant flowers and agricultural purposes for erosion prevention. However, it soon began spreading into wild areas. Japanese honeysuckle grows on the edges of woodlands and disturbed areas. In northern areas, it is deciduous and in warm areas, it is semi-evergreen or evergreen. It grows rapidly, covering native plants and winding around them, smothering and killing vegetation. It has tiny seeds that are transported by birds and other animals, further expanding its range. Japanese honeysuckle prefers full to partial sun but is also able to grow in shady environments.

Invasion

Japanese honeysuckle was first introduced to Long Island in the early 1800s for ornamental and agricultural purposes. After repeated introductions, Japanese honeysuckle began to spread in the wild. It can survive in a wide variety of habitats and is extremely fast-growing in full sun. It is invasive throughout most of the United States, including Hawaii, and is the most well-established ornamental vine in the country.

Map 1. Distribution of Japanese Honeysuckle (Lonicera japonica) in the United States and Canada.

Impact

Ecological, Human health

Japanese honeysuckle decreases the biodiversity of ecosystems and outcompetes native plants for light, space, and other resources. It grows rapidly and reproduces by sending out runners and tiny, easily dispersed seeds. Japanese honeysuckle vines grow around other vegetation, strangling shrubs and forming dense mats that smother everything beneath them. The thick vegetation prevents the growth of native plants, shrubs, and trees. Japanese honeysuckle invasions leave ecosystems susceptible to invasions by other invasive species, further decreasing biodiversity.

While Japanese Honeysuckle is not harmful to humans, it does harbor ticks that are vectors for a variety of diseases that infect humans. As one of the first green plants in the spring and evergreen in some areas, honeysuckle attracts deer when there is little other forage. While deer do not carry Lyme disease, they are a reservoir for black-legged ticks (Ixodes scapularis), also known as deer ticks, that carry diseases that infect humans including Lyme disease and Deer Tick Virus. Populations of black-legged ticks tend to increase in areas with Japanese honeysuckle. As it spreads into new areas, black-legged ticks spread with it. The expansion of Japanese honeysuckle and other invasive plants that provide habitat for ticks has led to an increase in tick-borne infections across the United States.

Mitigation/Eradication

Japanese honeysuckle is well-established across most of the United States, making controlling this invasive species challenging. Avoid planting Japanese honeysuckle and remove existing plants.

Like other woody invasive species, Japanese honeysuckle can be challenging and time-consuming to remove. Small patches of seedlings and small vines can be hand-pulled while large patches will require mowing at least twice per year along with the application of herbicides. The climbing vines can be difficult to remove by mowing and may need to be cut. When cutting established plants, apply glyphosate to the stump and cut branches to kill the plant and prevent regrowth.

Chemical herbicides such as glyphosate or triclopyr can be used for foliar application. Additional monitoring and application of herbicides will be necessary if using this method to ensure that regrowth is eliminated. There are no biological treatments available to control or eradicate Japanese honeysuckle.

Sources:

Allan BF, Dutra HP, Goessling LS, Barnett K, Chase JM, Marquis RJ, Pang G, Storch GA, Thach RE, Orrock JL. 2010. Invasive honeysuckle eradication reduces tick-borne disease risk by altering host dynamics. Proceedings of the National Academy of Sciences of the United States of America 107:18523–18527. Available from http://dx.doi.org/10.1073/pnas.1008362107.

Boyce RL. 2010. Invasive shrubs in Kentucky. Northeastern Naturalist 17. Available from http://dx.doi.org/10.1656/045.017.m701.

University of Florida IFAS. (n.d.). Lonicera japonica Japanese honeysuckle. Available from https://plants.ifas.ufl.edu/plant-directory/lonicera-japonica/ (accessed March 2022).

Figure 1. Japanese honeysuckle

Chuck Bargeron. University of Georgia. Available from https://www.forestryimages.org/browse/detail.cfm?imgnum=1150069 (Accessed March 2022).

Map 1. Distribution of Japanese Honeysuckle (Lonicera japonica) in the United States and Canada.

EDDMapS. 2022. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/

(Accessed March 2022).

Japanese Barberry (Berberis thunbergii)

Figure 2. Japanese Barberry

Photo by: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Description

Japanese barberry (Berberis thunbergii) is an invasive, woody shrub that grows 3 to 6 feet tall and 3 to 6 feet wide. It has small, oval, leaves that are green to dark red or purple. The stems have spines, giving the plant protection against grazing herbivores. Japanese barberry has small, yellow flowers in the spring and red fruit that lasts into fall and winter. Birds and other animals feed on the fruit and then deposit seeds as they move, introducing Japanese barberry into new areas and expanding its area of invasion. Barberry is a dense plant with many small twigs and branches that prevent other plants from growing in or under it.

Invasion

Japanese barberry was first introduced in the United States in 1875 as an ornamental plant. It was promoted as a replacement for Common barberry (Berberis vulgaris), also known as European barberry, which is a host for black stem rust, a fungal disease that afflicts wheat crops. Japanese barberry escaped cultivation and is now naturalized in the wild. It has spread throughout the Northern United States, into the Southeast, the plains, and Washington state.

Map 2. Distribution of Japanese barberry (Berberis thunbergii) in the United States and Canada.

