Invasive plants in Southern Ontario spread aggressively through forests, wetlands, meadows, and residential landscapes, crowding out native species and degrading habitat across the region. The area’s temperate climate, long growing season, and dense human population create ideal conditions for non-native plants to escape cultivation and establish in natural areas. Once rooted, many of these species are difficult and expensive to remove.
Many invasive plants arrived through horticulture, shipping, and agriculture before their aggressive behavior became clear. Others are still sold in nurseries, contributing to their continued spread into forests, shorelines, and roadsides. Birds, wind, water, and human activity carry seeds and root fragments into new areas each year.
Identifying invasive plants early is the most effective way to prevent them from taking over a property or natural area. Accurate identification determines the right control method and prevents wasted effort on the wrong species. Some invasive plants also pose health risks to humans and livestock, making correct identification essential.
In this guide, we break down 30 invasive plants in Southern Ontario by examining their appearance, habitat, spread, and control methods to help you identify and manage these species on your land.
Invasive Plants Southern Ontario: 30 Species To Know
1. Common Buckthorn (Rhamnus cathartica) – Most Widespread Invasive Shrub
Common buckthorn is the most widespread invasive shrub in Southern Ontario, forming dense thickets in forests, hedgerows, and roadsides. It grows 10 to 25 feet tall with glossy oval leaves and clusters of dark berries in fall. The plant spreads through bird-dispersed seeds that remain viable in the soil for years.
Common buckthorn leafs out early and holds leaves late, giving it a longer growing season than native shrubs. It also alters soil chemistry and harbors soybean aphids, a major agricultural pest. Dense infestations reduce habitat quality for wildlife and prevent native plants from regenerating.
Control requires cut-stump treatment with herbicide, since buckthorn resprouts vigorously after cutting alone to completely eliminate Common buckthorn. Foliar herbicide applications work on smaller plants, while basal bark treatment is effective on larger specimens. Repeated monitoring and treatment over several seasons are necessary because the seedbank persists long after the parent plants are removed.
2. Glossy Buckthorn (Frangula alnus) – Most Wetland-Invasive Shrub
Glossy buckthorn is a wetland-invasive shrub in Southern Ontario, forming dense thickets in bogs, fens, and wet meadows. It grows 10 to 20 feet tall with glossy oval leaves and small red berries that turn black as they ripen. The plant spreads through bird-dispersed seeds and root suckers that remain viable in the soil for years.
Glossy buckthorn thrives in wet soils where few other shrubs can grow, allowing it to dominate wetland habitats and reduce plant diversity. It leafs out early and holds leaves late, giving it a competitive advantage over native species. Dense infestations degrade habitat for amphibians, reptiles, and wetland birds.
Control requires cutting the plant and treating the stump with herbicide, since glossy buckthorn resprouts readily after cutting alone. Foliar herbicide applications work on smaller plants, while basal bark treatment is effective on larger specimens. Repeated monitoring and treatment are necessary because seeds remain viable in the soil for years.
3. Garlic Mustard (Alliaria petiolata) – Most Aggressive Woodland Invader
Garlic mustard is the most aggressive woodland invader in Southern Ontario, forming dense stands in forests, trailsides, and disturbed areas. It grows 1 to 4 feet tall with heart-shaped leaves and small white flowers in spring. The plant spreads through abundant seeds that remain viable in the soil for years.
Garlic mustard is particularly problematic because it releases allelopathic chemicals that suppress native plants and disrupt beneficial soil fungi. Its dense stands reduce habitat quality for wildlife and alter forest structure. The plant also spreads readily along trails and waterways, where human and water activity move seeds into new areas.
Control involves hand-pulling before seed set, mowing before flowering, and applying herbicide in spring or fall. Repeated treatments over several seasons are necessary to exhaust the seedbank in the soil. Removing plants before they flower prevents seed production and reduces future populations.
