Manitoba Maple seeds

“A landscape populated by nonnative plants is, for wildlife, the equivalent of a desert.” – Margaret Renkl, The Real Aliens in Our Backyard

Read more on invasive plants: Sustaining Biodiversity – A Strategic Plan for Managing Invasive Plants in Southern Ontario

Below are some of the invasive exotic species found in southern Ontario, according to observations from UFORA and our colleagues.

We use four categories to sort invasive species based on their greatest negative effect to local ecosystems.

The term “invades” refers to exotic species that become significant in a new area; “dominates” means that the species excludes most others; “native” is the same as indigenous.

Species marked with an (*) may be native to parts of Ontario, but they display aggressive behaviour that threatens natural biodiversity in certain areas, usually aided by human activities.

They are considered invasive exotic plants outside their natural range. We are in the process of revising this list for the Invasive Species Centre.  We’ll post the new list when it’s finished.


Image from Wikipedia

Common Buckthorn

Category 1
Aggressive invasive exotic species that can dominate a site to exclude all other species and remain dominant on the site indefinitely.

These are a threat to natural areas wherever they occur because they can reproduce by means that allow them to move long distances. Many of these are dispersed by birds, wind, water or vegetative reproduction. These are the top priority for control, but control may be difficult. Eradication may be the only option for long-term success.

Common Name Scientific Name Effect on Natural Area
Amur honeysuckle Lonicera maackii invades meadows & forest edges
Autumn olive Elaeagnus umbellata dominates forest edges
Black alder Alnus glutinosa dominates wetlands, changes by shading
Black swallow-wort Cynanchum nigrum dominates meadows & forest understory
Canada thistle Cirsium arvense dominates meadows, prairies & forest edges
Common buckthorn Rhamnus cathartica dominates understory, meadows & prairies
Common reed Phragmites australis * dominates wetlands & wet meadows
Crown vetch Coronilla varia dominates disturbed meadows
Curly pondweed Potamogeton crispus dominates open water areas in SW Ontario
Dames rocket Hesperis matronalis dominates open understory & meadows
Eur . fly honeysuckle Lonicera xylosteum invades meadows & forest edges
Eurasian water milfoil Myriophyllum spicatum dominates open water habitats
European birch Betula pendula dominates wetlands, changes by shading
European frog-bit Hydrocharis morsus-ranae dominates open water habitats
Floating heart Nymphoides peltatum dominates open water habitats
Flowering rush Butomus umbellatus dominates open marshes
Garlic mustard Alliaria petiolata dominates forest herb layer
Glossy buckthorn Rhamnus frangula dominates wetlands
Goutweed Aegopodium podagraria dominates forest understory
Himalayan balsam Impatiens glandulifera dominates forests & wet meadows
Japanese honeysuckle Lonicera japonica dominates forest understory in northern US
Manitoba maple Acer negundo * aggressively invades all habitat types
Morrow’s honeysuckle Lonicera morrowi invades meadows & forest edges
Multiflora rose Rosa multiflora dominates forest edges
Pale swallow-wort Cynanchum rossicum dominates meadows & forest understory
Purple loosestrife Lythrum salicaria dominates wetlands
Rough manna grass Glyceria maxima dominates wet meadows
Tartarian honeysuckle Lonicera tatarica invades meadows & forest edges
White mulberry Morus alba hybridizes with rare M. rubra



Image from Wikipedia

Japanese Knotweed

Category 2
Exotic species that are highly invasive but tend to only dominate certain niches or do not spread rapidly from major concentrations.

Many of these spread vegetatively, or by seeds that drop close to the parent plant. They may have been deliberately planted and persist in dense populations for long periods.

Control where necessary and limit their spread to other areas.