Impact

Ecological, Human Health

Japanese barberry is capable of significantly disrupting the ecosystems it invades. It forms extremely dense stands, competing with both native herbaceous plants and trees. The thickets have sharp spines which prevent grazing by herbivores, protecting it and preventing its control, leading to its rapid spread. Like many invasive plants, Japanese barberry spreads in disturbed areas and along the edges of woodlands. However, it also does well deep in the woods and can come to dominate this ecosystem.

Japanese barberry harbors black-legged ticks (Ixodes scapularis), also known as deer ticks, that are vectors for Lyme disease, Deer Tick Virus, and other tick-borne diseases. Recent research has indicated that populations of black-legged ticks increase in areas with dense Japanese barberry thickets. As Japanese barberry spreads into new areas, so too do black-legged ticks, leading to an increase in Lyme disease infections in these areas.

Mitigation/Eradication

Seedlings and young plants can be pulled by hand. Hand pulling will not work on older, well-established plants. Thick gloves should be worn to protect from the spines. Barberry can also be pulled out using a weed wrench or dug out. Subsequent treatment may be needed to pull runners that start new plants.

Established thickets can be removed mechanically. Professional mowing equipment is necessary as established plants will not be cut by lawnmowers. Plants will need to be moved repeatedly to kill the plant and prevent regrowth.

Chemical herbicides can be applied as a foliar treatment in the spring or late summer. Glyphosate can be applied directly to cut stumps and branches or as a basal bark treatment. There is currently no biological control method available.

Sources:

Abbey T. 2017, April. The Invasive Japanese Barberry. Available from https://extension.psu.edu/the-invasive-japanese-barberry (accessed March 2022).

Natural Resources Conservation Service. (n.d.). Brush Management – Invasive Plant Control Barberries. USDA. Available from https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs144p2_015219.pdf (accessed March 2022).

U.S. Department of Agriculture. (n.d.). Japanese Barberry. Available from https://www.invasivespeciesinfo.gov/terrestrial/plants/japanese-barberry (accessed March 2022).

Ward JS, Worthley TE, Williams SC. 2009. Controlling Japanese barberry (Berberis thunbergii DC) in southern New England, USA. Forest ecology and management 257:561–566. Available from https://www.sciencedirect.com/science/article/pii/S0378112708007238.

Figure 2. Japanese Barberry

Leslie J. Mehrhoff, University of Connecticut. Available at https://www.forestryimages.org/browse/detail.cfm?imgnum=5456971 (Accessed March 2022).

Map 2. Distribution of Japanese barberry (Berberis thunbergii) in the United States and Canada.

EDDMapS. 2022. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/ (Accessed March 2022).

Kudzu (Pueraria montana)

Figure 3. Kudzu

Photo by: David J. Moorhead, University of Georgia, Bugwood.org

Description

Kudzu (Pueraria montana) is a semi-woody perennial trailing and climbing vine. It grows prolifically–up to 60 feet in a season and up to a foot a day in the summer. The vines are supported by a massive root system. Roots can grow up to 12 feet deep and weigh several hundred pounds. Kudzu grows well in a variety of soils and forms a symbiotic relationship with bacteria that fixes nitrogen in the soil. Vines grow in all directions and form root crowns approximately every foot that can become separate plants. Climbing vines can grow almost a foot in diameter.

Invasion

Kudzu is native to China, Taiwan, Japan, and India. In the late 1800s, kudzu was imported and sold as an ornamental plant in the south. By the early 1900s, it was promoted as forage for livestock. In the 1930s, it was used for erosion control. By 1946, over 3 million acres of kudzu had been planted in the south. Unsurprisingly, it had become a nuisance plant by 1950 and was well on its way to becoming invasive in the southern United States.

Kudzu has invaded most of the eastern United States and the Pacific Northwest. It is currently expanding its range, due in part to climate change. Climate change has the potential to increase invasions by plant species as winters warm and more favorable conditions allow invasive plants to expand into new areas.

Map 3. Distribution of Kudzu (Pueraria montana) in the United States and Canada.

Impact

Ecological, Forestry, Agricultural

Kudzu can have significant ecological impacts on native plant biodiversity due to the density of its vines. Kudzu grows over top of native vegetation, shading it completely and preventing its growth. Because kudzu is a nitrogen-fixing plant, it can also impact nitrogen cycling in ecosystems.

While kudzu grows aggressively in the Southeastern United States, impacting native plant biodiversity, less is known about its impact in Midwestern and Mid-Atlantic states. It seems to grow slower and differences in seasonal temperature changes, soil composition, and growing season length may limit the severity of kudzu’s ecological and biogeochemical changes. More research is needed to determine the ecological impacts of kudzu in its recently invaded areas.

Kudzu negatively impacts forest resources by weighing down young trees and smothering seedlings. As a host for pest insects and agricultural diseases, it can also negatively impact agricultural resources.

Mitigation/Eradication

Kudzu is difficult to control due to its ability to grow up to a foot a day and send out underground runners that can form new plants. Containing a kudzu invasion will require a combination of mitigation methods. Complete eradication of kudzu in an area will require repeated, persistent treatments. In order to eradicate kudzu, every individual plant must be killed.