4. Dog-Strangling Vine (Vincetoxicum rossicum) – Most Aggressive Vine Invader
Dog-strangling vine is the most aggressive vine invader in Southern Ontario, forming dense mats that smother native vegetation in forests, meadows, and roadsides. It grows 3 to 6 feet tall with opposite oval leaves and small purple-brown flowers in summer. The plant spreads through wind-dispersed seeds and creeping rhizomes.
Dog-strangling vine is particularly problematic because it grows rapidly and outcompetes native plants for light and nutrients. Its dense mats smother native shrubs and tree seedlings, reducing forest biodiversity. The plant also traps monarch butterflies, which lay eggs on it that cannot survive.
Control involves repeated cutting before seed set, hand-pulling small infestations, and herbicide treatment for large stands. Combining mowing with late-season herbicide application weakens the rhizome system. Follow-up monitoring for several seasons is essential to prevent regrowth.
5. Phragmites (Phragmites australis) – Most Aggressive Wetland Grass
Phragmites is the most aggressive wetland grass in Southern Ontario, forming dense stands in marshes, roadside ditches, and shorelines. It grows 6 to 15 feet tall with hollow stems and feathery seed heads in late summer. The plant spreads through rhizomes and abundant seeds that remain viable in the soil for years.
Phragmites is particularly problematic because it displaces native wetland plants and reduces habitat quality for wildlife. Its dense stands alter wetland hydrology and reduce plant diversity. The plant also spreads readily along roadsides, where mowing equipment carries rhizome fragments into new areas.
Control requires repeated herbicide applications in late summer and early fall, combined with cutting or mowing to weaken the rhizome system. Repeated treatments over several seasons are necessary to exhaust the rhizome bank in the soil. Removing seed heads before they mature prevents wind-dispersed spread.
6. Japanese Knotweed (Fallopia japonica) – Most Destructive Invasive Plant
Japanese knotweed is the most destructive invasive plant in Southern Ontario, forming dense stands that damage foundations, pavement, and stream banks. It grows 6 to 12 feet tall with hollow bamboo-like stems and large heart-shaped leaves. The plant spreads primarily through rhizomes that can extend 20 feet from the parent plant.
Japanese knotweed is particularly problematic because its deep rhizome system makes hand-pulling ineffective unless the entire root is removed. It is especially aggressive along waterways, where rhizome fragments are carried downstream and establish new infestations. Dense stands reduce habitat quality for native wildlife and increase erosion.
Control requires a multi-year program because Japanese knotweed resprouts vigorously after cutting or herbicide treatment. Repeated herbicide applications in late summer and early fall are most effective, combined with cutting or mowing to weaken the rhizome system. Follow-up monitoring for several seasons is essential.
7. Giant Hogweed (Heracleum mantegazzianum) – Most Dangerous Invasive Plant
Giant hogweed is the most dangerous invasive plant in Southern Ontario because its sap causes severe skin burns and blistering when exposed to sunlight. It grows 8 to 15 feet tall with massive deeply lobed leaves and large white flower heads. The plant spreads through abundant seeds that remain viable in the soil for years.
Giant hogweed is particularly problematic along rivers, roadsides, and disturbed areas, where it forms dense stands that crowd out native vegetation. Its sap contains furanocoumarins that cause photodermatitis, a painful reaction that can leave scars lasting for years. Contact with the plant requires immediate medical attention.
Control requires protective clothing, gloves, and eye protection, plus careful removal of flower heads before seed set. Herbicide treatment or root cutting is effective when performed by trained professionals. Never attempt to remove giant hogweed without proper protective equipment.
8. Wild Parsnip (Pastinaca sativa) – Most Dangerous Roadside Invader
Wild parsnip is the most dangerous roadside invader in Southern Ontario because its sap causes severe skin burns and blistering when exposed to sunlight. It grows 2 to 5 feet tall with yellow umbrella-shaped flower clusters and deeply lobed leaves. The plant spreads through abundant seeds that remain viable in the soil for years.
Wild parsnip is particularly problematic along roadsides, trails, and field edges, where it forms dense stands that crowd out native vegetation. Its sap contains furanocoumarins that cause phytophotodermatitis, a painful reaction that can leave scars. Contact with the plant requires immediate medical attention.
