We all have a role to play in fighting invasive species
Giant hogweed (Photo by Henry Clark)
Invasive species are a serious threat to Canada’s native species and habitats, including in our parks and protected areas. Globally, invasive species are the second-most common threat associated with species extinctions (habitat loss is number one), and they are one of the top threats to Canada’s species at risk. In addition to causing the loss of biodiversity, invasive species can be a risk to human health and cost our economy between $16-34.5 billion each year.
The Nature Conservancy of Canada (NCC) is managing invasive species on many of our properties across Canada. Especially in southern Canada, where natural habitats have been fragmented and are close to our farms and cities, invasive species are more common and need to be managed to protect our native biodiversity. This list of the top 10 invasive plants includes some of the key species that we are managing on our properties, and that also pose a threat to other protected areas across Canada.
Canada’s top 10 invasive plants
Knapweeds: There are five invasive knapweed species in Canada that were unintentionally introduced into Canada from Europe in the late 1800s, probably in alfalfa and clover seeds. All species have slender stems with purple (or sometimes white) flowers and grow from a deep taproot. Spotted knapweed is an aggressive invader that is especially problematic in native grasslands in western Canada, and has recently spread to Manitoba.
Provinces/territories where this species is found: Yukon, BC, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia
Leafy spurge (Photo by Ed L/pawpaw67)
Leafy spurge: Leafy spurge was probably introduced to Canada in grain seeds from Europe. It has yellow-greenish flowers, and the leaves and stems have a white milky sap. It can rapidly spread in open habitats, such as prairies, and reduces the quality of rangelands. This species has been documented on NCC properties from Ontario to BC, and threatens key habitats, such as tall grass prairie in Manitoba. On NCC’s Big Valley property in Saskatchewan, beetles have been introduced as a biological control to contain the spread of the plant.
Provinces/territories where this species is found: Yukon, BC, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, PEI
Canada thistle: Despite its name, this invasive thistle is not from Canada, but has been established in North America for hundreds of years. The alternative name, creeping thistle, is a more apt name for this species. It has purple flowers and spiny leaves, and grows in open areas. In addition to crowding out native plants, Canada thistle reduces the quality of rangelands. The small seeds are dispersed by winds so it can quickly spread.
Provinces/territories where this species is found: Yukon, Northwest Territory, BC, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, PEI, Newfoundland and Labrador
Common reed (Photo by Wikimedia Commons)
European common reed: European common reed has rapidly spread in parts of eastern Canada, in wetlands and along beaches and lakeshores. It ready spreads along roads and highways before invading natural habitats. European common reed forms tall, dense thickets that shade out native vegetation. This invasive species is spreading westward, but there is still an opportunity to stop its spread into western Canada. NCC has been managing European common reed on several properties, including a successful control project on Pelee Island, in Ontario.
Provinces/territories where this species is found: Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, PEI, Newfoundland and Labrador
Japanese knotweed: Japanese knotweed was probably introduced as a garden plant. It can grow up to three metres in height and has nodes on its stems that resemble bamboo. Japanese knotweed is an aggressive invader that can form dense thickets and outcompete native vegetation. It has been identified by the International Union for the Conservation of Nature as one of the world’s worst invading species. This species had not been documented in the prairie provinces, until recent records from Alberta. Japanese knotweed is particularly problematic in Atlantic Canada, where it is taking over the edges of creeks and lakes.
Provinces/territories where this species is found: BC, Alberta, Ontario, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, PEI, Newfoundland and Labrador
NCC staff tackle invasive garlic mustard (photo by NCC)
Garlic mustard: Garlic mustard is a forest invader that was first recorded in New York in 1868. It is native to Europe and may have been brought to North America as a food and medicinal plant. Garlic mustard has spread to into forests throughout many parts of eastern North America, and has more recently been found in BC and Alberta. This species is one of the few invasive plants that spreads into healthy, intact forests and displaces native species. Each plant produces thousands of tiny black seeds that are viable in the soil for many years. NCC and our Conservation Volunteers are managing garlic mustard on many properties. Garlic mustard leaves can be picked and turned into a tasty pesto!
