Giant Hogweed is an invasive plant species that is dangerous to human health and pets, as well as the plant ecosystem.

It is native to the Caucasus region of Eurasia and was likely introduced to North America as an ornamental garden plant.

How does the plant spread? Giant Hogweed produces thousands of seeds, the majority fall to the ground close to the parent plant, but a large number can be easily spread along watercourses and by animals and people.

If you think you may have found Giant Hogweed in a City of Toronto Park, consider the following, as there are a variety of plants that can be mistaken for Giant Hogweed (Cow Parsnip, Queen Ann's Lace/Wild Carrot, Angelica):

Location:

  • Giant Hogweed is usually found along road edges, streambeds, and forest edges

OFAH Invasives tracking website

Height:

  • Giant Hogweed can get VERY tall reaching heights of up to 20 feet (6 metres)
  • Cow Parsnip is much smaller, reaching heights of up to 5-8 feet
  • Queen Anne's Lace/Wild Carrot reaches heights of 3-3.5 feet
  • Angelica can reach heights of up to 8 feet


Stalk appearance & diameter

  • Giant Hogweed has a green stalk with very noticeable reddish-purple blotches on it, even on very young plants.
  • The stalks are hollow and rigid with a diameter of 2-4 inches with very coarse bristly hairs sparsely scattered along the stem.

In comparison:

  • Cow Parsnip has a green stalk sometimes with a purplish colouring, it has hairs that are more downy looking and more abundant along the stem
  • Angelica has a stalk that is uniformly waxy green to purple with no hairs


Flower head

  • Giant Hogweed has small white flowers in an umbrella shaped head that is very large (approx 8-20 inches wide)

In comparison:

  • Cow parsnip produces similar looking flower clusters that are much smaller than Giant Hogweed, Cow Parsnip typically flowers before Giant Hogweed.
  • Queen Ann's Lace/Wild Carrot produces similar looking flower heads that are only about 3-4 inches wide
  • Angelica produces softball sized clusters of white or greenish-white flowers less than 1 foot in diameter


If there is no flower head:

  • Giant Hogweed usually don't flower until they are three years of age
  • Giant Hogweed typically has a very noticeable, large cluster of leaves at the base of the plant that is present even if there is no flowering stalk.
  • Giant Hogweed's basal cluster of leaves can reach heights of 3-6 feet in the first two years of growth


Leaves:

  • Giant Hogweed has large leaves (about 1-3 feet wide), divided in 3-5 segments that are lobed and coarsely toothed and pointed

In comparison:

  • Cow parsnip has similar leaves that are not as heavily toothed and pointed
  • Queen Ann's Lace/Wild Carrot has fine and wispy leaves
  • Angelica has leaves with dozens of small leaflets

Other available resources to assist in identification of the plant:

  • For photos that compare Giant hogweed characteristics with Cow Parsnip, see this PDF. Note that Angelica is very uncommon in Toronto and Poison Hemlock, also listed on this brochure is not known to be found in Toronto
  • www.weedinfo.ca - user can submit a photo and have the weed identified
  • For more detailed images and help Identifying Giant Hogweed here are some online resources, visit this link to Google Images.
  • What is Giant Hogweed? Please see link

If you believe that the plant is Giant Hogweed, contact 311. If the site has not already been identified as a location harbouring Giant Hogweed or a common look-a-like, 311 will submit a service request for a site inspection, by Parks, Forestry & Recreation. Please describe identifiable features in proximity to the site to assist staff in finding the location of concern within the park.

If you wish to report suspected Giant Hogweed on PRIVATE PROPERTY see: Bylaw enforcement - long grass and weeds - natural garden exemptions - on private property

What is Parks Forestry and Recreation doing about Giant Hogweed?When potential Giant Hogweed sighting is reported in a City park, staff will inspect sites to confirm identification. Qualified staff will visit the reported location and if they find that Giant Hogweed is present they will map the area (approximate) of its location. Where there is a potential risk to the public, a caution sign will be installed by Parks Forestry and Recreation staff. Depending on each situation staff may also consider construction of fencing to limit access to a site.