Mechanical methods of moving and cutting vines along with applying chemical herbicides can slow down their spread. Grazing is frequently used as a biological control method. All grazing animals can feed on kudzu, but cattle provide the most successful control. Grazing can eliminate an invasion in just a few years if it is consistently and closely grazed. Plants that survive grazing can be treated with herbicides. Repeated applications of chemical herbicides can be used to eradicate kudzu.

While small-scale eradication of kudzu by landowners is possible, complete eradication from the United States is not feasible.

Sources:

Bradley BA, Wilcove DS, Oppenheimer M. 2010. Climate change increases risk of plant invasion in the Eastern United States. Biological invasions 12:1855–1872. Available from https://doi.org/10.1007/s10530-009-9597-y.

Loewenstein NJ, Enloe SF, Everest JW, Miller JH, Ball DM, Patterson MG. 2014. The history and use of kudzu in the southeastern United States. System ACE, editor.

Miller JH. 1996. Kudzu eradication and management. In: Hoots:, Diane; Baldwin, Juanitta, comps. , eds. Kudzu the vine to love or hate. Kodak, TN: Suntop Press: 137-149. Available from https://www.fs.usda.gov/treesearch/pubs/985.

Figure 3. Kudzu

Moorhead, David. University of Georgia. Available at https://www.forestryimages.org/browse/detail.cfm?imgnum=1162002 (Accessed March 2022).

Map 3. Distribution of Kudzu (Pueraria montana) in the United States and Canada.

Callery Pear (Pyrus calleryana)

Figure 4. Callery Pear

Photo by: Chuck Bargeron, University of Georgia, Bugwood.org

Description

Callery pear (Pyrus calleryana) is an ornamental pear tree native to China, Korea, and Taiwan. It is one of the most widely used landscaping trees in the United States, despite its status as an invasive species. Varieties include popular cultivars Bradford Pear, Redspire, and Cleveland Select. The Bradford cultivar was introduced to the United States in 1908 in an unsuccessful attempt to produce pear trees resistant to the fire blight disease. Over 20 ornamental cultivars have been developed since the 1950s. The trees are fast-growing, have small fruit, and white blooms in the spring.

Invasion

After decades of extensive horticultural use, Callery pear escaped cultivation and started to appear in disturbed areas, fields, and grasslands. It becomes established quickly and out-competes native plants, transforming these ecosystems from grasslands into monoculture forests. Callery pears have also begun to invade woodlands and forests. The plant takes over the understory by growing fast and blocking sunlight to native, slower-growing plants and trees. Callery pear has escaped cultivation in at least 25 states and has been recently reported as a new invasive species in California, Michigan, Missouri, New Jersey, and West Virginia. There is evidence that the species is rapidly invading much of its horticultural range, especially in the Eastern United States.

Map 4. Distribution of Callery Pear (Pyrus calleryana) in the United States and Canada.

Impact

Ecological, Forestry, Agricultural

Callery Pear poses a significant threat to terrestrial ecosystems. It disrupts plant communities and displaces native plants. It is capable of hybridizing and reproducing in the wild. It spreads rapidly, taking over grasslands and forests. Callery pear is not only a threat to native plants but disrupts and transforms entire ecosystems along with resources and ecosystem services that humans rely upon. It is also an agricultural nuisance that easily spreads into crop fields. Callery pear is a species of high concern due to its widespread use in landscaping and its rapid spread once it has escaped cultivation. It is an increasing risk across the United States because it threatens the stability of ecosystems and ecosystem services including forestry and agricultural resources.

Mitigation/Eradication

Despite its invasive nature, many varieties of Callery pear are still widely sold and planted across the United States. There are currently no federal regulations to prevent their import, sale, or purchase, however, several states have added the Callery Pear to their invasive species lists.

Seedings can be hand-pulled. However, caution should be used removing saplings and older trees as they often grow protective thorns on the trunk and branches. Trees should be cut down and the stumps treated with herbicide. Tree bounties have become a popular means of removing Callery pear trees from urban and suburban landscaping and replacing them with native trees. Prescribed burning can help to control Callery pear invasions in wild prairie ecosystems.

Sources:

Culley TM, Hardiman NA, Hawks J. 2011. The role of horticulture in plant invasions: How grafting in cultivars of Callery pear (Pyrus calleryana) can facilitate spread into natural areas. Biological invasions 13. Available from http://dx.doi.org/10.1007/s10530-010-9864-y.

Gassett J, Alexy K, Mahala M. 2008. Kentucky Terrestrial Nuisance Species Management Plan. Kentucky Department of Fish and Wildlife Resources. Available from https://fw.ky.gov/More/Documents/KYTerrestrialNuisanceSpeciesPlan.pdf (accessed March 2022).

Miller JH, Chambliss EB, Loewenstein NJ. 2015. A Field Guide for the Identification of Invasive Plants in Southern Forests. United States Department of Agriculture, Forest Service, Southern Research Station. Available from https://play.google.com/store/books/details?id=lgvYvAEACAAJ.

Missouri Department of Conservation. 1/2018. Callery Pear. Missouri Department of Conservation. Available from https://mdc.mo.gov/sites/default/files/2020-05/Callerypearinvasive.pdf (accessed March 2022).

Vincent MA. 2005. On the spread and current distribution of Pyrus calleryana in the United States. Available from http://dx.doi.org/10.2179/0008-7475(2005)070[0020:OTSACD]2.0.CO;2.