Provinces/territories where this species is found: BC, Alberta, Ontario, Quebec, New Brunswick, Nova Scotia, PEI
Glossy buckthorn (photo by Calin Darabus)
Buckthorn (glossy false and European): These two invasive buckthorns are shrubs that were introduced to North America in the late 1800s as ornamentals and were widely planted as windbreaks along farm fields. Both species occur in a wide variety of habitats and form dense thickets that shade out native plants. The plant produces large numbers of seeds in berry-like black fruits that germinate quickly and prevent the regeneration of native trees and shrubs. Common buckthorn is also a primary host of the non-native soybean aphid, which is a serious agricultural pest.
Provinces/territories where this species is found: Alberta (European only), Saskatchewan (European only), Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, PEI
Common tansy: Common tansy is native to Europe and was introduced to North America in the 1600s as a horticultural and medicinal plant. It has yellow, button-like flowers and can grow to 1.5 metres in height. It has been documented from every region in Canada except Nunavut, but is having the greatest impact on stream banks and native grasslands in the prairies and central BC. In addition to outcompeting native plants, common tansy produces a toxic compound that can impact cattle and wildlife.
Provinces/territories where this species is found: Yukon, Northwest Territories, BC, Alberta, Saskatchewan, Manitoba, Ontario, New Brunswick, Nova Scotia, PEI, Newfoundland and Labrador
Volunteer works to remove dog-strangling vine, Carden Alvar, ON (Photo by NCC)
European swallow-wort/dog-strangling vine: European swallow-wort is native to eastern Europe and was first recorded near Toronto in 1899. It grows up to two metres long in dense thickets, or by growing on other plants. Monarch butterflies have been known to lay their eggs on this plant, but the larvae do not survive. European swallow-wort invades forests, stream banks, grasslands and globally rare alvar habitats. A moth from the Ukraine that can only survive on European swallow-wort has been approved for release in North America to act as a bio-control for this invasive plant.
Provinces/territories where this species is found: BC, Ontario, Quebec
Removing purple loosestrife from Maber Flats, BC (Photo by NCC)
Purple loosestrife: Identified by the International Union for Conservation of Nature as one of the world’s worst invading species, a single purple loosestrife plant can produce over 2 million seeds each year! This species was introduced to North America from Europe in the 1800s for ornamental and medicinal purposes. In fact, it is still sold as an ornamental plant in some places. Purple loosestrife crowds out most native vegetation and can create near-monocultures. From 1992 to 1994, two beetles and two weevils from Europe were released as a biological control and seem to be reducing the numbers of this plant.
Provinces/territories where this species is found: BC, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, PEI, Newfoundland and Labrador
Check out Canada's top 10 invasive plant species below:
Invasive plant species
How can you help?
We all have an important role to play in preventing the introduction and spread of invasive species. Many invasive plants, such as purple loosestrife and Japanese knotweed, started out in our gardens. We can all play a role in auditing our properties to ensure we aren’t growing plants that have a track record of escaping and invading natural habitats.
One of the most exciting aspects of conservation today is that we can play a role in identifying and reporting invasive plants, especially where they have recently expanded into new places. By reporting invasive plants to local weed inspectors, provincial invasive species councils or using phone apps such as iNaturalist, land managers can respond quickly and control invasive plants before they can gain a foothold and become a problem.
The 10 species listed above may not all be problem where you live, but they are some of most serious threats to NCC properties and other protected areas across Canada. Many other species, such as yellow flag iris, woodland angelica, scotch broom, Himalayan balsam and goutweed are key issues in certain places across the country.
We still have a long way to go in effectively managing invasive plants in Canada, but we all have role. By being able to identify these species, controlling them on our properties and reporting them as they spread into new areas, we can help to protect our Canadian species and habitats.
For more information about invasive species in your area and how to control them, please visit the website of your provincial/territorial invasive species council.