Invasive and noxious plants

Invasive plants are non-native trees, shrubs, grasses and flowers that have been spread by human activity, intentionally or accidentally. They are often capable of reproducing and spreading rapidly, outcompeting native species and upsetting the ecosystem balance. According to the International Union for the Conservation of Nature (IUCN), invasive alien species are the second most significant threat to biodiversity after habitat destruction.

In their new ecosystems, invasive alien species can become predators, competitors, parasites, hybridizers, and diseases of native and domesticated plants and animals. The impact of these alien species on native ecosystems, habitats and species is severe and often irreversible.

Prevention is key to controlling the spread of invasive plants. Consult the information below to learn more about preventing and controlling common ragweed and Japanese Knotweed, an exotic invader that has been identified in Westmount.

Sources: Environment and Natural Resources (Canada), Ministère des Forêts, de la Faune et des Parcs (Québec)

Common ragweed

(Ambrosia artemisiifolia)

Common ragweed — an invasive annual plant found throughout Quebec — is considered to be the main cause of allergic rhinitis or hay fever. More than 10% of the population is affected by the light, airborne pollen, which may cause sneezing, irritated nose and eyes, coughing and asthma. A single ragweed plant is capable of producing several million grains of pollen during its flowering stage from mid-July until the first frost.


Ragweed, which can reach up to 2m high, is recognizable by its serrated leaves and spiked flowers. Its appearance varies with its stages of growth. It tends to grow in poor, arid and disturbed soils, and is frequently seen along roads, streets and sidewalks, as well as on industrial land..


Ragweed is easy to pull up, especially in the early stages of growth. When there is too much to remove by hand, mowing will help prevent or delay the flowering stage.


Ragweed will not produce pollen if the plant is uprooted or cut before the flowering stage. Repeated mowing is necessary, however, to prevent the flowers from developing.


Ragweed is often found in difficult growing conditions. It does not compete well with other plants in fertile soil, including grass. By keeping your lawn and garden healthy, ragweed is less likely to find a niche.

Japanese Knotweed

(Reynoutria japonica or Fallopia japonica)

Japanese knotweed is an aggressive semi-woody perennial plant that is native to eastern Asia. It was introduced to North America in the 19th century as an easy-to-grow ornamental and was also planted on shorelines to prevent erosion because of its strong root system.

Now illegal to sell or propagate, the plant is very rarely seen in stores, but has been found under the label ‘Japanese Bamboo’. The IUCN has listed it among the 100 worst invasive species in the world.

Although Japanese Knotweed was once mostly found along road ways, ditches and along waterways, its past commercial popularity and aggressive root system means that it can now be found anywhere. Knotweed and other exotic invasive species often favour bare areas where there are little to no native species to compete with.


This invader is very persistent due to the lack of natural predators, its ability to spread aggressively from the smallest fragment, and its resistance to Canadian winters. Once established, it is extremely difficult to control.

  • Do not buy, plant or walk through Japanese Knotweed or any plant that may resemble it.
  • Any small fragment of the plant or root is enough to spread the plant.

The plant re-roots extremely easily and even the tiniest fragment will grow into a large plant. It is important to avoid disturbing these plants as unintentional propagation may occur. In wooded areas, always stay on pathways and never take cuttings for home or the cottage.


Japanese knotweed is often mistaken for bamboo because of its appearance.

  • large oval / heart-shaped leaves (3-6 inches long and 2-5 inches wide);
  • round, reddish-purple hollow stems with bamboo-like knots;
  • very rapid growth, reaching between 1 and 3 metres in height;
  • small greenish-white flowers that produce a small white fruit with wings that disperse seeds efficiently;
  • brown shiny seeds.


Before deciding on a removal method, consider the site conditions first to ensure the best results.

Recommendations for full-sun areas:

  • Cut the plants and solarize (cover with a dark tarp).
  • Re-seed the area with native plant seeds adapted to full sun

Recommendations for shaded areas:

  • Do repeated cutting, but without the solarization (tarping), as the heat produced will not be sufficient to effectively cook the roots.
  • Re-seed the area with native plant seeds adapted to full shade.

Repeatedly cutting knotweed low to the ground from April until October will stress the plant and its root system. This constant stress and absence of leaves to absorb nutrients will eventually result in the death of the plant.


Japanese knotweed spreads aggressively through an extensive rhizome network (horizontal plant stems growing underground). Stem growth is renewed each year from this deep underground system and its growth is rigorous. Once established, the root system is almost impossible to remove, as it can spread as deep as 10 feet beneath the surface.

  • Cut all Japanese Knotweed plants as low to the ground as possible.
  • Never allow to grow over 1 foot in height at any time or to go into flower or seed.
  • Never mow or roto-till Japanese Knotweed. This creates small fragments, each of which may re-root and create many new plants, further exacerbating the problem. Also check machinery, tools and clothing for plant fragments to avoid spreading the weed.
  • Make sure to pay special attention to cut any small satellite plants (plants growing from the main concentration).


The best way to eradicate Japanese Knotweed is to starve the plant of sunlight and energy by repeatedly cutting all shoots at the ground, as outlined above. Focusing on smaller new growth in priority will deter these new plants from growing bigger and stronger.

Re-planting the area with native vegetation once control measures are complete will help to suppress re-sprouting and assist in preventing new invaders from establishing.


Knotweed and its fragments should never be placed in the compost. Always double bag any part of the plant and place in the garbage.

  • Always dispose of knotweed (leaves, flowers, stems or root fragments) in double garbage bags in the regular garbage collection.
  • Never dispose of any part of the plant in compost (residential or municipal). Even small fragments of the plant will re-root and can often survive commercial processing of compost.
  • Dispose of any soil removed during these types of interventions also; it is contaminated and cannot be used elsewhere to avoid propagating the plant.