Will Parks, Forestry and Recreation remove the plant? Due to the widespread presence of this plant along waterways in Toronto, Parks, Forestry & Recreation is unable to remove every individual plant that is reported to be found on Park property. Priority will be assigned on a site specific basis whether to remove the plant, fence the area, or install a caution sign. In some places pesticides may also be used to control the spread of giant hogweed. Pesticides are only used by licensed applicators in accordance with applicable legislation.

How will Parks, Forestry and Recreation dispose of the plant? If it is necessary to remove the plant from the site, trained staff will dispose of the plant material in a manner to ensure that the vegetation is sufficiently biodegraded prior to adding it to a landfill or other method of compost. In most natural park situations, plants will be left on site to decompose naturally.

Where else is this plant found? This plant is found in parts of Britain, United States, and Canada.

Giant Hogweed is present in a variety of location in Southern Ontario including Mississauga, Milton, Markham, Toronto, and generally establishes along roadsides, forest edges, and streams.

For more detailed information on locations, Weed Info has a Google map of confirmed sightings in Southern Ontario.

What will happen if I touch the plant? Giant Hogweed has a clear watery sap that contains toxins causing photodermatitis (skin reacts becoming very sensitive to sunlight)

If skin is exposed to the sap as well as sunlight, severe skin rashes, blisters, and even blackening of the skin can occur

Reddening and swelling of the skin can be noticed 24 hours after contact, further inflammatory reaction can occur after up to 3 days. Peak sensitivity occurs 30 minutes to 2 hours following contact.

The reaction to Giant Hogweed sap depends on individual sensitivity, and in some cases effects can last for months, and skin can remain sensitive to UV light for years.

Sap contact with the eyes can cause temporary and possibly permanent blindness.

The toxicity of the sap depends on the part of the plant that is touched, sometimes just touching the surface of the plant may not cause a reaction, however sap can be picked up from the plant from brushing on the numerous blisters and hairs on the stems

My dog ran through a patch of Giant Hogweed, what do I do? If it is possible that your pet has had contact with the sap from Giant Hogweed it is very important not to touch your pet with bare hands.

There is potential that you could get the sap transferred to your skin via your pet's fur. Using protective gloves, immediately wash your pet with soap and water and seek veterinarian help immediately. Seek medical attention if you feel you have been exposed.


What can I do to prevent contact with Giant Hogweed? Familiarize yourself with what this plant looks like and the dangerous side effects and educate others. Please stay clear of Giant Hogweed in locations where it has been identified. Use park pathways and maintained areas, keep your dogs on leash, stay on trails and avoid walking through naturalized areas.

Other Links to background information on the plant:

Other Links to Identification Sources:

  • Weed Info - provides weed identification service where general public can send images and information to get a positive ED
  • A great information booklet produced by the Michigan department of agriculture, has lots of identification tips.

What should I do if I come in contact with Giant Hogweed?

  • Wash affected areas immediately
  • Keep the affected area out of the sun
  • Seek medical advice

What do I do if I find Giant Hogweed on my property?

  • Remove the plant or contact a professional to remove it for you.

If you choose to remove it yourself:

  • Wear protective clothing, including gloves, long sleeves, pants, and eye protection, rain coat and boots
  • Avoid getting sap on your skin
  • Immediately remove any flower heads to prevent seed growth and dispersal
  • Sever the plant roots 8 - 12 cm below the soil surface
  • Dispose of all plant parts in double-bagged garbage bag, seal the bag and DO NOT COMPOST or put it in your yard waste
  • Return to plant site periodically to remove any new plant growth
  • After completing the plant removal, wash the clothes that were worn when removing the plant to prevent possible transfer of toxins
  • Speak with a retailer about products permitted for control of giant hogweed
  • refer to the Giant Hogweed fact sheet from Invading Species website for further control suggestions (direct link)

Related information: