By Cynthia B. Lauer, Master Gardener, SCMG
Whether it’s periwinkle, English ivy, creeping bellflower, garlic mustard, dog-strangling vine, honeysuckle, or buckthorn among many others, chances are that you’ve done battle with an invasive plant. Digging, tarping, mowing, and cutting can be arduous and endless. What is it about invasives that make them so difficult to deal with?
Gardeners know that they out-compete the native species that provide essential food and habitat for insects. But learning the multiple ways that invasiveness works can be truly sobering. Understanding just how bad these plants really are should inspire efforts to suppress them wherever they are found.
What is an Invasive Plant?
An invasive plant is one that, after its introduction to a region, uses a range of characteristics to develop self-sustaining populations. In the absence of natural predators that controlled their spread in their original location, an invasive plant spreads so aggressively that it eventually threatens the environment, the economy, and the health of other organisms. Severe and often irreversible consequences include the loss of biodiversity, the degradation of terrestrial, marine and other aquatic ecosystems, the introduction of new predators against which native host species have no defense, and the loss of food sources. They disrupt water purification, carbon sequestration and climate regulation, and can act as new or additional hosts for crop diseases and pests.
Why Invasive Plants Succeed
Invasive plants have multiple means of dominating an environment. First, they have many physical traits that are superior to that of non-invasive species. One study compared their leaf-area, shoots, growth rates, size and fitness of invasive and non-invasive species and found that invasive species has significantly higher values than non-invasive species for all six qualities. Other research confirms that plant height, branch number, the below-ground to above-ground biomass ratio, and the reproductive capacity of invasive plants are all factors in their competitiveness.
Once introduced to a new region, invasive plants are highly adaptable. Their ability to change their physiology, chemistry, and behaviour ensures their success over a broad environmental range. When faced with changing environmental conditions, some invasives can change the timing of their flowering, their stem count, their leaf size, even their general biomass. They can remain undetected for a long time then proliferate when conditions are optimal. These “sleeper” species emerge from dormancy—anywhere from 40 years to over 300 years later—and quickly dominate a new territory.
The adaptability of invasives makes them tolerant to a range of environmental and climatic conditions caused by extreme weather events. Drought, wildfires, and floods in particular, facilitate the spread of invasive plants. Fragments of Japanese knotweed were distributed widely and established new populations after tropical storm Irene struck Vermont in 2011. Common reed grass (Phragmites australis), spreads much more rapidly with flooding.
Phragmites (Jason Ramsay-Brown)
Invasive plants also have an impressive array of reproductive traits. These include self-fertilization, asexual reproduction, and a high proportion of resources allocated to sexual reproduction. Seeds tend to be smaller than those of native plants increasing opportunities for their dispersal. Compared to native species, they are likely to germinate earlier and to better tolerate rising temperatures at the germination stage. Their seeds tend to persist in the soil over multiple seasons and in unfavorable conditions. Seeds aren’t the only means they have for reproduction. Invasives also propagate vegetatively through rhizomes below ground or stolons above ground.
As grave as these characteristics are, there is another one that may take the lead. Many invasive species are able to make the environment more receptive to invasion. In a process known as allelopathy, they produce biochemicals—known in ecology as a “novel weapon”—that negatively influence the germination, survival, and reproduction of other organisms. These substances are released into the soil through the plant’s roots, through the decomposition of its tissues, and they leach out after a rainfall. They change soil and water chemistry, modify nutrient cycling, alter nitrogen availability, and impact water availability. Allelopathy reduces the nematode population and suppressing mycorrhizal fungi affecting the quality and quantity of resources available to native plants.
field bindweed (Sterling College, CC BY 2.0)
One study found that allelopathy diminishes neighbouring plant performance by 25% on average. Well-documented in knotweeds (Reynoutria spp), garlic mustard (Alliaria petiolata), knapweeds (Centaurea spp), and Canada thistle (the non-native species, Cirsium arvense), allelopathy is not limited to these species. One 2021 study of 524 invasive plant species showed that almost 52% produced harmful chemicals with the potential to negatively affect native plants.
Natural or human-induced disturbances of ecosystems occurring with agriculture, forestry, or construction typically change resource availability, displace native plant species, and expose bare soil. This invites rapid colonization by non-native plants. As land is used, habitat edges are created that can be colonized by non-native species. To make matters worse, when multiple non-native species become established, it amplifies their impact and can facilitate secondary invasions.
The Horticulture Industry as a Pathway
A key means by which invasive plants are introduced is retail, wholesale, and online nurseries. Turfgrass mixes, wild seed mixes, aquarium plants, and plants for ornamental and landscaping uses can include invasive species. A 2022 Ontario Auditor General Report found that 50–60% of all invasives were intentionally introduced, a figure confirmed by the Canadian Food Inspection Agency.
The Canadian Coalition for Invasive Plant Regulation claims that “the ornamental/horticultural industry is the primary pathway for the introduction of non-native invasive plants entering Canada.” Specifically, 89% of invasive woody plants such as burning bush and Japanese honeysuckle, 65% of invasive vines like bittersweet and English ivy, and 31% of herbaceous plants such as creeping jenny and the ornamental grass, Miscanthus, are sold directly to consumers.
What You Can Do
Gardeners concerned about invasive plants can join campaigns to prevent their spread through the horticulture industry. We can support policies that aim to impose restrictions on importing and sales by plant breeders, propagators and producers. Codes of conduct can be adopted by retail outlets. Changing consumer behaviour through public awareness is particularly important means of preventing the spread of invasive plant species.
Featured Image: creeping bellflower (Robert Taylor, CC BY 2.0)
