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 and Centre-du-Québec
  • Montérégie
  • Montreal
  • 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:

Size of mature plant.

Source : DSP Capitale-Nationale

  • 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:

Flowers.

Source : MAPAQ

  • 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:

Stem.

Source : MAPAQ

  • 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:

Leaves.

Source : MAPAQ

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

Underside of leaves:

Underside of leaves.

Source : MAPAQ

  • 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

Cow parsnip. leaves of Cow parsnip. Stem of Cow parsnip. Stem 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 public 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:

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
  • If the plants bear seeds, avoid spreading them
  • Destroy the umbels by placing them in securely closed, sturdy plastic bags. Leave bags in the sun for a least a week
  • 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 du Développement durable, 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

Last update: May 31, 2016 2:15 PM

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