Control requires protective clothing, gloves, and eye protection, plus careful removal of flower heads before seed set. Mowing before seed set reduces spread, and herbicide treatment is effective on larger infestations. Never attempt to remove wild parsnip without proper protective equipment.
9. Purple Loosestrife (Lythrum salicaria) – Most Aggressive Wetland Invader
Purple loosestrife is an aggressive wetland invader in Southern Ontario, forming dense stands in marshes, stream banks, and wet meadows. It grows 3 to 6 feet tall with square stems and spikes of purple flowers in summer. The plant spreads through abundant seeds and root fragments that remain viable in the soil for years.
Purple loosestrife is particularly problematic because it displaces native wetland plants and reduces habitat quality for wildlife. Its dense stands alter wetland hydrology and reduce plant diversity. The plant spreads readily along waterways, where water carries seeds and root fragments into new areas.
Control involves hand-pulling before seed set, cutting flower spikes before they mature, and applying herbicide in late summer. Repeated treatments over several seasons are necessary to exhaust the seedbank in the soil. Biological control agents, including beetles, are also used in large wetland infestations.
10. Eurasian Watermilfoil (Myriophyllum spicatum) – Most Aggressive Aquatic Invader
Eurasian watermilfoil is the most aggressive aquatic invader in Southern Ontario, forming dense mats in lakes, rivers, and ponds. It grows 3 to 10 feet long with feathery leaves arranged in whorls around the stem. The plant spreads through stem fragments that root in new areas.
Eurasian watermilfoil is particularly problematic because it forms dense mats that block boat traffic, reduce oxygen levels, and displace native aquatic plants. Its rapid growth and vegetative spread allow it to dominate water bodies within a few seasons. Dense infestations harm fish habitat and reduce water quality.
Control involves hand-pulling small infestations, using benthic barriers to smother large patches, and applying aquatic-approved herbicides in severe cases. Cleaning boats and equipment between water bodies prevents the spread of stem fragments. Follow-up monitoring is essential to catch regrowth early.
11. Zebra Mussel (Dreissena polymorpha) – Most Aggressive Aquatic Invader
Zebra mussel is technically an animal, not a plant, but it is included here because it is one of the most damaging invasive species in Southern Ontario waters. It is a small freshwater mussel that attaches to hard surfaces in lakes, rivers, and pipes. The species spreads through microscopic larvae carried in ballast water and on boat hulls.
Zebra mussels are particularly problematic because they filter large volumes of water, removing plankton that native species depend on. They also clog water intake pipes and attach to boat hulls, docks, and native mussels. Dense infestations alter entire aquatic ecosystems and cause millions in infrastructure damage each year.
Control is largely preventive, including boat cleaning, drying, and draining between water bodies. There is no practical way to remove established zebra mussel populations from large water bodies. Public education and inspection stations help slow the spread to new lakes and rivers.
12. Emerald Ash Borer (Agrilus planipennis) – Most Destructive Invasive Insect
Emerald ash borer is an invasive beetle that has killed millions of ash trees across Southern Ontario since its discovery in 2002. The adult is a metallic green beetle about half an inch long, while the larvae feed under the bark and cut off the tree’s nutrient flow. The species spreads through infested firewood and natural flight.
Emerald ash borer is particularly problematic because it kills ash trees of all ages and species, eliminating an entire genus from forests and urban landscapes. Dead ash trees create hazards for people and property, and their loss alters forest structure. The insect has spread across most of Southern Ontario and continues to expand its range.
Control options include insecticide injections for high-value landscape trees and biological control agents released by government agencies. Ash trees cannot be saved once heavily infested. Removing dead ash trees before they fall is essential for safety, and replacing them with diverse native species restores forest resilience.
13. Norway Maple (Acer platanoides) – Most Common Shade Tree Invader
Norway maple is a common shade tree invader in Southern Ontario, forming dense stands in forests, roadsides, and urban areas. It grows 40 to 60 feet tall with large lobed leaves and clusters of winged seeds in fall. The plant spreads through wind-dispersed seeds that remain viable in the soil for years.
