The Invasive Plants Research Professional Interest Group of the American Society for Horticultural Science (ASHS) developed and executed a workshop at the 2022 ASHS Annual Conference to discuss the current state of invasive plants from a global to United States scale and then to strategize, debate, and find ways to mitigate this ever-growing challenge of plants with invasive tendencies. The objective of this workshop, titled “A Review of Invasive Offenders and Strategies to Mitigate Their Impact,” was to showcase problematic species, both nonnative and native, and then suggest solutions to mitigate their impact either proactively, reactively, or using a combination of proactive and reactive approaches. Because of the current misunderstandings within the scientific community, as well as among citizen scientists to everyday gardeners, workshop leaders M.A. Schnelle and L.A. Gettys encouraged speakers to present real-world problems as they pertain to invasive plants and then offer pragmatic solutions for academics, industry, and consumers.
Schnelle provided preliminary remarks and was followed by S.C. Marble, whose presentation “Just One of Many Nonnative Species That Are Bad…” focused on controlling small-leaf spiderwort (Tradescantia fluminensis), a once-popular ornamental groundcover that is now displacing natives in Florida, USA, and is also problematic in Australia and New Zealand. Marble discussed integrated control strategies, including the need for more herbicide options to prevent nontarget damage. The next presentation—“Strategies to Make Invasive Nonnative Plants Not so Bad…”—was given by K.W. Leonhardt, who discussed controlling sterility through breeding and polyploid induction. After outlining methods to induce sterility in ornamentals, such as golden shower tree (Cassia sp.) and umbrella plant (Schefflera sp.), Leonhardt reported that these efforts are driven by the Hawai’ian ornamentals industry, which feels that no viable alternative plants exist, and his work should focus on improvement of these familiar desired species. Wilson and Deng (2023) followed with “Ornamental Invasive Plants in Florida with Research-founded Alternatives,” a presentation that described sterile selections of nonnative plants, including lantana (Lantana sp.), heavenly bamboo (Nandina sp.), privet (Ligustrum sp.), and others, but also discussed educational outreach efforts to teach the public what to plant and what species to avoid. The next presentation, “A Rose Is a Rose Is a Rose… Or Is It? Ecotypes versus Locally Sourced Plants for Aquatic Habitat Restoration and Enhancement Projects,” was given by Gettys (2023), who emphasized that all members of the same species are not necessarily created equal, especially when planning restoration projects. She pointed out that screening ecotypes (genetically distinct populations adapted to specific environmental conditions) can lead to the discovery of broadly adapted species that may be better suited for restoration projects than locally derived populations; in other words, traits can be more important than provenance. The final presentation, “An Overview of Juniperus virginiana: A Resilient and Valuable Native Ornamental Which Can be Invasive Under Certain Conditions,” was delivered by Schnelle who mentioned the many positive attributes and uses for eastern redcedar (J. virginiana), not only for the ornamentals industry but for its potential medicinal value, food source for a plethora of animals in the wild, etc. Schnelle went on to explain that with the relatively recent advent of fire suppression and overutilization of this species, fields that are abandoned or not monitored can become eastern redcedar monocultures within 30 to 40 years, resulting in displacement of native flora and fauna and negative effects on groundwater recharge and stream flows. The workshop concluded with a final question and answer session in which all five speakers engaged with the audience.
Takeaway messages from the workshop
Global
On the worldwide stage, invasive species (accounting for plants, pathogens, and animals) cause damage valued annually at $1.4 trillion globally (City of Portland, OR, Bureau of Environmental Services 2021; Diagne et al. 2020a, 2020b). Approximately 17% of the world’s landmass is deemed highly vulnerable to nonnative plant invasions (Beyene et al. 2022). More than half of the world’s flora has been introduced to alien regions for use in gardening, agriculture, medicine, and other purposes (Guo et al. 2019; Naburga 2022; van Kleunen et al. 2018), but these nonnative ornamentals have caused a loss of biodiversity in native habitats in or near urban areas.
Plant mislabeling is thought to be responsible for prohibited invasive aquatic plants still being sold in Belgium (Van den Neucker and Scheers 2022), but intentional dissemination of nonnative species is extremely common. Horticultural industries throughout the world have played a prominent role in the introduction and spread of alien plants to new areas. For example, Rojas-Sandoval and Ackerman (2021) evaluated 616 alien plant species in 18 island groups across the Caribbean and found that 54% were intentionally introduced for ornamental purposes vs. any other reason to supplement indigenous species. Although invasive species are found in a multitude of plant families, the daisy (Asteraceae), legume (Fabaceae), grass (Poaceae), and madder (Rubiaceae) families include the largest numbers of problematic species (Huebner 2022; Humair et al. 2015). The consequences of invasive plants are many, including (but not limited to) a global upward trend in plant-fueled wildfires that result in economic, environmental, and societal consequences (Faccenda and Daehler 2021).
