Giant Hogweed at its peak … but don’t worry, it will die soon

Written by: Norm Miller, Communications Advisor

Giant Hogweed is an invasive species which has become very well-known for its height and potential health effects on humans. This is generally the time Giant Hogweed is at its peak for growth reaching heights of up to 4 metres, (14 feet), before going to seed and dying for the season, until reappearing the following spring.

Giant Hogweed (Heracleum mantegazzianum) has two major negative impacts:

  1. Due to its invasive nature, it poses a threat to biodiversity.
  2. It is a public health hazard. It produces a noxious sap that sensitizes the skin to ultraviolet light. This is known as photosensitivity, which can result in severe and painful burning and blistering. It is important to avoid any skin contact with this plant. If you are exposed to the sap then protect the area from the sun for at least 48 hours.

With Giant Hogweed at its peak for growth right now, Conservation Halton receives a significant number of calls and e-mails. People want to know what should they do about the plant, or what will Conservation Halton do about it.

If you see Giant Hogweed (or a plant you think may be Giant Hogweed), Conservation Halton would like you to report the sighting. You can report your sightings on the EDDS Maps Ontario website (Early Detection and Distribution Mapping System) which is a web-based mapping system for documenting invasive species distribution. Conservation Halton is happy to identify photos of plants you may think is Giant Hogweed, please e-mail your photos to Brenda Van Ryswyk,

Conservation Halton does track invasive species like Giant Hogweed as well as doing other long term environmental monitoring in our watershed. This provides current information on species diversity and abundance, which is used to assess changes in species diversity and abundance (biodiversity) over time.

Landowners are responsible for the removal of Giant Hogweed on their property. This time of year is far from the preferred time of year to attempt control. In fact it is the time when the risks are the highest, due to the large size of Giant Hogweed plants, and it is the least effective because it may already be producing seed. The plant that has flowered will soon go to seed and die. The plants that go to seed this year will not re-grow next year but there will be younger plants, as well as the seeds, to worry about.

The best thing to do at this time of year is likely to cut and bag the seeds to prevent their spread and mark on your calendar to return to that spot in April and May to treat any plants coming up at that time an prevent them from reaching the flowering stage.

The most effective time to try and control Giant Hogweed is when the plants are just starting to grow in the spring (April-May), and removal can prevent it from producing seeds. If you see Giant Hogweed now, you’re best to make note of the location, bag and remove the seeds if possible, and watch for the first signs of growth in the spring and take removal actions then.

Conservation Halton has a page on its website with more information about Giant Hogweed, including how to spot it, some common lookalikes, and how to remove it at

About Giant Hogweed

Giant Hogweed usually grows from 2.5 to 4 metres (8 to 14 feet) high with leaves up to 1 metre (3 feet) in breadth. It has a thick, 5 to 10 centimetres (2 to 4 inches) hollow stem. Its stem and the undersides of its leaves are covered in coarse hairs. You can click here to see Conservation Halton’s factsheet on Giant Hogweed.

Giant Hogweed has several lookalikes, including a smaller, harmless plant which has a similar (but greatly smaller) white flower called Queen Anne’s Lace. If you are unsure whether you have Giant Hogweed, feel free to seek confirmation from an expert. At Conservation Halton, you can contact Brenda Van Ryswyk, by phone 905-336-1158, ext. 2282 or email

For general inquiries, questions on invasive species or to report a sighting anywhere in the province, you can use the Invasive Species Hotline: 1-800-563-7711 or the EDDMaps, For more information on Giant Hogweed, some tips on its control, and other invasive species, please visit Conservation Halton’s website,


Leave a comment

Filed under Nature's Spaces

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s