Giant Hogweed, copyright 2010 Andy Fyon, www.ontariowildflower.com

Giant Hogweed

 

Andy's Northern Ontario Wildflowers

Giant Hogweed

(Heracleum mantegazzianum)

 

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Giant Hogweed:

There has been a increased awareness about the dangers of having Giant Hogweed (Heracleum mantegazzianum) in our gardens. The main concern is the reaction that the clear sap can cause with human skin when exposed to Sunlight. Avoid at all times and watch children.

Distinction:

Giant hogweed may be mistaken for Wild carrot (Daucus carota), poison hemlock (Conium maculatum), water-parsnip (Sium suave), spotted water-hemlock (Cicuta maculata) and Cow parsnip (Heracleum maximum). The pure enormous size of Giant Hogweed is the easiest distinguishing feature.

Description:

Giant Hogweed: perennial or biennial; also known as giant cow parsnip. Stable into Growing Zone 3.

Family: Parsley or carrot family (Apiaceae or Umbelliferae)

Flower: White or rarely pinkish; occurs as a compound umbel up to 1 m across under ideal conditions; those observed on Manitoulin Island were about 0.5 m across; the umbel consists of 30 to 50 branches; each branch ends in a simple umbel with approximately 30 to 40 flowers; each umbel has a flat bottom and gently rounded top; flower petals are up to 12 mm long; June to August.

Leaves: Leaves are very large, up to 1m across; compound with 3 large deeply cut leaflets; each leaflet has deep irregular lobes; coarse, sharp teeth on all margins; leaves are attached to the stem by a petiole.

Stem: The flower stems are enormous - standing up to up to 5 m tall and up to 10 cm in diameter under ideal conditions; the plants observed on Manitoulin Island were about 2 m tall and 3 cm in diameter (as best I could guess!); stems are hollow except at the nodes; stems and petioles have an obvious reddish-purple streaks and specks and may be nearly solid purple near the base; lower stem may be very rough with pointed, irregularly-spaced bumps.

Seed: Seed remains viable for 7 years; hence, an eradication program must be sustained for 7 years!!! Plant produces 1,500 to 100,000 seeds that are flattened, up to 1 cm long, oval, have a broadly rounded base, and have broad marginal ridges

Habit: Forms rosettes up to 1 m tall during the first year; in the second year, produces flower stem and very large rosette of huge leaves; produces flower umbels in the third year.

Height: Tall - up to 5 m under ideal conditions.

Habitat: Introduced from Europe and has grows along roadsides, steam banks and waste areas.

Interest: Giant Hogweed flowering only once in its lifetime. It reproduces only by seed. The watery, clear sap causes severe dermatitis in most people when exposed to Sunlight. The sap contains photosensitizing compounds (furanocoumarins), which are activated on humans when exposed to Sunlight. Avoid contact!  Effects can last for months and skin can remain sensitive to UV light for years (see references below - this summary from weedinfo.ca).

Location: Gore Bay golf club, Manitoulin Island
Date: July 11, 2010

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References that provide detailed information about Giant Hogweed:


Giant hogweed, copyright 2010 Andy Fyon, www.ontariowildflower.com

Giant Hogweed plant compared to a cedar fence. The fence is about 0.75 m tall (waist height to an a adult).

Location: Manitoulin Island
Date: July 11, 2010

Compound inflorescence umbel masses of the giant hogweed flowers. The entire flower array is about 1.5 m across.

Location: Manitoulin Island
Date: July 11, 2010

Giant hogweed, copyright 2010 Andy Fyon, www.ontariowildflower.com

Giant hogweed, copyright 2010 Andy Fyon, www.ontariowildflower.com

Compound inflorescence of the giant hogweed flower umbel The umbel is about 0.5 m across.

Location: Manitoulin Island
Date: July 11, 2010

Underside of the compound flower umbel of the Giant Hogweed plant. The umbel is about 0.5 m across.

Location: Manitoulin Island
Date: July 11, 2010

Giant hogweed, copyright 2010 Andy Fyon, www.ontariowildflower.com

Giant hogweed, copyright 2010 Andy Fyon, www.ontariowildflower.com

Giant hogweed leaves, copyright 2010 Andy Fyon, www.ontariowildflower.com

Leaves of the Giant Hogweed plant (above and opposite). Note the size of the leaves compared to the cedar fence.

Location: Manitoulin Island
Date: July 11, 2010

Flower stem of the Giant Hogweed plant. Note the reddish, purple colour of the stem.

Location: Manitoulin Island
Date: July 11, 2010

Giant hogweed flower stem, copyright 2010 Andy Fyon, www.ontariowildflower.com

Giant hogweed early growth leaves, copyright 2010 Andy Fyon, ontariowildflower.com

Leaves of first year occur in the top right corner. Second to fourth year growth starting in lower left part of image. Note the boot toe in the lower right corner of the photo.

Photo by Elizabeth Fyon
Location: Manitoulin Island
Date: August 5, 2010

Giant hogweed seed head, copyright 2010 Andy Fyon, ontariowildflower.com

 

Seed of the Giant Hogweed.

Location: Manitoulin Island
Date: August 5, 2010

Giant hogweed seed, copyright 2010 Andy Fyon, ontariowildflower.com

Andy Fyon and giant hogweed, copyright 2010 Libby Fyon.

Andy Fyon (cool hat) standing for scale beside a Giant Hogweed plant.

Photo by Elizabeth Fyon
Location: Manitoulin Island
Date: July 11, 2010

 

 

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For more information email: andy@ontariowildflower.com
URL: http://www.ontariowildflower.com/giant_hogweed.htm
© 2010-2012 Andy Fyon

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Andy Fyon

September 29, 2012

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