Figure 4. Callery Pear

Bargeron, Chuck. University of Georgia. Available from https://www.forestryimages.org/browse/detail.cfm?imgnum=2308117 (Accessed March 2022).

Map 4. Distribution of Callery Pear (Pyrus calleryana) in the United States and Canada

Purple Loosestrife (Lythrum salicaria)

Figure 5. Purple Loosestrife

Photo by: Becca MacDonald, Sault College, Bugwood.org

Description

Purple loosestrife (Lythrum salicaria) is a perennial flowering plant native to Europe, Asia, and Northern Africa that has invaded throughout North America. It can grow up to 6 feet tall and a single clump can be 5 feet in diameter with magenta flowers. Purple loosestrife grows well in any habitat with wet soil, including wetlands, marshes, ponds, lakeshores, stream and river banks, and ditches. It is also able to establish in damp soils, allowing it to spread to meadows and pastures.

The ability of Purple loosestrife to grow in a wide variety of habitats, along with its high production of seeds, make it an aggressive invader. Purple loosestrife produces a large volume of light, tiny seeds that can survive for years before germinating and can even germinate in standing water. The small seeds are dispersed in a variety of ways–by floating in water, by wind, and through human activity. The roots of Purple loosestrife also produce many buds allowing for a secondary means of reproduction.

Invasion

Purple loosestrife is native to Europe and Asia. It was introduced to the United States multiple times by different means. It was first introduced in the early 1800s from a ship ballast contaminated with the seeds. It was also planted as a medicinal herb used to treat diarrhea, dysentery, and digestive issues and as an ornamental flower. By the 1830s, it was already naturalized and well-established in New England. It continued to spread along waterways and by humans. It has invaded almost all US states with the highest concentration along the East Coast with scattered populations in the Western states. Purple loosestrife spreads rapidly with a central New York wetland experiencing a 1000x increase in acreage containing purple loosestrife over 23 years.

Map 5. Distribution of Purple loosestrife (Lythrum salicaria) in the United States and Canada.

Impact

Ecological

Purple loosestrife out completes other plants and can rapidly displace native plant species once established. Purple loosestrife forms a dense canopy that prevents the growth of other plants, and its abundant seed production outpaces that of native plants.

Purple loosestrife also impacts wildlife. Stands of Purple loosestrife can prevent nesting of native waterfowl, and other aquatic animals, such as amphibians and turtles may be impacted. While dense stands of Purple loosestrife can negatively impact wetland habitats for some native wetland birds, others seem to prefer nesting in it over other aquatic plants and some have been observed using the plant to build their nests.

Purple loosestrife can impact ecosystem structure and functioning. The density of stands of the plant traps sediment and raises the water table. This can change waterways and impede waterflow. Purple loosestrife invasions can even contribute to the succession of diverse meadows to willow forests. Purple loosestrife does not burn well in seasonal fires. This fire resistance combined with its ability to vegetatively reproduce from buds gives it an advantage over other riparian plants such as sedges that do burn, leading to dramatic changes in plant communities.

Mitigation/Eradication

In order to mitigate the spread of Purple loosestrife, boats and equipment used in water should be thoroughly cleaned and drained to ensure that no seeds are unintentionally transported. While Purple loosestrife is a banned plant, supposedly sterile cultivars in a variety of flower colors are still sold in nurseries in some states. No cultivars of Purple loosestrife should be planted due to the risk of it escaping cultivation.

Small infestations can be removed manually by hand pulling. The crown of the root must be fully removed to prevent regrowth. Mechanical management using hand held tools such as shovels to dig out plants and weed pullers can be effective for small areas. Mowing is not recommended because plants will regrow and seeds will be spread, worsening the invasion.

Chemical control using herbicides such as glyphosate or triclopyr is effective for small infestations, but these chemicals are not suitable for large infestations as they are non-selective. Aquatic specific herbicide formulations must be used for aquatic invasions. Chemical application must be repeated for multiple years to eliminate all adult plants and seeds.

Biological control methods can be used on invasions larger than an acre using four species of beetles which are successful and well-established biological control agents. Two lead feeding beetles Galerucella calmariensis and G. pusilla defoliate plants, target apical buds, and diminish seed production of Purple loosestrife. The weevil species Nanophyes marmoratus feeds on the flower buds and seeds and the weevil species Hylobius transversovittatus feeds on the foliage and roots of Purple loosestrife.

Sources:

Cornell University Cooperative Extension, Sea Grant. 2019, July 2. Purple Loosestrife. Available from http://nyis.info/invasive_species/purple-loosestrife/ (accessed March 9, 2022).

Reeves K. 2010. Exotic Species: Purple Loosestrife. Available from https://www.nps.gov/articles/purple-loosestrife.htm (accessed March 2022).

U.S. Department of Agriculture, U.S. Forest Service. (n.d.). Lythrum salicaria. Available from https://www.fs.fed.us/database/feis/plants/forb/lytsal/all.html (accessed March 2022).

Figure 5. Purple Loosestrife

MacDonald, Becca. Sault College. Available from https://www.forestryimages.org/browse/detail.cfm?imgnum=5530447 (Accessed March 2022).

Map 5. Distribution of Purple loosestrife (Lythrum salicaria) in the United States and Canada.