Common Name Scientific Name Effect on Natural Area
Bird-foot trefoil Lotus corniculatus dominates meadows & prairies
Black locust Robinia pseudo-acacia invades meadows
Cow vetch Vicia cracca dominates meadows & prairies
Japanese knotweed Polygonum cuspidatum dominates wet meadows & moist forests
Kentucky bluegrass Poa pratensis dominates prairies
Lilac Syringa vulgaris dominates shallow limestone areas
Moneywort Lysimachia nummularia dominates wet forest understory
Mossy stonecrop Sedum acre invades alvars
Norway maple Acer platanoides dominates forest canopy
Oriental bittersweet Celastrus orbiculatus dominates forests and forest edges
Periwinkle Vinca minor dominates forest understory
Scilla Scilla siberica dominates in forest understory
Scots pine Pinus sylvestris invades meadows
Siberian elm Ulmus pumila invades prairies, causes changes by shading
Sycamore maple Acer pseudoplatanus dominates forest canopy
Tree of Heaven Ailanthus altissima replaces native early-successional forest
White bedstraw Galium mollugo invades meadows
White poplar Populus alba invades meadows
White sweet-clover Melilotus alba dominates meadows & prairies
Yellow sweet-clover Melilotus officinalis dominates meadows & prairies



Image from Wikipedia

Bittersweet Nightshade

Category 3
Exotic species that are moderately invasive but can become locally dominant when the proper conditions exist.

Control where necessary and limit their spread to other areas.

Common Name Scientific Name Effect on Natural Area
Absinth sage Artemisia absinthum invades disturbed meadows
Bittersweet nightshade Solanum dulcamara invades forests & wetlands
Bouncing bet Saponaria officinalis invades meadows
Bugleweed Lycopus europaeus invades wetlands, displaces native species
Common barberry Berberis vulgaris invades forests
Common hawkweed Hieracium vulgatum invades meadows
Common vetch Vicia sativa invades meadows
Crack willow Salix fragilis invades wetlands, displaces willows
Creeping buttercup Ranunculus repens invades meadows
Creeping thyme Thymus praecox invades meadows
Cypress spurge Euphorbia cyparissias invades meadows
English ivy Hedera helix invades forest understory
Eulalia Miscanthus sinensis dominates wet meadows
European stinging nettle Urtica dioica ssp . dioica dominates forest understory
False spiraea Sorbaria sorbifolia invades meadows & forest understory
Field bindweed Convolvulus arvensis dominates meadows
Hoary-alyssum Berteroa incana invades prairies
Horse-chestnut Aesculus hippocastanum creates dense shade
Hybrid willow Salix x rubens invades wetlands, displaces willows
Japanese barberry Berberis thunbergii invades forests
Japanese hop Humulus japonicus invades wet meadows
Lily-of-the-valley Convallaria majalis frequently escapes garden for forests
Marsh cress Rorippa amphibia invades wetlands in SE Ontario
Mother-of-thyme Acinos arvensis invades alvar habitats
Nodding thistle Carduus nutans invades meadows & prairies
Orange hawkweed Hieracium aurantiacum invades meadows
Orchard grass Dactylis glomerata invades meadows & prairies
Pale hawkweed Hieracium x floribundum invades meadows
Quack grass Elymus repens dominates meadows & prairies
Russian olive Elaeagnus angustifolia invades meadows & shrub communities
Singleseed hawthorn Crataegus monogyna dominates shrubs in meadows & prairies
Slender vetch Vicia tetrasperma invades meadows
Spindle-tree Euonymus europaeus invades forest understory & edges
Spotted knapweed Centaurea maculosa invades meadows & prairies
Summer cypress Kochia scoparia invades meadows
Tall fescue Festuca arundinacea dominates moist meadows & prairies
Tansy Tanacetum vulgare invades disturbed meadows
Teasel Dipsacus sylvestris dominates meadows & prairies
Velvet-leaf Abutilon theophrasti invades meadows
White willow Salix alba invades wetlands, displaces willows
Wild parsnip Pastinaca sativa invades meadows
Winged euonymus Euonymus alata invades forest understory shrub layer
Yellow bedstraw Galium verum invades meadows & prairies
Yellow hawkweed Hieracium caespitosum invades meadows
Yellow rocket Barbarea vulgaris invades meadows



European Mountain-Ash flowers

European Mountain-Ash

Category 4
Exotic species that do not pose a serious threat to natural areas unless they are competing directly with more desirable vegetation.

