Directory

Ontario

Birdsfoot Trefoil (Lotus corniculatus)
This plant forms a thick mat, choking out all other groundcovers.  It was introduced as livestock fodder and erosion control and is a particular problem along the St. Lawrence.

Canada Thistle (Cirsium arvense)
Despite its name, this weed hails from Europe.  It is one of the first species to become established after a fire.  Although 80 types of insects feed on it, its spread continues unabated.

Cornflower (Batchelor's Button) (Centaureacyanus)
This common plant has a long-lasting blue flower that likes pristine grassland habitats.   It is often included in commercial wildflower seed mixes.

Crown Vetch (Coronilla varia)
A prolific goundcover planted along our highways to control erosion.  It will climb over shrubs, shading them out.  It should only be planted where it can be contained.

Dame's Rocket (Hesperis matronalis)
Sometimes mistakenly called wild phlox, the Europena immigrant escaped cultivation and went wild in the 19th century.  It excludes native species along roadsides and forest borders.

Garlic Mustard (alliara petiolata)
This is the "purple loosestrife" of the forests.  This weed dominates a forest very easily, and is extremely difficult to get rid of once it is established.   It dominates a woodland and excludes all other forest floor plants.

Periwinkle (Vinca minor)
This is a pretty shade-tolerant evergreen groundcover that can persist for generations and crowd out native vegetation.  Please only use it where it cannot escape into local wooded areas.

Purple Loosestrife (Lythrum salicaria)
The effect of this plant of marshes has been well publicized, and many know of its effects.  Our wetlands may never be the same again, although there have been some encouraging strides made in control of this plant.  Given free reign, purple loosestrife chokes out all other plant life in a marsh.

Pampas Grass (Cortaderiaselloano)
This is a huge ornamental that grows over 7 feet high and has plumes that are a favourite of florists.  It hails from Argentina and is invading our roadside ditches and sand dunes where it crowds out everything native.

Burningbush (Euonymus alatus)
A favourite garden shrub known for its brilliant autumn foilage, it has the potential to become a problem, although it has only escaped cultivation occasionally.

Common Buckthorn (Rhamnus cathartica)
This European shrub tolerates a wide range of moisture and light conditions and seeds itself widely.

English Holly (Ilex aquifolium)
Another garden favourite which is not welcome in the wild.  It has infiltrated some old growth forests in British Columbia (Canada)

Japanese Yew (Taxus cuspidata)
Loved by both landscapers and deer.  The latter are keeping it in check at the moment but there are fears that it may become a real weed because of its intense use.

Russian Olive (Elaegnus angustifolia)
Welcomed on the prairies where hardy shrubs and trees are rare, it is now choking out native cottonwoods and willows that wildlife rely on for nesting sites and insect food.

Scotch Broom (Cytisus scoparius)
Introduced as an ornamental and later used to stabilize sand dunes, it is invasive and also flammable to the point of inhancing forest fires.

Tamarisk (Tamarix ramossissima et al)
Prized by gardeners for its pink flowers, it was planted along rivers to prevent erosion and has the potential to invade the prairies of Western Canada.

Tatarian Honeysuckle (Lonicera tatarica)
An escapee in Ontario, although it has not ventured much beyond suburbia.  It can turn prairies into scrubland and is so dense that tree seedlings cannot compete.

Dog-strangling Vine (Cynanchum vincetoxicum)
A member of the Milkweed family, this Europena native was brought to Canada during the Second World War to be evaluated as a possible filler for lifejackets (the seeds are bouyant).  It escaped from research plots and is now established at several sites in Ontario and Quebec.  It forms a tangled mat of twining stems that scramble over and choke out shrubs and trees.  Native plants cannot compete for space.  It is difficult to eradicate even with weedkillers.  Look for a 6-8 foot twining vine with oval leaves, clusters of purple flowers and seedpods similar to those of milkweed.

Amur Maple (Acer ginnala)
One to watch, this Japanese oriental is a prolific seeder and has the potential to become a major invasive in southern parts of Canada.

Norway Maple (Acer platanoides)
One of the most extensively planted shade trees, especially in urban areas, this tree is naturalizing in woodlands and displacing the native Sugar Maple.  It has very dense shade which greatly inhibits understory growth.  Eventually, there are virtually no plants growing at all underneath these trees.

Scots Pine (Pinus sylvestris)
This tall-growing "Christmas tree" has been used widely in reforestation projects and is slowly becoming naturalized.

Tree of Heaven (Ailanthus altissima)
This hardy, fast-growing tree is so rugged that it sprouts in sidewalk cracks and survives in poor soil with little water.

White Poplar (Populus alba)
Used extensively in the 1800s as a windbreak, today it is invading grasslands and meadows.

European Frogbit (Hyddrocharis morsus-ranae)
In 1932 this free-floating herb escaped from an Ottawa experimental farm into the Rideau Canal.  It is now found as far away as Lake Erie.  It forms dense mats that block out sunlight to other aquatic plants.  It also inhibits boat traffic (perhaps this is a good thing?).

Eurasian Watermilfoil (Myriophyllum spicatum)
This herb can grow in water 30 feet deep.  When it reaches the surface it branches into a dense canopy that inhibits underwater plants.  Swimmers also dislike it.

Places Outside of Ontario

Florida

Brazilian Pepper. Also called Florida Holly. Its berries are used by the French as a color additive to whole black pepper kernels. In Florida it is all over the place and pushing out natives.

Mellaleuca tree. Taking over many habitats in Florida and crowding out native species.

With thanks to Pamala Watts of the Hamilton Naturalists' Club.