Invasive species are one of the leading threats to the world’s biodiversity (IUCN). Many can be found in our own yards. How can we identify them and what should we do when we find them? 

First, we need to look at the difference between native and non-native species and apply this to the plant world. Environment and Climate Change Canada defines native species as “species that are indigenous to a particular area or region. Typically, these species have evolved over thousands of years, adapting to their surroundings, and have become an important part of the local ecosystem”. Native plants have co-evolved with other species, predators, diseases and climate factors within their environment, and are an important part of a balanced ecosystem, providing essential food and shelter for wildlife.

Non-native species do not naturally occur in an area and were brought either intentionally or unintentionally by humans. They have not co-evolved with the natural environment and are not in balance with the ecosystem. 

Invasive species are non-native species that spread aggressively and cause harm. They cause damage to infrastructure, interfere with agricultural crops, contribute to wildfire risk, block wildlife movement, and threaten Indigenous ways of life. Estimated costs to North America amount to $26 billion per year (Crystal-Ornelas et. al., 2021). The Ontario Invasive Plant Council defines an invasive plant as “one that has been moved from its native habitat to a new area (possibly for garden/domestic use), and reproduces so aggressively that it displaces species within native plant communities, the result being economic, ecological or social [cultural] disruption”.

How did they get here and why are they so pervasive?

Many invasive plants were intentionally brought to North America long ago, when European and other settlers arrived with their favourite plants and the desire to establish a familiar homestead in the new world. Over generations, these plants have become familiar and favoured in our landscapes, and there is a continued demand for nurseries to keep supplying them to gardeners and landscapers. Not all introduced plants are invasive, but the ones that are have flourished in the horticulture industry because of their desirable traits, such as low maintenance, aggressive spread and beauty. Due to lack of regulation, they are still widely available and even recommended by the horticulture industry; meanwhile conservation authorities, cities, municipalities and even the Auditor General of Ontario have published long lists of unregulated invasive plant species that gardeners and landscapers must avoid in order to protect biodiversity.

Plants have various methods of spreading, both sexually and asexually. Plants such as burning bush (Euonymus alatus) and berberis (Berberis thunbergii) spread sexually through fruit and seed production. Birds, attracted to the bright red berries of these shrubs, consume the fruits and disperse the seeds across meadows and forested areas, where they can germinate and outcompete native vegetation. Plants like periwinkle (Vinca minor) and lamuim (Lamium maculatum) spread vegetatively (asexually), allowing even small stem fragments to take root and establish elsewhere. 

Unintentional introductions occur when plants hitchhike on boats, vehicles, machinery, firewood, garden soil, people's clothing or footwear and when gardeners share plants that are hosting hitchhiker seeds and reproductive vegetative parts.

How do they displace native species?

Invasive plants possess several traits that contribute to their success in displacing native species. Many introduced plants are unpalatable to North American herbivores, allowing them to establish and reproduce without significant pressure. They often exhibit both sexual and asexual reproduction mechanisms which ensure maximum dispersal, rapidly grow to maturity and easily adapt to environmental stresses. They tend to thrive in colonies, monopolizing space and resources. They alter soil chemistry, block seed germination, or form dense mats that choke out other species.

A few examples;

• Burning bush (Euonymus alatus) is now found growing in dense colonies in woodlands throughout northeastern North America. These colonies are blocking the establishment of forest plants and altering the entire ecosystem of the forest. This plant is now listed as invasive in 21 states.

• Escaped garden plants like periwinkle form dense mats on the forest floor, inhibiting the germination of woodland plants and tree seedlings, halting natural forest succession.

• The vibrant biodiversity of a meadow habitat suffers from the establishment of invasive plants like butterfly bush (Buddleja davidii), which excel at seed production and dispersal, outcompeting native plants and providing little support to native insects.

How do I know if my garden plants are invasive?

Thanks to the dedicated work of many ecologists, conservationists, invasive species councils and other organizations, a wealth of resources is available (see below) to assist gardeners in identifying invasive and problematic plants throughout North America. 

The photos above only include six of the most problematic plants that we commonly use in our landscapes. A more inclusive list would contain:

• Norway and Amur maples (Acer platanoides, Acer ginnala)

• Japanese barberry (Berberis thunbergii)

• Butterfly bush (Buddleja davidii)

• Siberian pea shrub (Caragana arborescens)

• Burning bush (Euonymus alatus)

• Winter creeper (Euonymus fortunei)

• English ivy (Hedera helix)

• Yellow archangel (Lamium galeobdolon)

• Japanese honeysuckle (Lonicera japonica)

• Creeping Jenny (Lysimachia nummularia)

• Japanese spirea (Spiraea japonica)
  

A common thought among gardeners is that keeping a contained pot or small garden of invasive plants will stop them from escaping and spreading; while this may be true for non-fruiting plants in the short term, we must think in the long term. Who will be guarding this small patch of invasives in 50 years? 100 years? 200 years? What will the forest look like after this once small patch of periwinkle has had 100 years to spread into the surrounding forest?

What should I do if I’ve found invasive plants in my garden?

If you find one or some of these plants in your landscape, don’t be discouraged!

Start small. Come up with a plan to slowly remove patches of invasive plants and replace them with native species that perform a similar function or have a similar aesthetic.

• When removing invasive plants, there are species specific recommendations for timing of removal and proper disposal that must be followed in order to minimize escape to other locations. For example, burning bush must be removed BEFORE it goes to seed in the fall; orange daylilies and periwinkle need to be placed in black plastic bags and set in the sun for many days before the roots will be killed off. 

• There are “Grow Me Instead” guides for most provinces that recommend native plants to use as replacements for invasive garden plants.

• Native plant libraries and swaps are popping up all over the country, like the Ottawa Wildflower Seed Library! Meet your neighbours and share information and knowledge about the right plants for the right place.

How do I prevent spreading invasive plants to new locations?

• Clean the soil from all of your garden equipment; there may be vegetative parts or seeds hitchhiking in the soil and they can be unintentionally spread.

• Beware of “wildflower mixes” sold in stores and nurseries; oftentimes, they contain a mix of non-native and invasive plants.

• If you want to share plants with your friends and neighbours, first make sure they are not on the “unwanted” list and then clean residual soil from the roots before transplanting to the new location.

• Check out the many resources online about invasive plants, visit a local native plant nursery to learn about the fascinating plants that grow in your ecosystem, get to know the native bees, butterflies and birds you share space with and talk to your neighbours about the dangers of invasive plants.

Think about your garden in the long term. Your ecosystem will thank you!

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Resources:

• CCIPR: Canadian Coalition for Invasive Species Regulation

• Ontario Invading Species Awareness Program:

  • Terrestrial Plants

  • Invasive Ground Covers

• Ontario Invasive Plant Council:

  • Grow me Instead

  • Best Management for Goutweed

• Office of the Auditor General of Ontario: Management of Invasive Species

• Credit Valley Conservation Authority: Priority Invasive Plants

• New York State Department of Environmental Conservation: Prohibited and Regulated Invasive Plants

• Clean North: Invasive Garden Plants

• Toronto Master Gardener: Periwinkle Fact Sheet