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What is an invasive species?

Invasive plants are species that are not native to a region or country. Invasive plants have the biological ability and / or lack of natural control agents to compete with, and replace, native species in natural habitats.

Some invasive alien plants were introduced by people when they moved to North America. These plants were introduced for food, medicine, or just to remind the folks of their homeland.  Other alien plants arrived hidden in soil that was once commonly used as ship ballast, in bales of hay used as fodder, in natural packing materials, and in bags of crop seeds.

Not all invasive plants are alien to Canada. Some native Canadian species such as Manitoba maple (Acer negundo) and common milkweed (Asclepias syriaca) extended their Canadian ranges in response to human activities. Manitoba maple now grows in areas well beyond its original area because it grows quickly and seeds itself readily in a variety of disturbed habitats, especially in and around urban centres.  Milkweed has spread widely because of the large expanses of open fields that were created by the cutting the eastern forests.

This page provides information on some invasive plants present in Ontario that have a significant detrimental impact on native ecosystems.

Information about Invasive Plants

A Canadian Wildlife Service publication describes plants that are invasive to natural habitats in Canada.

Invasive plants of natural habitats in Canada:
An integrated review of wetland and upland species and legislation governing their control.

Link to tours.

The following information is synthesized from that report.

The impact of invasive plants is varied, but at the very least, invasive plants threaten our native plants.  More information about the impact of invasive plants is found at:

Link to tours.

Upland invasive plants

Wetland invasive plants

Principal Upland Invasive Alien Plants:

The following is a list of alien species that occupy upland habitats that are considered to represent the most significant threat to natural areas.

Principal Upland Invasive Alien Plants:

Common buckthorn (Rhamnus cathartica)

Garlic mustard (Alliaria petiolata)

Glossy buckthorn (Rhamnus frangula)

Leafy spurge (Euphorbia esula)

Moderate Upland Invasive Alien Plants:

Canada thistle (Cirsium arvense)

St. John'swort (Hypericum perforatum)

Smooth brome grass (Bromus inermis)

Tatarian honeysuckle (Lonicera tatarica)

Yellow sweet clover (Melilotus spp.)

White sweet clover (Melilotus spp.)

Minor Upland Invasive Alien Plants:

Absinth (Artemisia absinthium)

Alfalfa (Medicago sativa)

Black locust (Robinia pseudo-acacia)

Canada blue grass (Poa compressa)

Celandine (Chelidonium majus)

Crested wheat grass (Agropyron pectiniforme)

Dame's rocket (Hesperis matronalis)

Dog-strangling vine (Cynanchum spp.)

English ivy (Hedera helix)

European birch (Betula pendula)

Goutweed (Aegopodium podagraria)

Himalayan blackberry (Rubus discolor)

Hoary-alyssum (Berteroa incana) 

Kentucky blue grass (Poa pratensis)

Lilac (Syringa vulgaris) 

Manitoba maple (Acer negundo)

Mother-of-thyme (Acinos arvensis) 

Mullein (Verbaseum thapsus)

Multiflora rose (Rosa multiflora) 

Nodding thistle (Carduus nutans)

Norway maple (Acer platanoides) 

Periwinkle (Vinca minor)

Ragweed (Ambrosia artemisiifolia)

Scotch broom (Cytisus scoparius)

Scots pine (Pinus sylvestris)

Siberian peashrub (Caragana arborescens)

Spotted knapweed (Centaurea maculosa)

Teasel (Dipsacus spp.)

White bedstraw (Galium mollugo) 

White mulberry (Morus alba)

White poplar (Populus alba)

Wild marjoram (Origanum vulgare)

Potentially Upland Invasive Alien Plants:

The following are listed as potentially invasive aliens of upland natural areas:

Amur honeysuckle (Lonicera maackii)

Goatsbeard (Tragopogon dubius)

Amur maple (Acer ginnala)

Great ragweed (Ambrosia trifida)

Blueweed or Viper's Bugloss (Echium vulgare)

Hedge parsley (Torilis japonica)

Bouncing bet (Saponaria officinalis)

Hemp-nettle (Galeopsis tetrahit)

Burnet-saxifrage (Pimpinella saxifraga)

Impatiens (Impatiens glandulifera)

Cat's-ear (Hypochoeris radicata)

Japanese barberry (Berberis thunbergii)

Climbing euonymus (Euonymus fortunei)

Japanese honeysuckle (Lonicera japonica)

Climbing nightshade (Solanum dulcamara)

Japanese knotweed (Polygonum cuspidatum)

Coltsfoot (Tussilago farfara) 

Mossy stonecrop (Sedum acre)

Crown-vetch (Coronilla varia)

Ox-eye daisy (Leucanthemum vulgare)

Cypress spurge (Euphorbia cyparissias)

Sheep sorrel (Rumex acetosella)

English holly (Ilex aquifolium)

Short-rayed aster (Aster brachyactis)

Eulalia (Miscanthus sinensis)

Siberian elm (Ulmus pumila)

European mountain-ash (Sorbus aucuparia)

Spurge-laurel (Daphne laureola)

Field scabious (Knautia arvensis)

Winged euonymus (Euonymus alata)

Foxglove (Digitalis purpurea)

Winter cress (Barbarea vulgaris)

Principal Wetland Invasive Alien Plants:

The following is a list of alien species that occupy wetland habitats that are considered to represent the most significant threat to natural areas.

Eurasian watermilfoil (Myriophyllum spicatum)

European frog-bit (Hydrocharis morsus-ranae)

Flowering-rush (Butomus umbellatus)

Glossy buckthom (Rhamnus frangula)

Purple loosestrife (Lythrum salicaria)

Reed canary grass (Phalaris arundinacea)

Minor Wetland Invasive Alien Plants:

The following invasive are considered to present only a minor or local threat to wetland natural areas.

Angelica (Angelica sylvestris)

Black alder (Alnus glutinosa)

Curly pondweed (Potamogeton crispus)

European birch (Betula pendula)

Floating heart (Nymphoides peltatum)

Great manna grass (Glyceria maxima)

Marsh cress (Rorippa ainphibia)

Moneywort (Lysimachia nummularia)

Scots pine (Pinus sylvestris)

Yellow flag (Iris pseudacorus)




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For more information email: ajfyon@vianet.on.ca
URL: http://www.ontariowildflower.com/invasive_plants.htm
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Date last modified:

Andy Fyon

April 19, 2003

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