Tree-of-heaven (Ailanthus altissima)

Figure 6. Tree-of-heaven

Photo by: Chuck Bargeron, University of Georgia, Bugwood.org

Description

Tree-of-heaven (Ailanthus altissima) is a highly invasive and rapidly growing deciduous tree native to China and Taiwan. Tree-of-heaven can grow up to 80 feet tall and up to 6 feet in diameter. It grows in dense colonies of clones. Female trees can produce as many as 300,000 wind-dispersed seeds per year, allowing for rapid spread.

Established trees send up root suckers that allow them to spread as far as 50 feet. A cut or damaged tree will sprout from the stump and roots, making them extremely difficult to kill. Trees start producing seeds at 2 years old. Tree-of-heaven produces chemicals that prevent other plants from growing near them.

Invasion

Tree-of-heaven was first introduced into the United States in the late 1700s in Philadelphia. It was later introduced to the West Coast in the 1850s. It was planted as an ornamental shade tree able to tolerate poor soil. It was planted down the East Coast from New York City to Washington D.C. By 1900, Tree-of-heaven had already fallen out of favor because of its bad odor, root sprouting, and weediness, but by then it was already spreading in urban, agricultural, and forested areas. It has spread to almost all U.S. states except for Alaska, Hawaii, and the High Plains.

Tree-of-heaven can grow almost anywhere, in poor or fertile soil, in full sun or partially shaded, and along streams and rivers. It has spread throughout urban and suburban areas, and in disturbed areas along roads, railroads, fences, edges of woodlands, and forest clearings. It will quickly colonize disturbed areas, but cannot complete in shaded forests.

Map 6. Distribution of Tree-of-heaven (Ailanthus altissima) in the United States and Canada.

Impact

Ecological, Forestry, Agricultural

Tree-of-heaven is an aggressive invader of ecosystems. It has an extensive root system that produces suckers and it is capable of producing a huge volume of seeds per year. Because of its high level of reproduction and extremely fast growth rate, tree-of-heaven can dominate ecosystems, outcompeting native plants for space, sunlight, water, and other resources. Tree-of-heaven also produces allelopathic chemicals in its bark, leaves, and roots that prevent other plants from becoming established nearby, effectively eliminating any competition from native plants. Because of its rapid spread and growth rate, and its ability to prevent the growth of competing plants, tree-of-heaven can completely transform an ecosystem in a short period of time.

The ability of the tree-of-heaven to invade a variety of habitats means that it is not only a threat to ecosystems, but forestry and agricultural resources. Tree-of-heaven is also the preferred host of the spotted lanternfly, an emerging invasive insect that damages crops including grapes, fruit trees, hops, and ornamental plants.

Mitigation/Eradication

Tree-of-heaven invasions are extremely difficult to control. Because of the adaptability of the tree to many environments, its multiple means of reproduction, and rapid growth rate, the effectiveness of mitigation methods are limited. Due to the extensive root system and the ability to send out shoots across long distances, mechanical methods are less effective than chemical methods. This is because once cut or damaged, a tree-of-heaven will send off many suckers, worsening the invasion. For this reason, cutting and prescribed burning are not recommended. Seedlings can be hand-pulled or dug up in small patches, but care must be taken to ensure that all roots are removed to prevent new plants from growing.

Chemical treatment with herbicides is the most effective treatment method for tree-of-heaven invasions. Herbicides must be applied to the roots during the growing season. Mature trees along with root suckers need to be treated in at least two consecutive years. Sites must be monitored for regrowth. Foliar treatments along with basal bark and stem cut applications are effective, but cut stump application is not. Herbicides with glyphosate or triclopyr are safest and most effective because they pose low risk to nontarget plants through root uptake. Even with subsequent applications of herbicides, tree-of-heaven is difficult to control. Well established stands of tree-of-heaven can only be eradicated through repeated treatments and vigilant monitoring of sites.

Sources:

Jackson DR, Wurzbacher S, Gover A. (n.d.). Tree-of-Heaven. Available from https://extension.psu.edu/tree-of-heaven#:~:text=To%20control%20tree%2Dof%2Dheaven,will%20only%20injure%20aboveground%20growth. (accessed March 2022).

Iowa State University Extension and Outreach. (n.d.). Tree of Heaven Invasive Species Profile. Available from https://naturalresources.extension.iastate.edu/forestry/iowa_trees/trees/tree_of_heaven.html (accessed March 2022).

U.S. Forest Service. 2014. Field Guide for Managing Tree-of-heaven in the Southwest. U.S. Department of Agriculture. Available from https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5410131.pdf (accessed March 2022).

WA State Noxious Weed Control Board. 2020. Tree-of-Heaven and Spotted Lanternfly. WA State Noxious Weed Control Board. Available from https://www.nwcb.wa.gov/pdfs/Tree_of_heaven_and_spotted_lanternfly_small.pdf (accessed March 2022).

Figure 6. Tree-of-heaven

Bargeron, Chuck. University of Georgia. Available from https://www.forestryimages.org/browse/detail.cfm?imgnum=1150026 (Accessed March 2022).

Map 6. Distribution of Tree-of-heaven (Ailanthus altissima) in the United States and Canada.

