Giant Hogweed — you NEED to know this dangerous plant

Greetings, friends!  I live in the Great Lakes region but for some strange reason I had never heard of this horrifying plant until this year.  When Michigan herbalist, jim mcdonald, posted in April on his Facebook page his “obligatory annual GIANT HOGWEED post” (find it at the bottom of this post), it was the first time I’d heard of it.  Then at the end of May, after stumbling upon a story in a Detroit news outlet titled (alarmingly),  “Michiganders warned of dangerous hogweed“, I decided that I needed to learn about this plant as thoroughly as possible and share what I have found.

The short summary is this: The sap of this plant will give a person a horrifyingly nasty and painful skin rash that will last for weeks.  If the sap gets into a person’s eyes, it will cause blindness (one source says temporary blindness).  The toxin (which is in the class of phytochemicals called furanocoumarin) in the sap  is activated by UV rays (phytophotodermatitis).  According to Midwest Invasive Plant Network, giant hogweed is a perennial herbaceous plant that, by the 4th or 5th year, produces a 7-15 foot flower stalk.  Before then its form is a rosette that grows bigger and bigger each year until it is ready to produce that humongous flower stalk and gigantic  flower umbels.

Note: there are several “well-known” plants that are related to giant hogweed (Heracleum mantegazzianum).  Some of them are common hogweed, wild parsnip, cow parsnip, Queen Anne’s lace (wild carrot) and poison hemlock.

Why is this plant so dangerous?  According to this CBC report, “The toxins in the sap can create what is known as “phytophotodermatitis” — basically an extreme sensitivity to sunlight. The effects of the toxins are not felt immediately, but once activated by UV rays, they can damage skin cells and cause lesions that look similar to burns.”

Here is more information about giant hogweed:

• The scientific name is Heracleum mantegazzianum.
• The range for this plant is:  In the western half of North America, it is found in Oregon, Washington and British Columbia.  In the eastern half of N. Am., it is found in only some of the states/provinces in the Great Lakes region as well as in most of New England and in North Carolina.  According to this USDA map, the states in the eastern US that have giant hogweed are as follows: (from west to east) IL, MI, NC, PA, NY, CT, MA, and ME (more states from other sources listed below).  All of eastern Canada is highlighted on the map as having giant hogweed present.  If you live in one of those affected areas, I encourage you to click on the link to get to the map; then below the map you will see that most of the states affected have a link to county distributions.  Not all counties within the states have giant hogweed present.  According to Midwest Invasive Plant Network and this pdf, northern Wisconsin, northern Indiana and eastern Ohio also have giant hogweed present.  According to this USDA Forest Service webpage, this plant is found also in Maryland, New Jersey, Vermont and New Hampshire.
• What it looks like as a shoot and small plant (early stages of growth): It grows in a rosette for several years before it sends up a flower stem to produce seeds (after which the plant dies; it is a monocarpic plant).  The seedling leafs are spiked and have a pronounced jagged appearance.
• Profile of the stem: Like most of the plants in the Apiaceae family, it has a hollow stem.  It can have noticeable purple splotches on the stem.  The stem is covered in coarse hairs (bristle).
• Profile of the mature leaves: “On mature plants, leaves are divided into three equal or almost equal parts which are then divided into a further 3 parts (ternate)” and  “Lateral leaflets have blade touching main stem with no petioles (leaf stalk)” Source “Hairs are also found under the leaves. The plant produces a clear, watery sap that is found in the hairs and stem.” Source
• Profile of the flowers: white flowers in multiple, large umbels (think umbrellas); 50-150 rays per main umbel Source
• Habitat: likes moist soil such as near rivers
• Time of year for its life cycle: blooms in June in Ontario; my guess is that it blooms in spring in places south of Ontario.  But remember that blooming is the last stage of life for this plant.  As stated previously, it spends 4 – 5 years as a rosette so be on the look out for the rosettes since they can be as toxic but not as noticeable as the large flower stalk.