Norway maple is particularly problematic because it grows rapidly and outcompetes native tree species for light and nutrients. Its dense canopy shades out native understory plants, reducing forest biodiversity. The plant also releases allelopathic chemicals that suppress competing vegetation.
Control involves cutting the tree and treating the stump with herbicide to prevent resprouting. Repeated mowing over several seasons weakens the root system. Removing seed-producing trees before seeds ripen prevents wind from spreading seeds to new areas.
14. Tree of Heaven (Ailanthus altissima) – Most Invasive Urban Tree
Tree of heaven is a highly invasive urban tree in Southern Ontario, forming dense stands in cities, roadsides, and disturbed areas. It grows 40 to 80 feet tall with compound leaves and clusters of winged seeds in fall. The plant spreads through wind-dispersed seeds and root suckers that form dense colonies.
Tree of heaven is particularly problematic because it grows rapidly and outcompetes native tree species for light and nutrients. Its dense stands reduce urban biodiversity and alter habitat structure. The plant also releases allelopathic chemicals that suppress competing vegetation and produces a strong odor.
Control involves cutting the tree and treating the stump with herbicide to prevent resprouting. Repeated mowing over several seasons weakens the root system. Removing seed-producing trees before seeds ripen prevents wind from spreading seeds to new areas.
15. Black Locust (Robinia pseudoacacia) – Most Aggressive Nitrogen-Fixing Invader
Black locust is an aggressive nitrogen-fixing invader in Southern Ontario, forming dense stands in forests, roadsides, and disturbed areas. It grows 40 to 80 feet tall with compound leaves and clusters of white flowers in spring followed by seed pods. The plant spreads through seeds and root suckers that form dense colonies.
Black locust is particularly problematic because it grows rapidly and outcompetes native tree species for light and nutrients. Its dense stands reduce forest biodiversity and alter habitat structure. The plant also fixes nitrogen in the soil, changing nutrient cycles in ways that favor additional invasive species.
Control involves cutting the tree and treating the stump with herbicide to prevent resprouting. Repeated mowing over several seasons weakens the root system. Removing seed-producing trees before pods ripen prevents seeds from spreading to new areas.
16. Multiflora Rose (Rosa multiflora) – Most Aggressive Thorny Invader
Multiflora rose is one of the most aggressive invasive shrubs in Southern Ontario, forming impenetrable thickets that spread across pastures, roadsides, and forest edges. It grows 6 to 10 feet tall with arching stems covered in curved thorns and clusters of small white flowers in late spring. The plant produces abundant red rose hips in fall that are spread by birds and mammals.
Multiflora rose was once widely planted for erosion control and wildlife cover before its invasive nature became clear. It spreads through seeds, root suckers, and stems that root where they touch the ground, allowing a single plant to form a large colony within a few years. Dense thickets exclude livestock, reduce pasture productivity, and crowd out native vegetation.
Control requires persistent effort because multiflora rose resprouts readily after cutting. Repeated mowing over several seasons weakens the plant, while cut-stump or foliar herbicide treatment delivers faster results. Removing seed-producing plants before fall prevents birds from spreading seeds to new areas.
17. Autumn Olive (Elaeagnus umbellata) – Most Prolific Seeder
Autumn olive is a prolific seeding invasive shrub in Southern Ontario, forming dense thickets in forests, roadsides, and old fields. It grows 10 to 20 feet tall with silvery-green leaves and small fragrant cream-colored flowers in spring. The plant produces thousands of red speckled berries each fall that are spread by birds.
Autumn olive spreads rapidly through bird-dispersed seeds and can colonize disturbed areas within a few years. Its ability to fix nitrogen allows it to thrive in poor soils where native plants struggle, giving it a competitive advantage that leads to dense monocultures. These thickets reduce habitat quality for wildlife that depends on native plants.
Control requires cutting the plant and treating the stump with herbicide to prevent resprouting, since autumn olive regrows vigorously from cut stems. Foliar herbicide applications work on smaller plants, while basal bark treatment is effective on larger specimens. Follow-up monitoring for several seasons is essential because seeds remain viable in the soil for years.