Poison hemlock (Conium maculatum)

Figure 7. Poison Hemlock

Photo by: Eric Coombs, Oregon Department of Agriculture, Bugwood.org

Description

Poison hemlock (Conium maculatum) is a problematic invasive species because it is a lethal plant that is extremely toxic to humans and livestock. The entire plant is poisonous, including the stems, leaves, and fruit with most of the toxins concentrated in the roots, lower stem, and seeds. It is identifiable by its hollow green stem with purple spots and streaks, fern-like leaves, and umbrella-shaped clusters of small, white flowers. The plant has a musty “mousy” smell.

Invasion

Poison hemlock is native to Europe and was introduced to the United States as an ornamental plant called “winter fern” in the 1800s. As a member of the carrot family, it has been mistaken for wild carrot, parsley, and parsnips, with deadly consequences. Once introduced, it quickly became established and spread rapidly throughout all states except Hawaii. Poison hemlock infests shady disturbed areas with damp soil, including roadsides, along with fields and meadows, ditches, marches, pasture, rangeland, and the edges of cultivated fields.

Map 7. Distribution of Poison hemlock (Conium maculatum) in the United States and Canada.

Impact

Ecological, Human Health, Agricultural

Poison hemlock invasions impact ecological stability, have negative consequences for human health and incur agricultural costs and losses. It reduces flora and fauna species biodiversity, degrades habitat available for wildlife, reduces forage available for cattle and horses, contaminates hay, and can even decrease land value.

Due to the toxic nature of poison hemlock, it is frequently responsible for livestock poisoning. The concentration of toxins is highest in the leaves in the spring, so poisoning is most likely to occur in early spring when little other vegetation is available. While drying reduces the potency of the toxins, hay containing dried poison hemlock plants is not safe. Chronic non-lethal exposure during pregnancy can lead to livestock born with birth defects and malformations. Poison hemlock invasions cause increased agricultural costs through hay contamination, land degradation, mitigation expenses, and decreased profits through loss of forage and loss of livestock to poisoning.

Poison hemlock is also a human health risk. There have been recorded cases of human poisonings by invasive poison hemlock. Human poisoning has occurred due to mistaking poison hemlock for anise, parsley, or parsnips and eating the seeds, leaves, and roots. Even very small amounts can cause respiratory paralysis, coma, and death if ingested and not treated within 3 hours.

Mitigation/Eradication

The transportation, propagation, and sale of this plant is prohibited. When removing poison hemlock, both the above and below group parts of the plant must be destroyed using mechanical and chemical methods. Biological control in the form of grazing livestock is not possible due to the toxicity to animals and poison hemlock have few insect predators.

Calling a professional is recommended due to the high toxicity of the plant. Protective gloves, clothing, and eyewear must be worn to ensure that toxic sap does not get on the skin or in the eyes. First-year plants without flowers can be treated with chemical herbicides and frequently mowed. Herbicides will only work on small plants in their first year. In their second year, plants will produce seeds and can spread quickly. If plants are manually removed by hand-pulling or digging, the roots must be fully removed. Once removed, place whole plants in plastic bags. Plants can be sprayed with herbicides while flowering or the entire plant can be dug out, but care should be taken to ensure that flowers are not left on the plant or spread onto the ground. Flower/seed heads can be cut off and placed in a plastic bag. Do not mow without first removing flowers or seed heads. Dead plants can remain toxic for up to 3 years, so continue to use protective equipment when removing dead plant material.

Sources:

Agricultural Extension Service/University of Tennessee. 1980, January. Poisonous Plants of the Southern United States. Available from https://carteret.ces.ncsu.edu/wp-content/uploads/2013/05/Poisonour-Plants-of-the-Southern-United-States.pdf?fwd=no.

Gupta A, Rager A, Weber MM. 2020. Poison hemlock. Available from https://extension.umn.edu/identify-invasive-species/poison-hemlock#:~:text=Poison%20hemlock%20(Conium%20maculatum)%20is,edges%2C%20farms%20and%20bike%20paths. (accessed March 2022).

Herron L. 1972. Some Plants of Kentucky Poisonous to Livestock. Available from http://www2.ca.uky.edu/agcomm/pubs/id/id2/id2.htm.

US Forest Service. 2015. Field Guide for Managing Poison Hemlock in the Southwest. United States Department of Agriculture. Available from https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5410121.pdf (accessed March 3, 2022).

Figure 7. Poison Hemlock

Coombs, Eric. Oregon Department of Agriculture. Available from https://www.forestryimages.org/browse/detail.cfm?imgnum=5435830 (Accessed March 2022).

Map 7. Distribution of Poison hemlock (Conium maculatum) in the United States and Canada.

Japanese knotweed (Polygonum cuspidatum)

Figure 8. Japanese Knotweed

Photo by: Jan Samanek, Phytosanitary Administration, Bugwood.org

Description

Japanese knotweed (Polygonum cuspidatum) is an extremely fast-growing herbaceous member of the buckwheat family native to Japan, China, and Korea. It has bamboo-like stems and is often mistaken for bamboo. It is even sometimes referred to as Japanese bamboo. Japanese knotweed can grow up to 15 feet tall, growing more than 3 inches per day in the spring and summer. It is an extremely versatile plant tolerating sun and deep shade, high and low temperatures, and high soil salinity. Despite being commonly found along streams and rivers, it is drought tolerant. Like many invasive plants, it quickly invades disturbed areas. Like other successful invasive plants, Japanese knotweed spreads by both rhizomes and seeds, allowing it to rapidly reproduce, shade out, and prevent the growth of native plants.