Climate change, rising carbon dioxide levels, and increasingly prevalent world trade are key forces that contribute to the rise in global plant invasions (Clements et al. 2022). Climate change can facilitate invasive plant invasions by altering background environmental conditions, exacerbating disturbances via extreme climatic events, and through human responses to climate change (Coville et al. 2021; Turbelin and Catford 2021). As a result of climate change, altered environmental conditions define every species as a potential new invader or colonizer, and species that adapt and regenerate via seeds are the ones most likely to survive (Huebner 2022). Habitat suitability modeling may aid in anticipating the potential spread of invasive plant species (particularly as a result of range expansion) and their deleterious effects on global ecosystems (Wang and Wan 2021).
It is important to recognize that provenance does not guarantee plant “behavior.” In other words, some native species have the potential to exhibit unchecked, aggressive growth, which can disrupt ecosystems and reduce floral diversity as effectively as invasive nonnative species. Yazlik and Ambarli (2022) showed that nonnative and dominant native species in Turkey carry a comparable risk of invasiveness. They suggest that dominant native species, shown to have negative environmental and socioeconomic impacts in their habitats, be included in priority lists for management strategies regardless of their origin or native status.
United States
Estimates of the economic ramifications of invasive species (plants, pathogens, and animals) in the United States range from $40 billion to $120 billion annually (City of Portland, OR, Bureau of Environmental Services 2021; Diagne et al. 2020a, 2020b). Nearly 40% of invasive plants in the United States were originally introduced as ornamentals (Lehan et al. 2013). These same species affect almost every ecosystem of the country and continue to colonize new areas (Allen and Bradley 2016). Beaury et al. (2021) searched the internet and a database of nursery catalogs and found that 61% of 1285 species recognized as invasive in the United States remain in the plant trade, including 50% of state-regulated plants and 20% of federal noxious weeds. Vendors selling invasive plants were identified throughout the continental United States, and researchers deduced that this ready availability of invasive species was due to disjointed state regulations that fail to protect both ecosystems and economies. In addition to causing habitat disruption in natural areas, invasive plants are commonplace in horticultural settings (M.A. Schnelle, personal observation) and thus require management in the built environment (Deparis et al. 2022).
Nonnative plants become invasive and disrupt natural ecosystems using a variety of strategies. For example, Xu et al. (2021) found that invasive species use a more flexible strategy in establishing associations with soil microbes, which allowed them to acclimate and overcome the soil legacy of the native species. This process could potentially accelerate establishment in natural ecosystems. Also, Woods and Sultan (2022) showed that post-introduced evolution by lady’s thumb (Polygonum cespitosum) led to the evolution of traits that maximized propagule production, thus leading to invasiveness. However, numerous researchers have shown that some species have the potential to be invasive regardless of their origin, including plants native to the respective country or region (Anderson et al. 2021; Gettys 2019; Gettys and Schnelle 2019; Marble 2018; Schnelle 2019, 2021; Schnelle and Gettys 2021). Range-shifting native plants may behave like invasive plants and colonize new environments, resulting in potential negative consequences (Wallingford et al. 2020). Also, Yazlik and Ambarli (2022) suggested that dominant native species shown to have negative environmental and socioeconomic impacts in their “home” habitats should be included in priority lists for management strategies regardless of their origin.
Conclusions
Many researchers believe humans are responsible for the sixth mass extinction in history as a result of changing global climate, eliminating species directly, spreading pathogens, fragmenting habitats, and introducing nonnative species (Barnosky et al. 2011). Stroud et al. (2022) commented that university students are exposed to limited plant content, particularly with regard to identification, compared with material focused on animals. Consequently, they argue that fewer plant scientists are being mentored and even fewer scientists are being educated about plants, which they characterize as a self-accelerating cycle coined “the extinction of botanical education.” Stroud et al. (2022) argue that this separation or disconnection from the natural world makes people blind to the biodiversity crisis and hampers our ability to restore it. The global impacts and costs resulting from nonnative plant invasions are expected to increase over the next decades (Garcia-Diaz et al. 2021). For example, Seebens et al. (2021) used a modeling approach and predicted that naturalized invasive species could increase 36% by 2050.
Nearly all scientists agree that it is imperative to identify priorities, targets, and actions for the long-term social and ecological management of invasive nonnative species (Garcia-Diaz et al. 2021). However, as reported in this workshop, native status does not automatically mean a plant will exhibit “good behavior.” It is therefore critically important to recognize that some native plants can grow as aggressively—and cause similar damage to diverse ecosystems—as nonnative species.
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