18. Burning Bush (Euonymus alatus) – Most Common Landscape Escape
Burning bush is a common landscape escape in Southern Ontario, widely planted for its brilliant red fall foliage before its invasive tendencies became widely recognized. It grows 6 to 15 feet tall with corky ridges along its stems and small oval leaves that turn bright red in autumn. The plant produces orange-red fruits in fall that are spread by birds.
Burning bush spreads from landscape plantings into forests, roadsides, and natural areas, where it forms dense stands that crowd out native shrubs and wildflowers. Its seeds are dispersed by birds, allowing it to establish far from the original planting. Many nurseries still sell the species, contributing to its continued spread across the region.
Control involves removing the plant and treating the stump with herbicide to prevent resprouting. Hand-pulling works on seedlings and small plants, while cut-stump treatment with glyphosate or triclopyr is effective on established bushes. Replacing burning bush with native alternatives like red chokeberry or winterberry prevents future spread.
19. Amur Honeysuckle (Lonicera maackii) – Most Shade-Tolerant Honeysuckle
Amur honeysuckle is a shade-tolerant invasive shrub in Southern Ontario, forming dense stands beneath forest canopies. It grows 10 to 15 feet tall with opposite oval leaves and pairs of white to yellow flowers in spring. The plant produces abundant red berries in fall that are spread by birds.
Amur honeysuckle leafs out earlier in spring and holds leaves later in fall than native shrubs, giving it a longer growing season and a competitive edge. It also releases allelopathic chemicals that suppress the growth of native plants, further reducing biodiversity. Dense infestations reduce habitat quality for wildlife and alter forest structure.
Control requires cutting the plant and treating the stump with herbicide, since Amur honeysuckle resprouts vigorously after cutting alone. Foliar herbicide applications work on smaller plants, while basal bark treatment is effective on larger specimens. Follow-up monitoring for several seasons is essential because the seedbank persists for years.
20. Morrow’s Honeysuckle (Lonicera morrowii) – Most Common Honeysuckle Invader
Morrow’s honeysuckle is one of the most common invasive honeysuckles in Southern Ontario, forming dense thickets in forests, edges, and disturbed areas. It grows 6 to 8 feet tall with gray-green leaves and pairs of white flowers that turn yellow as they age. The plant produces red berries in summer that are spread by birds.
Morrow’s honeysuckle spreads rapidly through bird-dispersed seeds and can colonize forests, roadsides, and old fields within a few years. It leafs out early and holds leaves late, giving it a competitive advantage over native shrubs. Dense thickets shade out native wildflowers and tree seedlings, reducing forest biodiversity.
Control requires cutting the plant and treating the stump with herbicide, since honeysuckles resprout readily after cutting alone. Foliar herbicide applications work on smaller plants, while basal bark treatment is effective on larger specimens. Repeated monitoring and treatment over several seasons are necessary because seeds remain viable in the soil for years.
21. Tartarian Honeysuckle (Lonicera tatarica) – Most Ornamental Honeysuckle Invader
Tartarian honeysuckle is an ornamental invasive honeysuckle in Southern Ontario, widely planted for its pink or white flowers before its invasive nature became clear. It grows 6 to 10 feet tall with blue-green leaves and pairs of fragrant flowers in spring. The plant produces red berries in summer that are spread by birds.
Tartarian honeysuckle spreads from landscape plantings into forests, roadsides, and natural areas, where it forms dense stands that crowd out native shrubs and wildflowers. It leafs out early and holds leaves late, giving it a competitive advantage over native species. Dense infestations reduce habitat quality for wildlife and alter forest structure.
Control involves removing the plant and treating the stump with herbicide to prevent resprouting. Hand-pulling works on seedlings and small plants, while cut-stump treatment with glyphosate or triclopyr is effective on established bushes. Replacing Tartarian honeysuckle with native alternatives like native honeysuckle vines prevents future spread.