Invasion

Japanese knotweed was introduced as an ornamental plant in the late 1800s. It was also historically used for erosion control. Japanese knotweed prefers disturbed areas, and once established, can spread rapidly. It forms large monoculture stands, outcompeting native plants and disturbing plant communities. By the late 1930s, Japanese knotweed was already becoming problematic and was viewed as a nuisance species.

Japanese knotweed has spread across most of the United States, except for the Southwest, Florida, and Hawaii. Because of its heat and drought tolerance, it has the potential to continue spreading and to tolerate the extremes of climate change well.

Map 8. Distribution of Japanese knotweed (Polygonum cuspidatum) in the United States and Canada.

Impact

Ecological

Japanese knotweed is fast-growing and spreads rapidly, forming dense thickets of vegetation that shade out and prevent the growth of native plant species. Infestations can reduce plant species diversity and reduce wildlife habitat. Japanese knotweed invasions lead to other environmental issues, such as erosion and bank instability in riparian areas, due to the lack of other vegetation. Once it becomes established in an area, Japanese knotweed is difficult to eradicate.

Mitigation/Eradication

Japanese knotweed is difficult to control and even harder to eradicate. Its rapid growth combined with extensive root system, and ability to grow in a wide range of conditions make it a species of high concern. Eradication is usually a slow process that involves a combination of methods. Because Japanese knotweed grows from rhizomes as well as seeds, removal of the entire root system is critical. If any part of the root is left in the ground, a new plant will form from it. The entire root system must be killed, not just the part of the plant above ground. For this reason, a combination of mechanical and chemical control methods are recommended.

Small populations can be controlled by cutting the canes above ground and digging up the roots. Because of the vegetative propagation abilities of Japanese knotweed, all parts of the plant should be placed in plastic bags and should never be composted. Plants should never be mowed as this will only further spread the invasion. Smothering with weed fabric or thick black plastic sheeting is another method that can be used.

Chemical herbicides can be an effective control method for Japanese knotweed. Glyphosate can be used, but applications must be repeated for 3-5 years. Herbicides are best applied between the flowering of the plant and the first frost. There are currently no biological control methods available.

Sources:

Cornell University Cooperative Extension. 2019, July. Japanese Knotweed. Available from http://nyis.info/invasive_species/japanese-knotweed/ (accessed March 2022).

Cygan D. (n.d.). Control Methods for Japanese knotweed. New Hampshire Department of Agriculture, Markets & Food. Available from https://www.agriculture.nh.gov/publications-forms/documents/japanese-knotweed-control.pdf (accessed March 2022).

University of Kentucky Forestry Extension. (n.d.). Japanese Knotweed. Available from https://forestry.ca.uky.edu/japanese-knotweed (accessed March 2022).

Figure 8. Japanese Knotweed

Samanek, Jan. Phytosanitary Administration. Available from https://www.forestryimages.org/browse/detail.cfm?imgnum=5205098 (Accessed March 2022).

Map 8. Distribution of Japanese knotweed (Polygonum cuspidatum) in the United States and Canada.

Water Hyacinth (Eichhornia crassipes)

Figure 9. Water Hyacinth

Photo by: Josh Hillman, FloridaNature.org, Bugwood.org

Description

Water hyacinth is the world’s most successful aquatic invasive plant and has spread from its native South America throughout Central and North America, and to Asia, Europe, and Africa. Its success as an invader is due in part to its ability to disperse long distances, its vegetative propagules, and aggressive clonal reproduction. Its desirability as an ornamental plant helped it spread around the world.

Invasion

Native to South America, the water hyacinth has been introduced to and successfully invaded areas of Asia, Africa, Europe, North America, Central America, and the Caribbean). It can be found in a majority of US states, including Hawaii. It was introduced to the United States in 1884 as an ornamental plant and spread rapidly. It can be found in most states with the exception of the high plains and Southwest.

Map 9. Distribution of Water Hyacinth (Eichhornia crassipes) in the United States and Canada.

Impact

Ecological, Fisheries, Recreational

Water hyacinth has major ecological impacts on the sensitive aquatic ecosystems it invades. It often forms densely populated colonies that block out sunlight below, reducing water quality and dissolved oxygen concentrations and impacting aquatic wildlife populations. Water hyacinth out competes native plant species due to its ability to rapidly reproduce clonally and send out vegatative propagules.

Water hyacinth also impacts water resources, fisheries, and recreation. It clogs waterways making fishing, boating, and recreational water activities difficult if not impossible. The density of plants can create dams in waterways, increasing the likelihood and severity of flooding. Water hyacinth infestations also impact human health by lowering water quality and creating the ideal breeding grounds for mosquitos that carry disease.

Mitigation/Eradication

Near water, water hyacinth can be mechanically removed. Caution should be used as stem fragments can regrow, so mowing or otherwise damaging the plants may spread vegetative propagules. In water, water hyacinth can be removed manually by raking it from the surface of the water. This can prove challenging over large areas that are heavily infested. The Neochetina beetle relies upon Water hyacinth as its main food source, so it is often used as a biological control method. Biological methods are not appropriate in all environments because they often require releasing another non-native species into an ecosystem.

