Non-native plants can be an eco-catastrophe, degrading sensitive habitats and diminishing biodiversity. Where and how to draw the battle lines in the fight against alien species is now a topic of heated debate.

By Lorraine Johnson
Purple loosestrife

Purple loosestrife. Credit:

It was a lesson in making connections. On a warm autumn morning, a small group of garden writers, plant nursery workers and ecologists arrived for a tour of High Park at the invitation of the Toronto and Region Conservation Authority (TRCA). As we marvelled at the lush growth all around us, TRCA’s Colleen Cirillo explained our mission: “Not many people make the connection between what they grow in their yards and the non-native plants that spread into and degrade natural areas.” Our walk would bring that link to the fore.

We were on a hunt for evidence of invasive alien garden plants taking over the rare oak savannah of this west Toronto park. We didn’t need to look far. At the top of a gentle hill a few minutes into our hike, we confronted a tableau straight out of an ecologist’s nightmare: the invasive alien dog-strangling vine climbing up and completely covering the equally invasive alien shrub buckthorn, and the invasive alien Oriental bittersweet vine growing right beside, up the bark and branches of – you guessed it – another invasive alien, the black locust tree. Rough justice of a twisted sort, really: aliens battling for supremacy. Which of them won the battle would not matter because the loser was already clear: the park’s natural areas.

The botanical world is full of migrations – of plants expanding their territories with the help of animals, wind and serendipity. Humans, of course, are now the most influential force in such migrations. We move plants from one continent to another, far outside their native ranges, sometimes deliberately, sometimes accidentally.

By a wide margin, the majority of these immigrant plants have little impact on their new homes, simply staying put in the confines of gardens or agricultural fields where they are cultivated. But some are the ecological equivalents of bomb blasts, altering the structure and function of native ecosystems, diminishing biodiversity, displacing native plants (including some already rare ones), and severing complex and necessary interactions and relationships between native plants and animals. Concern about these invaders is widespread in ecological and horticultural circles. Which plants cause the most damage, however, and how to deal with those that do – whether through regulation, public education or outright bans – is a subject generating passionate debate.

Invasives & Alternatives

Norway maple (Acer platanoides)
Introduced to North America in the mid-1700s, Norway maple is a ubiquitous shade tree in Ontario cities, commonly used as an ornamental tree and by municipalities to replace American elms as street trees when Dutch elm disease killed them in the 1950s and ’60s. This medium-sized tree looks very similar to the native sugar maple but has leaves with five to seven lobes (versus sugar maple’s three to five) that are wider than they are long. Fast-growing and hardy, Norway maple does well in tough urban conditions, is adaptable to a wide range of soils and grows well in deep shade. However, it creates dense stands in natural areas, suppressing the growth of native groundcovers and the regeneration of native trees.
Alternatives: red maple, sugar maple, silver maple, hackberry

Autumn olive (Elaeagnus umbellata)
Native to China, Korea and Japan, the autumn olive was introduced as an ornamental shrub or small tree (growing up to six metres high) in the early 1900s. Its greyish foliage and fragrant yellow flowers quickly made it popular. In the fall, wildlife eats the tree’s prolific red fruits, and birds spread them long distances. Autumn olive grows rapidly into a dense thicket, displacing native species.
Alternatives: serviceberry, chokeberry

Russian olive (Elaeagnus angustifolia)
Brought to North America in the early 1900s, this small tree (growing to approximately seven and a half metres high) quickly gained popularity for its striking silvery foliage and drought tolerance. Its landscape use was promoted during the drought of the 1930s, and it has since escaped beyond cultivated areas, moving onto riparian land and wet meadows. It displaces native trees such as cottonwood and willow, on which numerous wildlife species depend.
Alternatives: serviceberry, chokeberry

Black locust (Robinia pseudoacacia)
Native to the eastern United States (from Pennsylvania and southern Indiana south to Georgia and Louisiana), the leguminous black locust tree has compound leaves with rounded leaflets typical of nitrogen-fixing species. Its white, pea-like flowers, which appear in late spring and early summer, are fragrant clusters that dangle from branches and develop into pods. This species is often planted along roadsides, whence it escapes into nearby woodland edges, fields and riparian areas, creating dense stands and displacing native vegetation.
Alternative: honey locust

Oriental bittersweet (Celastrus orbiculatus)
Native to Japan, Korea and China and brought to North America in the mid-1800s, this vine is often mislabelled at nurseries as American bittersweet (Celastrus scadens), leading customers to think they are buying a native plant. Bearing prolific yellow and red fruit spread by birds, the woody vine can be distinguished from the native bittersweet by the placement of the fruits: on Oriental bittersweet, they appear along the stem; on American bittersweet, they are at the tip of stems. The vine is most problematic in meadows, thickets and young forests, where it climbs up trees and shrubs, constricting the hosts’ stems.
Alternatives: virgin’s bower clematis, Virginia creeper, American bittersweet

English ivy (Hedera helix)
With its glossy, dark-green, waxy leaves and shade tolerance, English ivy has been a popular groundcover since it was first introduced to North America centuries ago. Native to Eurasia, it spreads into forests, completely covering the ground and shading out native species, and sometimes growing up tree trunks.
Alternatives: wild strawberry, foamflower, wild ginger, bearberry

Yellow flag iris (Iris pseudacorus)
Invasive in wetlands, yellow flag iris has showy golden flowers and grows to approximately a third of a metre high. It outcompetes the native aquatic plants of shallow wetlands, spreading by roots and seeds.
Alternatives: blue flag iris, vervain, marsh marigold, sweet flag

Invasive alien plants are species that have been accidentally or deliberately introduced to areas beyond their native range, and whose spread negatively affects biodiversity, society or the economy. They are difficult to control, tend to grow aggressively, spread quickly, set many seeds or clone themselves rapidly, and have few pests or diseases to limit their growth.