22. Japanese Barberry (Berberis thunbergii) – Most Common Thorny Invader
Japanese barberry is a common invasive bush in Southern Ontario, forming dense thorny thickets in forests, pastures, and residential landscapes. It grows 2 to 6 feet tall with small spoon-shaped leaves and sharp spines along its stems. The plant produces bright red berries in fall that persist through winter and are spread by birds.
Japanese barberry is particularly problematic because it changes soil chemistry, raising pH and nitrogen levels in a way that favors its own growth over native plants. It also harbors blacklegged ticks, the species that transmits Lyme disease, at higher densities than native vegetation. These traits make it one of the highest-priority invasive shrubs for removal.
Control requires a combination of mechanical and chemical methods. Hand-pulling works on small plants, while cut-stump treatment with glyphosate or triclopyr is effective on established bushes. Repeated monitoring and follow-up treatment are necessary because barberry resprouts vigorously from root fragments left in the soil.
23. European Buckthorn (Rhamnus cathartica) – Most Persistent Hedgerow Invader
European buckthorn is a persistent hedgerow invader in Southern Ontario, forming dense stands along fencerows, roadsides, and forest edges. It grows 10 to 25 feet tall with oval leaves and clusters of dark berries in fall. The plant spreads through bird-dispersed seeds that remain viable in the soil for years.
European buckthorn is particularly problematic because it leafs out early and holds leaves late, giving it a competitive advantage over native shrubs. It also alters soil chemistry and harbors soybean aphids, a major agricultural pest. Dense stands reduce habitat quality for wildlife and prevent native plants from regenerating.
Control requires cut-stump treatment with herbicide, since buckthorn resprouts vigorously after cutting alone. Foliar herbicide applications work on smaller plants, while basal bark treatment is effective on larger specimens. Repeated monitoring and treatment over several seasons are necessary because the seedbank persists long after the parent plants are removed.
24. Common Reed (Phragmites australis) – Most Destructive Wetland Grass
Common reed is the most destructive wetland grass in Southern Ontario, forming dense stands in marshes, roadside ditches, and shorelines. It grows 6 to 15 feet tall with hollow stems and feathery seed heads in late summer. The plant spreads through rhizomes and abundant seeds that remain viable in the soil for years.
Common reed is particularly problematic because it displaces native wetland plants and reduces habitat quality for wildlife. Its dense stands alter wetland hydrology and reduce plant diversity. The plant also spreads readily along roadsides, where mowing equipment carries rhizome fragments into new areas.
Control requires repeated herbicide applications in late summer and early fall, combined with cutting or mowing to weaken the rhizome system. Repeated treatments over several seasons are necessary to exhaust the rhizome bank in the soil. Removing seed heads before they mature prevents wind-dispersed spread.
25. Reed Canary Grass (Phalaris arundinacea) – Most Aggressive Wetland Grass
Reed canary grass is an aggressive wetland grass in Southern Ontario, forming dense monocultures in marshes, wet meadows, and stream banks. It grows 2 to 6 feet tall with flat leaves and dense seed heads in early summer. The plant spreads through rhizomes and abundant seeds that remain viable in the soil for years.
Reed canary grass is particularly problematic because it forms dense mats that crowd out native wetland vegetation and reduce habitat quality for wildlife. Its rapid growth and vegetative spread allow it to dominate wetland habitats within a few seasons. Dense infestations reduce plant diversity and alter wetland hydrology.
Control involves repeated mowing before seed set, applying herbicide in late summer, and smothering with black plastic for small patches. Combining methods over several seasons is necessary to exhaust the rhizome bank and seedbank. Follow-up monitoring is essential to catch regrowth early.
26. Yellow Flag Iris (Iris pseudacorus) – Most Aggressive Aquatic Invader
Yellow flag iris is an aggressive aquatic invader in Southern Ontario, forming dense stands in wetlands, ponds, and stream banks. It grows 3 to 5 feet tall with sword-shaped leaves and bright yellow flowers in late spring. The plant spreads through rhizomes and abundant seeds that float on water.
