Beware of Heracleum mantegazzianum – Giant Hogweed

No, that’s not a spell from Harry Potter. Although it sounds like something from Harry Potter, Heracleum mantegazzianum is better known as Giant Hogweed (still not helping my case.) Oddly enough it actually belongs to the carrot family (Apiaceae). You do need to know that it is an invasive perennial herb and it has potential negative health impacts to humans and pets.

Giant Hogweed (Heracleum mantegazzianum) is a plant that is native to Asia. Specifically the Caucasus Mountains in Southwest Asia. The plant grows to a large size and produces large umbels (umbrella-shaped clusters) of white flowers (like a super-sized Queens Mum – the plant that is.) It was thought to have been introduced to North America as an ornamental garden feature in the early 1900’s.

By Fritz Geller-Grimm – Own work, CC BY-SA 3.0,

It’s considered an invasive species as it has no known pests or diseases to counter it’s spread here in North America. It out-competes native plants, reduces biodiversity and degrades the quality of riparian habitats that it favours.

Here in Ontario[^1] the plant has flourished and is recognized as a threat to Ontario’s biodiversity, agriculture, and human health. In 2010, Ontario listed Giant Hogweed (one of 168 invasive species listed) as a provincially noxious weed under the Weed Control Act. I believe similar issues have raised in the U.S. where it is also prevalent and growing. Giant Hogweed has many States on alert for the plant[^2].

EDDMapS. 2018. Early Detection & Distribution Mapping System. The University of Georgia – Center for Invasive Species and Ecosystem Health. Available online at


Many counties, in both the U.S. and Canada, have issued public warnings, telling residents to stay away from giant hogweed. Giant hogweed sap can cause a condition called phytophotodermatitis. This condition can make skin extremely sensitive to sunlight and can result in severe burns and blisters.

USDA APHIS PPQ – Oxford, North Carolina , USDA APHIS PPQ,

It’s interesting to note that many plants can cause phytophotodermatitis[^3]. Would you believe that carrots (makes sense given the family), celery, many of the citrus fruits, wild forms of dill, parsley and parsnips can also cause phytophotodermatitis?

Phytophotodermatitis occurs when the parent compound, psoralen, is mixed with UV radiation on exposed skin. Psorlalen is a parent compound of furocoumarins which is a photosensitizing chemical component produced by certain plants [^4].

There are two things that occur when dealing with activated furocoumarins. One is that these photochemical reactions actually damage cell membranes and DNA. The second is that a postinflammatory pigment alteration may occur after the blister phase of this phototoxic reaction. Many people will develop hyperpigmentation at the site of the burn.

What do you do when encounter or spot Giant Hogweed?

First off, keep in mind the weed is a public health issue to both people and pets. If you are around the plants you should wear protective clothing, gloves and eye protection. You should avoid getting the sap on your clothing.

When disposing of the plant (call your county prior to disposal), it is recommended that you not mix it with regular compost. Keep in mind that any seeds left behind can germinate for up to 15 years after the parent plant has been killed. Your local municipality will probably have protocols in place to deal with Giant Hogweed as it’s not easy to remove or eradicate.

If you get any sap on you wash it off with cold water immediately and get out of the sun. A toxic reaction can begin as soon as 15 minutes after contact. Call a doctor if you’re experiencing a severe reaction.

In general, it’s best to avoid Giant hogweed.  If you do happen to encounter it in the wild – BE CAREFUL!

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About jasonbu 56 Articles
I'm a wannabe extrovert and an Interweb nerd who has spent the past 25+ years in the software industry. My first 10 years at Microsoft. I've served in the Canadian Military; I'm a humanist; I love coffee; love all things science and love spending time with my family.