These can often be tolerated in restoration projects if they are already present. They may eventually be replaced through natural succession or management.

Control where necessary and limit their spread to other areas.

Common Name Scientific Name Effect on Natural Area
Alfalfa Medicago sativa invades meadows & prairies
Amur maple Acer ginnala frequently planted
Black medick Medicago lupulina invades meadows
Butter-and-eggs Linaria vulgaris invades meadows
Carolina poplar Populus x canadensis frequently misidentified
Catnip Nepeta cataria invades meadows
Creeping bellflower Campanula rapunculoides invades forest edges & meadows
Creeping bugleweed Ajuga reptans persists in forest understory & edges
Elecampane Inula helenium invades meadows
European Mountain-ash Sorbus aucuparia invades forests
Foxtail grasses Setaria sp. invades meadows
Goat willow Salix caprea frequently substituted for S. discolor
Ground ivy Glechoma hederacea competes with meadow & prairie species
Guelder rose Viburnum opulus ssp. opulus replaced cranberry bushes in S. Ontario
Japanese spurge Pachysandra terminalis persists in forest understory & edges
Leafy spurge Euphorbia esula can dominate prairies
Musk mallow Malva moschata invades meadows
Orange Day lily Hemerocallis fulva dominates meadows
Peppermint Mentha x piperita invades meadows
Perennial rye grass Lolium perenne competes with meadow & prairie species
Privet Ligustrum vulgare invades forest edges
Purple willow Salix purpurea invades wetlands
Rabbit-foot clover Trifolium arvense invades meadows
Red clover Trifolium pratense invades meadows
Scotch elm Ulmus glabra invades disturbed forests
Sheep sorrel Rumex acetosella invades meadows
Smooth brome Bromus inermis resists conversion to native meadow & prairie
St. John’s wort Hypericum perforatum can dominate meadows
Sweet coltsfoot Tussilago farfara invades wet meadows & riverbanks
Tansy groundsel Senecio jacobaea invades meadows
True forget-me-not Myosotis scorpioides dominates shaded seepage areas
Western snowberry Symphoricarpus albus var laevigatus frequently misidentified
White clover Trifolium repens invades meadows
Wild marjoram Origanum vulgare invades disturbed meadows
Yellow flag Iris pseudacorus invades wetlands



Common Hop

Common Hop

Potentially Invasive Exotic Species to Monitor
Some of these species have the potential to become invasive exotics in Ontario.

They can reproduce aggressively on occasion but have not been shown to be a serious threat to natural areas in the province.

Some are very similar to indigenous species and could simply have been overlooked.

Common Name Scientific Name Risk to Natural Areas
Bird cherry Prunus avium does not dominate natural areas
Common hop Humulus lupulus locally invasive in Ontario
Common mugwort Artemisia vulgaris highly invasive in NYC natural areas
Dyer’s woad Isatis tinctoria new invader to Bruce Peninsula
European ash Fraxinus excelsior sometimes misidentified as Black ash
European aspen Populus tremula very similar to Bigtooth aspen
European linden Tilia cordata does not dominate natural areas
European red elder Sambucus racemosa often misidentified as American red elder
European white alder Alnus incana ssp. incana often misidentified as Speckled alder
Exotic cattail species Typha sp. sold in nurseries in northeast US
Fanwort Cabomba caroliniana invasive in New England lakes
Hydrilla Hydrilla verticillata invasive in mid-Atlantic US states
Mezer’s Daphne Daphne mezereum has invaded moist forests in S. Ontario
Minor naiad Najas minor invasive in waterways in US northeast
Nipplewort Lapsana communis
Perfumed cherry Prunus mahaleb
Porcelain-berry Ampelopsis brevipedunculata invasive in northeast US
Red osier dogwood Cornus sericea confusion with Eurasian red dogwoods
Reed canary grass Phalaris arundinacea* European strain may dominate wet meadows
Sweet violet Viola odorata very aggressive garden plant
Water-chestnut Trapa natans invasive in mid-Atlantic US states
Waterweed Egeria densa invasive in Massachusetts wetlands