Yellow flag iris is particularly problematic because it displaces native wetland plants and reduces habitat quality for wildlife. Its dense stands alter wetland hydrology and reduce plant diversity. The plant also spreads readily along waterways, where water carries seeds and rhizome fragments into new areas.
Control involves digging out rhizomes by hand for small infestations, cutting flower heads before seed set, and applying aquatic-approved herbicide for large stands. Repeated treatments over several seasons are necessary to exhaust the rhizome bank. Follow-up monitoring is essential to catch regrowth early.
27. Himalayan Balsam (Impatiens glandulifera) – Most Aggressive Riverside Invader
Himalayan balsam is an aggressive riverside invasive plant in Southern Ontario, forming dense stands along streams, ditches, and wet meadows. It grows 3 to 10 feet tall with hollow stems, serrated leaves, and pink to purple flowers in summer. The plant spreads through explosive seed pods that scatter seeds up to 20 feet from the parent plant.
Himalayan balsam is particularly problematic because it outcompetes native vegetation along waterways and increases erosion when it dies back in winter. Its rapid growth and high seed production allow it to dominate riverbanks within a few seasons. Dense stands reduce habitat quality for native plants and insects.
Control involves hand-pulling before seed set, which is effective because the plant has shallow roots. Repeated mowing or cutting before flowering prevents seed production. Removing plants before seed pods mature is essential to prevent spread along waterways.
28. Periwinkle (Vinca minor) – Most Persistent Ground Cover Invader
Periwinkle is a persistent ground cover invader in Southern Ontario, forming dense mats in forests, roadsides, and residential landscapes. It grows 6 to 18 inches tall with glossy evergreen leaves and purple flowers in spring. The plant spreads through creeping stems that root at the nodes and form dense colonies.
Periwinkle is particularly problematic because it forms dense mats that smother native wildflowers and prevent tree seedlings from establishing. Its tolerance for shade allows it to thrive beneath forest canopies, where it slowly displaces native understory plants. The plant also spreads readily from garden plantings into natural areas.
Control involves hand-pulling or raking before seed set, repeated mowing, and herbicide treatment for large stands. Combining mechanical removal with late-season herbicide application weakens the root system. Follow-up monitoring for several seasons is essential because periwinkle regrows from root fragments.
29. English Ivy (Hedera helix) – Most Aggressive Climbing Vine
English ivy is an aggressive climbing vine in Southern Ontario, forming dense mats on trees, walls, and forest floors. It grows 30 to 90 feet long with lobed evergreen leaves and small greenish flowers in fall. The plant spreads through bird-dispersed seeds and creeping stems that root at the nodes.
English ivy is particularly problematic because it climbs trees and can eventually kill them by blocking sunlight and adding weight to branches. Its dense ground mats smother native wildflowers and prevent tree seedlings from establishing. The plant also spreads readily from garden plantings into natural areas.
Control involves cutting vines at the base of trees, pulling ground mats by hand, and applying herbicide to regrowth. Repeated treatments over several seasons are necessary because English ivy regrows from root fragments. Removing vines before they produce seeds prevents birds from spreading seeds to new areas.
30. Kudzu (Pueraria montana) – Most Aggressive Vine in Ontario
Kudzu is an aggressive vine that has recently been confirmed in Southern Ontario, forming dense mats that smother native vegetation in forests, roadsides, and disturbed areas. It grows 30 to 100 feet long with compound leaves and clusters of purple flowers in late summer. The plant spreads through seeds and creeping root crowns that form dense colonies.
Kudzu is particularly problematic because it grows up to a foot per day in warm conditions and quickly covers trees, buildings, and fences. Its dense mats smother native plants and reduce habitat quality for wildlife. The plant is difficult to eradicate once established because it regrows from deep root crowns.
Control requires repeated cutting, grazing, or herbicide treatment over several seasons to exhaust the root system. Combining methods is usually necessary because kudzu resprouts vigorously. Early detection and rapid response are critical to preventing kudzu from spreading across the region.
7 Tips For Managing Invasive Plants In Southern Ontario
- Identify before removing: Confirm the plant is invasive before cutting or spraying, since some native species look similar to invasive plants. Consult local extension services or a field guide for accurate identification.
