Weeds or invasive plants


Giant Hogweed (Heracleum mantegazzianum), a plant native to the Caucasus Region and Central Asia, was introduced to North America for horticultural purposes and has been found in Québec since 1990. An exotic invasive species, it can quickly grow to heights of 1.5 to 2 metres. Its sap, which oozes from every part of the plant, contains furanocoumarins, toxins that render the skin photosensitive (i.e., sensitive to light). Clear and watery, this sap does not cause pain to the touch, but can produce dermatitis up to 48 hours after skin contact and exposure to the sun. The resulting burn-like lesions are very painful. Inflammation and temporary blindness can occur. Moreover, skin sores can leave brown or white spots that can last for months, even years after healing.

Giant hogweed is topped by white umbel inflorescences (cluster of flowers), i.e. flower stems of equal length that all start at the same point. Resembling an umbrella, it is part of the umbelliferous family. The umbel’s diameter measures 20 to 50 cm. Approximately 2.5 cm in diameter, the main stem is green dotted by many purple spots. Furthermore, the stem is hollow and covered in white hairs. The giant hogweed can be found on shorelines, in ditches, along railroad tracks, and by the roadside.

Giant hogweed should not be confused with cow parsnip (also known as Indian celery or Pushki), a plant indigenous to Québec whose umbel is considerably smaller, i.e. 15 to 20 cm in diameter). Its leaves are also much smaller and far less chiselled. You should avoid touching giant hogweed. Should this occur, however, wash the affected areas with soapy water and cover them to prevent exposure to strong light or sun.

To remove this plant, you can rip it out, taking care to protect yourself with rubber gloves, a raincoat and goggles. Afterwards, carefully wash the equipment you used to remove the plants as well as the protective clothing. Dig up to 20 cm down, in order to remove the plant’s roots. Do not use as compost, dispose of where it could grow back, or cut down with an edger since the sap could splash onto the skin. You can signal the presence of giant hogweed by contacting the Réseau de surveillance des plantes exotiques envahissantes (invasive exotic plant monitoring network) at rspee.glu.org or the Town of Rosemère’s eco-consultant at 450 621-3500, ext. 3305.

For additional information:

Can you recognize it?

Ragweed is an annual plant belonging to the same family as the daisy and camomile. It is not to be confused with poison ivy, which will cause a skin rash upon contact.

Ranging in height from 5 to 90 cm, it features serrated foliage and hairy stems. It provokes no allergic reaction to the touch.

This plant appears in June, blooms in August (stalks of tiny yellowish flowers), and produces an outstanding quantity of seeds for the following year (as many as 1,000 per plant) before it dies. It is said that these seeds can germinate after being buried for more than 40 years!

What can you do?

In June and July, rip out these small undesirable plants that make thousands ill every year. Ragweed can easily be ripped out by hand. It’s the pollen from its tiny flowers that causes respiratory allergies in August.

Ragweed favours environments that are disturbed, poor and sunny. Since other plants don’t do well there, ragweed easily takes over. This undesirable plant quickly colonizes areas that have become barren, like the edge of the street or a patch of lawn weakened by white grubs or chinch bugs. If you hope to eliminate it completely, you will have to modify the environment to make it more appealing to other plants. For example, by improving the quality of the soil, grass will take better.

Cutting ragweed down with the lawnmower before it blooms is a simple way of controlling it in large areas of greenery, but take care: if you mow your lawn regularly, the plant will eventually spread low on the ground, and bloom anyway. The best way of eliminating it is still to rip it out while maintaining a healthy lawn that will be more resistant.

If you have problems eliminating ragweed on your property, do not hesitate to contact the Town’s eco-consultant, at 450 621-3500, ext. 3305.

Practical tips if you’re allergic:

If possible, during ragweed season, plan outings at the end of the day, take a walk after a rainfall, avoid drying your clothes outdoors, and wash your hair and hands before going to bed. In addition, leave your windows closed; use an air conditioner or an air exchanger with an anti-pollen filter. Check your car’s air purifier as well!

Ragweed and poison ivy are two different plants with different effects on health. If you are unsure about their differences, see the Poison Ivy information sheet on our website.

Practical references:

Poison ivy (toxicodendron radicans) is a plant whose irritating sap causes rashes. Crawling or climbing, it prefers moist, shaded or semi-shaded locations by wooded areas or waterways. While it can assume various forms, it is important to recognize it in order to avoid it.

This plant’s leaves comes in clusters of 3 oval leaflets whose edges can be jagged or smooth, and occasionally both. Often shiny, the leaves are dark green in summer.

Even if you haven’t been in contact with the plant, you could have an allergic reaction if you’ve been in contact with a piece of clothing, a shoe or a pet that has been. In fact, the oily sap (oleoresin) adheres to various items or fabrics, but it is possible to eliminate it by washing the surfaces properly with soapy water. If you believe that your skin has been in contact with poison ivy, wash immediately with cold water.

Unfortunately, there is no easy way to get rid of poison ivy, and manual removal remains the most effective method. Before tackling this task, however, you must take a number of precautions to protect yourself. For example, 24 to 48 hours before you start, spray a low-impact herbicide like Safer’s "TopGun” on all of the plant’s visible leaves to weaken it. Note that, despite this measure, the plant will remain allergenic, and you must not touch any part of it with your bare hands, even if the leaves appear dry (roots, stems, live or dead leaves, fruit). Once you are well covered from head to foot, make sure you pull out as many stems as possible with your gloved hands, even those that run deep into the ground or under other shrubs. Dispose of all of these stems and other parts of the plant in a bag along with the gloves you used and, if need be, your clothing as well. Then put all the tools, accessories and footwear used into a waiting pail of hot, soapy water.

If any pieces of stem remain, chances are that the plant will resurface, but if you check the location often and pull out the poison ivy as soon as it appears instead of waiting for it to gain strength, you will most likely succeed in eradicating it. Spring is a good time for pulling out poison ivy, since there are fewer leaves and therefore, less of the surface is in contact with the sap. The use of a herbicide is not a preferred solution as it will only be effective if it is followed by manual removal.

A small percentage of the population has no allergic reaction to poison ivy. Whatever you do, never burn poison ivy, and if you need help identifying it, contact the Town’s eco-consultant, at 450 621-3500, ext. 3305.

For additional information:

The European buckthorn and the glossy buckthorn are exotic varieties originating in Europe. The buckthorn can colonize large areas in a short period of time. Its dense foliage creates shade and keeps other native trees from growing, which is why it is considered harmful.

The buckthorn is found mainly along roads as well as at the back of properties and in wooded areas. It is easily recognizable, given its simple shiny leaves with their parallel veins, its purplish grey bark and clusters of berry-like fruits that change from green to red to purplish black depending on their degree of maturity. It is easier to identify in late October to early November when other trees have already shed their leaves.

Finally, to cut the trunk of buckthorns that measure more than an inch in diameter, you must obtain a permit from the Public Works Department.





What’s happening

  • 2017 Elections
  • Roadwork Info
  • Water Meter Reading
  • Citizen Participation
  • Paws for Hope