Identifying and getting rid of giant hogweed

Description

Giant hogweed is an invasive and poisonous exotic plant. It is a health and environmental risk.

In humans, contact with giant hogweed sap, together with exposure to light (natural or artificial ultraviolet rays), causes skin lesions similar to burns. Furthermore, when this plant grows somewhere, it spreads fast and inhibits the growth of other plants in the area.

Distribution of giant hogweed

Giant hogweed first appeared in Québec in 1990. To date, it has been spotted in several regions:

Bas-Saint-Laurent
Capitale-Nationale
Chaudière-Appalaches
Estrie
Lanaudière
Laurentides
Laval
Mauricie et Centre-du-Québec
Montérégie
Montréal
Outaouais
Saguenay–Lac-Saint-Jean

Giant hogweed grows in cool, moist environments, such as:

Along river banks
Along ditches
Along railway tracks
Along roadsides
In meadows
In vacant lots

Identifying giant hogweed

Knowing how to recognize giant hogweed is important in order to avoid contact with it as much as possible. Giant hogweed has the following characteristics:

Size of mature plant:

The plant can measure 2 to 5 metres tall.
After having being mowed down, it can measure 30 cm to a metre.
The leaves of giant hogweed can reach 1.5 metres in width and 3 metres in length.

Flowers:

The flowers of giant hogweed are white.
They grow on a single stem, forming clusters of rounded flowers called umbels.
Each umbel is 25 to 50 cm in diameter.

Stem:

The main stem of giant hogweed is strong and hollow.
It is 4 to 10 cm in diameter.
It is covered with rough white hairs scattered all over the stem but mostly at the base of the leaf stalk.
It has extensive and prominent reddish-purple blotches.

Leaves:

The leaves are divided into 1 to 3 deeply cut, pointed leaflets.

Underside of leaves:

The underside of giant hogweed leaves are smooth or slightly scaly.

Differentiating between giant hogweed and cow parsnip

It is important not to confuse giant hogweed with cow parsnip.

Cow parsnip looks a lot like giant hogweed. However, cow parsnip is not invasive. It poses no risk to the environment.

Cow parsnip is indigenous to Québec, meaning the plant is native to the province.
Cow parsnip does not grow taller than 3 metres.
Each umbel is 15 to 20 cm in diameter.
The stem is entirely covered with felt-like, soft white hairs.
Some parts of the stem may have a red tinge. Spots on the stem are not as colourful as those of giant hogweed.
The back of the leaves have a felt-like texture and are abundantly covered with white soft and woolly hairs.

leaves of Cow parsnip. — Leaves of cow parsnip.

Stem of Cow parsnip. — Stem of cow parsnip.

Underside of leaves of Cow parsnip. — Underside of leaves of cow parsnip.

Source : MAPAQ. Click on images to enlarge

Identifying lesions caused by giant hogweed

The sap of giant hogweed contains toxic substances that are activated by sunlight and also by artificial UV rays. Exposure to light causes inflammation of skin that comes into contact with giant hogweed sap. The inflammation is characterized by:

Redness and swelling of the skin
Blisters
Superficial or more serious (first or second-degree) burns

To know what to do if your skin comes in contact with the sap of this plant, see the Treatment section of the Burns Caused by Giant Hogweed page.

Getting rid of giant hogweed

Given the harmful effects of giant hogweed on the environment and health, it is very important to prevent it from spreading.

As such, avoid sowing, planting, multiplying or transporting giant hogweed.

Before proceeding with getting rid of giant hogweed, you must do the following:

Correctly identify the plant so that you do not confuse it with cow parsnip
Find the clonal colonies (group of plants), at the beginning of spring ideally
Protect yourself in order to avoid burns caused by contact with sap from the plant

Handling the plant

If you must handle giant hogweed, protect yourself properly:

Cover all parts of your body with non-absorbent garments (synthetic and waterproof materials): pants, long-sleeved shirt, rubber gloves covering the wrists and forearms
Pay particular attention to the hems of your clothes: protect your wrists, ankles and neck
Protect your eyes and face with a visor

Battling giant hogweed

Plants growing in isolation or covering a small area of land

You can cut the root of young plants with a sharp round shovel. Start at the beginning of the spring and repeat every two weeks in order to weaken the plants
Repeat the cutting each year over several years in order to completely get rid of the plant
Use a tool that rotates and tills soil, such as a rototiller, to get rid of small plants growing in abundance
To limit regrowth, if the area has few obstacles, cover it with geotextile fabric once the plants have been eliminated and the clonal colony is minor

Mature plants or ones covering a large area

Cut about 15 cm from the ground
Repeat at least 2 or 3 times over the spring
Cut the root to a depth of about 20 cm under the soil surface. Use a sharp round shovel
Work the soil, turning it at a depth of about 24 cm, for example. This will limit regrowth of plants
Dry the cut plants and plant residues by placing them in plastic bags exposed to the sun for at least a week. The bags can then be thrown in the trash.
If the plants bear seeds, avoid spreading them
Do not compost any part of the plant
To prevent giant hogweed from invading your land again, you can grow new plants in the area cleared. Preferably choose plants that are native to your area and grow quickly. Check with your local garden center

Chemical measures

The use of herbicides or chemical controls are solutions to be considered only as a last resort
If you plan to use chemical controls, consult an expert or a specialized company. This way, you avoid harming your health and the environment

After handling giant hogweed

Remove your clothing and take off your gloves by turning them inside out
Make sure that clothes worn during handling of plants do not come into contact with other clothes or objects in order not to contaminate them with sap
Wash your clothes in the washing machine before wearing them again
With water and soap, wash tools (shears or trimmers for example) that came into contact with the plant’s sap
Wash your hands and face with soap and water

Other precautions

Call Info-Santé 811:

If you think you have been in contact with giant hogweed
If you want more information on what to do in case you come into contact with giant hogweed

Contact your municipality or the Ministère de l’Environnement et de la Lutte contre les changements climatiques at 1-800-561-1616:

For information about control or elimination procedures
To indicate the presence of giant hogweed (in French only)