- Remove before seed set: Cut or treat invasive plants before they produce seed to prevent birds and wind from spreading seeds to new areas. This is the single most effective way to slow the spread of invasive species.
- Treat cut stumps immediately: Most invasive shrubs and trees resprout vigorously after cutting alone. Apply concentrated herbicide to freshly cut stumps within minutes to kill the root system and prevent regrowth.
- Time herbicide applications for late summer and fall: Invasive plants translocate energy to their roots in late summer and fall, making herbicide applications more effective during this window than in spring or early summer.
- Replace with native alternatives: Plant native species after removing invasive plants to provide habitat and prevent reinfestation. Native plants compete with invasive seedlings and support local wildlife.
- Monitor and re-treat for several seasons: Invasive plants regrow from seeds and root fragments left in the soil. Scout treated areas several times per season and re-treat regrowth while it is still small.
- Clean equipment between areas: Mowers, trimmers, and hand tools can spread seeds and root fragments to new areas. Clean equipment after working in infested zones to prevent introducing invasive plants to clean sites.
Invasive Plants Southern Ontario (FAQs)
1. What are the most invasive plants in Southern Ontario?
Common buckthorn, garlic mustard, dog-strangling vine, phragmites, and Japanese knotweed are among the most invasive plants in Southern Ontario. Each spreads aggressively and displaces native vegetation across forests, wetlands, and roadsides.
2. Why are invasive plants a problem in Southern Ontario?
Invasive plants in Southern Ontario crowd out native species, reduce biodiversity, alter soil chemistry, and damage ecosystems. Some species are toxic to humans or livestock, while others block waterways or increase erosion along rivers and coastlines.
3. How do invasive plants spread in Southern Ontario?
Invasive plants spread through bird-dispersed seeds, wind-dispersed seeds, water-carried fragments, and garden escapes. Many species were introduced through horticulture, agriculture, and shipping, and continue to spread from planted landscapes into natural areas.
4. What is the most dangerous invasive plant in Southern Ontario?
Giant hogweed and wild parsnip are the most dangerous invasive plants because their sap causes severe skin burns and blistering when exposed to sunlight. Contact with these plants requires immediate medical attention and removal should only be attempted with protective equipment.
5. Can I remove invasive plants from my property?
Yes, landowners can remove invasive plants from their property using digging, cutting, herbicide treatment, or smothering. Timing matters, and repeated follow-up is usually necessary because most species regrow from seeds or root fragments.
6. Are invasive plants in Ontario illegal to plant?
Ontario regulates certain invasive species under the Invasive Species Act, restricting their sale, planting, and movement. Check the official Ontario invasive species list and local regulations before purchasing or planting any species that may be invasive.
7. What should I plant instead of invasive plants in Southern Ontario?
Plant native Ontario species like red chokeberry, winterberry, native viburnums, dogwoods, and native wildflowers after removing invasive plants. Native species provide habitat for wildlife, compete with invasive seedlings, and require less maintenance over time.
Conclusion
Common buckthorn is the most widespread invasive shrub in Southern Ontario, forming dense thickets that alter soil chemistry and harbor agricultural pests. Garlic mustard follows closely as the most aggressive woodland invader, while dog-strangling vine and phragmites dominate meadows and wetlands across the region.
Giant hogweed and wild parsnip pose the greatest health risks because their sap causes severe skin burns on contact. Japanese knotweed, purple loosestrife, Eurasian watermilfoil, and zebra mussels round out the list of species that damage infrastructure, waterways, and native ecosystems.
Effective management requires identifying invasive plants early, removing them before seed set, treating cut stumps with herbicide, and replacing them with native alternatives. Follow-up monitoring for several seasons is essential because seeds and root fragments remain viable in the soil long after the parent plants are gone.
Start your invasive plant control program this season, prioritize the most aggressive species on your property, and plan for follow-up work each year. Consistent effort over several seasons protects Southern Ontario’s forests, wetlands, and waterways from these destructive invaders.