According to the Canadian Food Inspection Agency (CFIA), roughly 1,229 (or 24 percent) of Canada’s flora is made up non-native species. Of these, CFIA estimates that 486 are weedy or invasive. Over the past 400 years, Canada’s flora has expanded by roughly three alien species per year, of which one, on average, becomes invasive. Among the provinces, Ontario has the highest number of invasive alien plants – 441 species – and they are considered one of the greatest threats to regional biodiversity.

Although dates of entry to Canada and the pathways of introduction are poorly documented for many species, CFIA notes that of the 245 invasive alien plants for which this information is known, 73 species were intentionally introduced as ornamental or landscaping plants. Another 86 arrived as hitchhikers with other plant products. The implication is that the seemingly benign plant choices gardening enthusiasts make are contributing to the ecological damage invasive plants cause.

And that damage is extensive: A 2006 study commissioned by the Office of the Auditor General of Canada estimated that just 11 invasive alien species cost fisheries, agriculture and forestry $187 million per year, primarily in control expenses and lost productivity. A 2002 report by A.V. Stronen for the Canadian Wildlife Service found that invasive plants contributed to putting more than 25 plant species at risk of becoming endangered, including eastern prairie white-fringed orchid (threatened by invasive reed canary grass), drooping trillium (threatened by garlic mustard) and hoary mountain-mint (threatened by honeysuckle).

NurseriesNative plants are readily available commercially. The best sources are specialty native plant nurseries, where knowledgeable staff are able to answer any questions and offer advice.

Pterophylla: This nursery near Walsingham carries many Carolinian native plants (trees, shrubs, perennials) and grows its stock from indigenous seed. 519-86-3985;;

Wildflower Farm: Based near Coldwater, Wildflower Farm sells a wide range of native perennial seedlings and does a brisk mail-order business in native seed and seed mixes (for dry conditions, for example, or for deer resistance).

Grand Moraine Growers: Along with perennials, trees, vines and shrubs, this Alma-based nursery sells grasses, sedges and rushes.

Acorus Restoration: Three hundred and fifty native species for wetlands, woodlands, prairies and meadows are available from this Walsingham

Native Plants in Claremont: This Claremont nursery supplies perennials, shrubs and grasses indigenous to Ontario and grown from seed collected locally.

Grow Wild! This Claremont-based nursery sells a variety of native perennials.

TRCA has a long list of sites within the Greater Toronto Area (GTA) alone where the destructive impact of invasive plants commonly sold in nurseries can be readily seen: privet invading Rattray Marsh; large areas of goutweed in the floodplains of the Duffin’s Creek watershed; lily-of-the-valley in the ravines and forests of the Humber watershed; periwinkle completely overtaking the ground layer of some high-quality woodlands on the King Campus of Seneca College; wintercreeper in the lower Humber.

Invasive alien plants entered public consciousness in the 1990s, when media reports described purple loosestrife as invading Canada’s wetlands creating monocultures in these sensitive and biologically important ecosystems. In the news gardeners were exhorted to rip the purple patches out of their plots and nurseries urged to stop selling this bully. The Ontario Ministry of Natural Resources and nonprofit groups set up telephone hotlines for people to report sightings of the loosestrife menace, and groups formed to eradicate it from natural areas.

But even as Ontarians rallied to fight this one species, nurseries continued to sell, and gardeners continued to plant, a host of equally harmful invaders. Dan Bissonnette, program coordinator at the Naturalized Habitat Network in Windsor, cannot contain his anger when he describes how he first became concerned about invasive plants. “I looked at a plant inventory list for Point Pelee National Park, and about one-third of the plants were exotics, many of them invasive,” he says. “With my horticultural background, I recognized that about three-quarters of these species are still sold on a regular basis in nurseries.” Invasive non-natives such as Russian olive, Norway maple and English ivy, he exclaims, “are staples of the landscape industry!” He goes so far as to call the commercial trade in invasive plant species “a war on biodiversity. Every spring, it’s a new wave of battle when unsuspecting gardeners go to nurseries and buy them.”


To learn more about plants native to your area, visit the following websites:

Evergreen ( This national nonprofit organization’s mission is to bring nature back to cities, and one of its useful tools is a native plant database that allows you to search for plants indigenous to your region.

North American Native Plant Society ( Promoting the planting of native species, this charitable organization focuses on education and hosts workshops, as well as publishing an informative newsletter.

The following websites are good sources of information on invasive plants in Ontario:

Ontario Invasive Plant Council ( This nonprofit organization is composed of various agencies and groups involved in invasive species issues and serves as a hub for sharing information.

Invading Species Awareness Program ( Created by the Ontario Federation of Anglers and Hunters in partnership with the Ontario Ministry of Natural Resources, this program promotes public education and volunteer action to address the threats from invasive alien species.

For more information on native plants to use in the garden as alternatives to invasive alien plants, see Lorraine Johnson’s 100 Easy-to-Grow Native Plants for Canadian Gardens and The New Naturalized Garden.

Lorraine Johnson