Herbicides can be used as a chemical control method, although caution must be exercised when using chemicals in an aquatic habitat. Further, there is the risk of oxygen depletion post treatment due to decomposing plant material. Oxygen depletion has the potential to kill fish and other aquatic organisms.

Sources:

Gezie et al. 2018. Potential Impacts of Water Hyacinth Invasion and Management on Water Quality and Human Health in Lake Tana Watershed, Northwest Ethiopia. Biological Invasions 20:2517–34. Available from https://doi.org/10.1007/s10530-018-1717-0.

How to Control Water Hyacinth. (n.d.). Available from https://aquaplant.tamu.edu/management-options/water-hyacinth/ (accessed March 5, 2022).

Toft JD, Simenstad CA, Cordell JR, Grimaldo LF. 2003. The effects of introduced water hyacinth on habitat structure, invertebrate assemblages, and fish diets. Estuaries 26:746–758. Available from https://doi.org/10.1007/BF02711985.

Zhang Y-Y, Zhang D-Y, Barrett SCH. 2010. Genetic uniformity characterizes the invasive spread of water hyacinth (Eichhornia crassipes), a clonal aquatic plant. Molecular ecology 19:1774–1786. Available from http://dx.doi.org/10.1111/j.1365-294X.2010.04609.x.

Figure 9. Water Hyacinth

Hillman, Josh. FloridaNature.org. Available from https://www.forestryimages.org/browse/detail.cfm?imgnum=1237079 (Accessed March 2022).

Map 9. Distribution of Water Hyacinth (Eichhornia crassipes) in the United States and Canada.

Eastern Red Cedar (Juniperus virginiana)

Figure 10. Eastern Red Cedar

Photo by: Howard F. Schwartz, Colorado State University, Bugwood.org

Description

Eastern Red Cedar (Juniperus virginiana) is native to the United States and was once rare. Due to human impacts, red cedar is now invading new areas and expanding its range. Eastern red cedar is being planted outside of its native range for forestry and landscaping purposes, giving it opportunities to escape cultivation and establish itself in the wild in areas outside of its original native range. Eastern red cedar is expanding into grasslands, prairie, and rangelands and transforming these ecosystems into forests. Seasonal fires historically controlled the spread of Eastern red cedar in grasslands and pasture, but fire suppression has led to its expansion.

Invasion

Eastern red cedar is native to the East Coast, from Florida to Texas, Missouri, and Kentucky. It has spread throughout the Eastern United States and is expanding west. While range-spreading species are not necessarily considered invasive, Eastern Red Cedar has been introduced to new areas by humans, resulting in its expansion and subsequent ecological consequences.

Map 10. Distribution of Eastern Red Cedar (Juniperus virginiana) in the United States and Canada.

Impact

Ecological, Agricultural, Human Health

Eastern red cedar is expanding well beyond its native range due to human introductions for forestry and horticultural purposes. Once Eastern red cedar is established, it expands quickly and turns prairie into forest. Eastern red cedar displaces native wild game animals and grassland birds, and causes a decline in the diversity of small mammals. The establishment of Eastern red cedar forest leads to a reduction in plant diversity. Eastern red cedar forests require more water than grasslands and can lead to changes in water availability and stream morphology. This expansion of Eastern red cedar not only has ecological impacts, but affects agriculture and human health.

Eastern red cedar also reduces the availability of forage for livestock leading to loss of profit for ranchers. The transformation of grassland to forest increases the risk and severity of wildfires and their danger to infrastructure and human health. Eastern red cedar is a host for cedar-apple-rust, which is a disease that infects apple trees, making the spread of Eastern red cedar of agricultural concern.

Eastern red cedar is also involved in the spread of Lone star ticks (Amblyomma americanum) into Midwestern states. Lone star ticks are vectors for diseases that infect humans including rickettsiosis (Rocky Mountain spotted fever), spotted fever group (SFG), anaplasmosis, ehrlichiosis, other emerging pathogens, and AlphaGal Syndrome.

Mitigation/Eradication

As grass fires historically prevented Eastern red cedar from expanding into grasslands, prescribed burning can control red cedar and restore native grasslands. However, the window for safe burning can be limited and fire cannot be used on a large scale. A combination of methods is therefore needed. Mechanical removal of trees is needed in some areas in addition to or in lieu of prescribed burning. Chemical herbicides may also be used on the roots or as a foliar application, however, Eastern red cedars will not sprout from the stump so mechanical methods are typically sufficient.

Sources:

Graper D. 2019, April. Woody Weeds: Eastern Red Cedar. Available from https://extension.sdstate.edu/woody-weeds-eastern-red-cedar (accessed March 2022).

Noden BH, Dubie T. 2017. Involvement of invasive eastern red cedar (Juniperus virginiana) in the expansion of Amblyomma americanum in Oklahoma. Journal of vector ecology: journal of the Society for Vector Ecology 42:178–183. Available from http://dx.doi.org/10.1111/jvec.12253.

University of Nebraska. (n.d.). Eastern Red Cedar. Available from https://neinvasives.com/species/plants/eastern-redcedar (accessed March 2022).