Invasive Plant Species

Invasive plant species pose a significant threat to urban forests across Canada, impacting biodiversity, ecosystem health, ecological functions and services of green spaces and natural urban areas while also causing significant economic impact. According to the 2021 national survey led by the Invasive Species Council, Canadian municipalities estimated annual expenditures on invasive plant species ranged from $95.8 million to $400 million (Vyn & Invasive Species Centre, 2022). Effective and integrated prevention and management of invasive plant species is crucial to maintaining the integrity and resilience of urban forests to ensure biodiversity conservation and a steady flow of economic and ecological benefits provided by urban forests. 

Due to the history of land use change (e.g., previously agricultural land or disturbed land converted to urban), the introduction of, and preferences for introduced plant species, urban areas tend to have numerous non-native plants, some of which are invasive. The introduced invasive species are either planted or spontaneously established across various land uses, threatening the biodiversity of natural areas. For example, some invasive plants such as Japanese knotweed (Reynoutria japonica), Garlic mustard (Alliaria petiolata), Dog-strangling vine (Vincetoxicum rossicum), European buckthorn (Rhamnus cathartica), Himalayan blackberry (Rubus armeniacus), and Tree of heaven (Ailanthus altissima) proliferate through urban areas, monopolizing natural areas while outcompeting native flora and disrupting local ecosystems and ecological functions (City of Toronto, 2013; Ministry of Forests, 2024; Ontario’s Invading Species Awareness Program, n.d.; Stanley Park Ecology Society, 2013; Saskatchewan Invasive Species Council, n.d.).

Invasive plant species often alter soil biological properties (e.g., mycorrhizal properties), chemistry, and nutrient availability, which hinders the establishment of native species, promoting further expansion and establishment of invasive plants (Ehrenfeld, 2003; Kourtev et al., 2002). Invasive plants modify vegetation composition, growing conditions, and light conditions, and decrease water availability for native plants, making environments less hospitable for native species (Lamarque et al., 2011). Another risk from invasive plants involves the possibility of them hybridizing with native plants and eventually eliminating native genetic potential (Mooney & Cleland, 2001). These cumulative impacts ultimately change the composition and structure of natural vegetation in urban areas (Delavaux et al., 2023). It has been shown that non-native, invasive trees reduce the diversity of native insects and could facilitate the spread of non-native pests (Branco et al., 2019). Moreover, invasive plant species can significantly change ecosystem functions and negatively impact human health (Vila et al., 2011).

Some of the planted or self-established trees in Canadian cities are also listed as invasives and include: Black Locust (Robinia pseudoacacia), European black alder (Alnus incana), White mulberry (Morus alba), Scots pine (Pinus sylvestris), Norway maple (Acer platanoides), Manitoba maple (Acer negundo), Tree of heaven (Ailanthus altissima), Autumn olive (Elaeagnus umbellata), Japanese barberry (Berberis thunbergii), and European buckthorn (Rhamnus cathartica). However, many other invasive trees, shrubs, and grasses occur in Canadian urban areas and pose a threat to natural ecosystems. 

Managing Invasive Plant Species in Urban Forests

Invasive species management is multifaceted and includes various tools and methods ranging from policy, control, and eradication measures to public education and prevention actions. In Canada, different levels of government have introduced weed and/or invasive species control legislation prohibiting plant introduction and distribution. At the national level, provincial control measures with federal regulations can be adopted to control the spread of invasive species in Canada (Government of Canada & Environment and Climate Change Canada (ECCC), 2024; Sherman & Ontario Invasive Plant Council, 2015). For example, the government of Ontario prohibits and restricts the import, possession, transport, or release of 42 invasive species under the Invasive Species Act (Government of Ontario, 2023). Not all invasive species have the same impact; a few species cause most of the damage. In British Columbia, six invasive species alone caused an estimated $65 million in damage in 2008 (Invasive Species Council of British Columbia, 2024b). 

Invasive Plants Control

The control and eradication of invasive plant species range from mechanical and chemical techniques to burning, flooding, biological control, and other control methods. Mechanical removal methods such as hand-pulling, mowing, and cutting can effectively manage small infestations. More recently, controlled goat grazing has been used to manage invasive species in natural areas (Rathfon, 2021; City of Mississauga, 2021). 

Herbicides and other chemical controls may also be used selectively to control invasive species, mainly when mechanical methods are impractical. However, the application of these chemicals must comply with local regulations and be conducted by certified professionals (Invasive Species Centre, 2021; Wisconsin Department of Natural Resources, n.d.). Also, the use of certain chemicals to control weeds and invasive plants has been forbidden by many municipalities across Canada (The Ontario Pesticides Act, 2024; City of Vancouver, n.d.; Health Canada, 2024).

Biological control methods are also used to introduce natural predators or pathogens specific to the invasive species to manage its populations. This approach requires research and careful ecological risk assessment to avoid unintended ecological impacts (Invasive Species Centre, 2021). For example, after conducting over a decade of testing, the Canadian Food Inspection Agency, as part of a larger integrated management strategy for invasive Phragmites (Phragmites australis), approved the release of two stem-boring moths (Archanara neurica and Lenisa geminipuncta) to control the spread of this species. Phragmites is one of Canada’s most disruptive invasive species (Ducks Unlimited Canada, 2024) that has been threatening wetlands’ biodiversity, but has also been rapidly spreading along roads, including urban areas. However, biocontrol measures alone are not enough to eradicate invasive phragmites. They are expected to gradually reduce species dominance and habitat disturbances, allowing native plants and animals to recover (Ducks Unlimited Canada, 2024).

Early Detection and Rapid Response

Early identification and control of invasive species is another essential tool that is based on regular monitoring and community reporting of invasive plant observations. One such approach is early detection and rapid response (EDRR) of invasive species. In British Columbia, for instance, EDRR activities are conducted in partnership with the Canadian Food Inspection Agency and involve key land managers and stakeholders across B.C. and neighbouring regions (British Columbia Inter-Ministry Invasive Species Working Group (IMISWG), 2014). Moreover, invasive species monitoring and management through remote sensing, drone-based imaging, and data processing and analysis have emerged as practical solutions to map and detect large plant invasions in urban forests (Singh et al., 2024). 

Education

Educating the community about the impacts of invasive species and encouraging the use of native plants in landscaping can significantly aid in prevention and control efforts. Across Canada, different levels of government, non-profit organizations, regional groups, and conservation authorities have been educating the public and eradicating or preventing invasive plants from spreading in urban areas (Government of Alberta, 2014; Invasive Species Council of British Columbia, 2024). These groups often initiate and engage in developing invasive species strategic management plans and best management practices (BMPs), as part of urban forest management (Ontario Invasive Plant Council, 2024). Community members and the public are also actively involved in controlling invasive species and removing them from natural areas. Examples of community efforts in coordination with municipalities, NGOs, or on their own exist from the east to the west coast (Invasive Species Centre, 2024). 

Collaborative efforts between municipalities and academia and the continuous adaptation of best management practices based on current research and local conditions also enhance invasive species control effectiveness and contribute to urban forest sustainability (Sherman & Ontario Invasive Plant Council, 2015).

Municipalities have integrated invasive plant management strategies into their urban forest management plans to preserve the ecological health and resilience of urban green spaces and natural areas (City of Toronto, 2024a; Government of Alberta, 2024; Patterson, 2015). Many municipalities started prioritizing planting native and non-invasive and introduced tree species to lead by example and stop the spread of invasive species (Patterson, 2015). However, some invasive species, such as Norway maple (Acer platanoides), are still planted and produced by tree nurseries.