If videos are helpful for you to commit this plant’s features to memory, please check out the following videos by Chris Phyto:

• Video of giant hogweed’s leaves
• Video of giant hogweed’s umbels
• Video of young giant hogweed leaves (Chris compares these to some common hogweed plants nearby and talks about the differences)

Links to check out, especially if you live in an area where giant hogweed is present; it’s good to see the various photos of the plant:

• CBC: 5 things you need to know about toxic hogweed (has a video report and includes what to do if you do get sap on your skin)
• MIPN info sheet on giant hogweed
• This USDA Forest Service webpage has a TON of information on giant hogweed.
• Lots of clear photos and information on this pdf from Ontario Invasive Plant Council
• Very helpful info sheet from The Ohio State University
• Here is another info sheet from the gov’t of New Brunswick, Canada.
• Photo in post from Washington Post

Plant geek info:

Family: Apiaceae (or Umbelliferae)

Genus: Heracleum 

Species: mantegazzianum

Common names: giant hogweed, cartwheel-flower, giant cow parsnip,”hogsbane” or giant cow parsley (source)

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Below is the post from jim mcdonald:

Jim Mcdonald

April 17 · Edited

the obligatory annual GIANT HOGWEED post; please feel free to share or copy and paste it onto any and all of the hundred thousand threads on the topic:

Giant hogweed possesses the same photosensitizing compounds found in parsnip (and not just wild parsnip, as “garden parsnip” and “wild parsnip” are the *same* species, Pastinaca sativa). It’s the sap that’s so bad; if you just touch the plant you won’t melt like the wicked witch in the Wizard of Oz… But *don’t* try pulling it up or chopping it down. The plant is freakin’ huge, so pretty easy to ID. It’s not like you won’t notice it.

Cow parsnip, hogweed and wild parsnip are of a wholly different breed than poison ivy; the sap contains furocoumarins and is photosensitizing, it increases the effects for sun exposure. Here’s a nice overview, as related to wild parsnip (but, again, it applies to garden & cow parsnip and hogweed):

“This section is adapted from the excellent article on wild parsnip burns by David Eagan (1999). Wild parsnip is of concern because humans develop a severe skin irritation from contact with its leaves. Plants have chemicals called psoralens (more precisely, furocoumarins) that cause phyto-photodermatitis: an interaction between plants (phyto) and light (photo) that induce skin (derm) inflammation (itis).

Once the furocoumarins are absorbed by the skin, they are energized by uv light on both sunny and cloudy days. They then bind to DNA and cell membranes, destroying cells and skin. Parsnip burns usually occur in streaks and elongated spots, reflecting where a damaged leaf or stem moved across the skin before exposure to sunlight.

Wild parsnip burns differ from the rash caused by poison ivy in several aspects. First, everyone is sensitive to wild parsnip and you do not need to be sensitized by a prior exposure to develop burns or blisters. You can brush against wild parsnip plants and not be affected. Parsnip is only dangerous when the plant sap from broken leaves or stems gets on your skin. Lastly, the wild parsnip’s “burn” is usually less irritating that poison ivy’s “itch.” The worst of the burning pain caused by wild parsnip is usually over within a couple of days while the rash and itch of poison ivy can last a long time.

In cases of mild exposure to wild parsnip, affected areas turn red and fell sunburned. In severe cases, the skin first turns red and then blisters form. The arms, legs, torso, face, and neck are most vulnerable and affected areas may feel like they have been scalded. Blisters form a day or two after sun exposure and soon after the blisters rupture and the skin starts healing. But for many people the ordeal is not over as dark red or brownish “scars” remain in the burned areas for several months to years. Animals can also get parsnip burns if they have little hair and lightly pigmented skin, characteristics that allow the chemical and sunlight to reach the skin.

The burning sensation can be relieved by covering the affected areas with a cool, wet cloth. Try to delay blisters from rupturing as long as possible as blisters protect the skin by keeping it moist and clean while the areas heal. For those cases with extensive blistering, consult a doctor.

Tips to avoid exposure include wearing gloves, long pants, and long-sleeved shirts. Planning control activities for the early evening will minimize sunlight and thus activation of the blistering process. If you are exposed to the plant juice, wash the contaminated areas thoroughly as soon as possible.”