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Giant hogweed

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General Information

A member of the parsley and carrot family, giant hogweed (Heracleum mantegazzianumcan) be distinguished from most other plants in that family by its massive size. It is a perennial herbaceous plant with enormous, dark green leaves and hollow stems with purple blotches that can reach a height of 2.5 to 5 metres when in flower. The flowers are borne in large umbrella-shaped clusters which can be up to 0.8m across. The seeds are winged and oval and approximately 1cm long. Each giant hogweed plant can produce up to 50,000 seeds. Giant hogweed produces toxic compounds which, when human skin comes into contact with it, can cause "phyto-photodermatitis". This makes the skin extremely sensitive to sunlight resulting in painful blistering and scarring of the skin, akin to third degree burns.

Download giant hogweed control in BC document

Examples of burns below:

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 Hogweed_flower.jpg

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Growth Habit

Giant hogweed is originally from Asia and was introduced as an ornamental.  Spreading by seed, giant hogweed has escaped into numerous backyards, ravines, parks, abandoned lots, streams, woods, and roadsides.  It can crowd out other plants and take over natural areas, especially in moist areas such as streamsides.  Somewhat shade tolerant, giant hogweed can also thrive in full sun and has been known to even invade healthy turf. Giant hogweed grows best in rich, moist soils in both open and partially-shaded areas. It will take from 3 to 8 years before it produces seed, depending on how good the growing conditions are. Giant hogweed produces copious seeds, grows from a dense taproot which keeps producing leaves and is also very dangerous to human health, making it difficult to remove. In the UK, giant hogweed is considered toxic waste.

Management

Control must be conducted very carefully so as not to touch the leaves and get the sap on your skin. Waterproof clothing including gloves, boots and safety goggles should be used. The best way to rid an area of a hogweed infestation is to excavate the plants with the soil. As this may not be an option for most individuals, another option is to repeatedly remove all vegetation, cut the root crown at an angle (below the soil surface) with a sharp spade, and mulch the area with deep soil or bark, planting with shrubs. Remember to use extreme caution when handling giant hogweed. If the plants have already gone to seed, it is possible to simply cut the seed heads and bag them for disposal, then cut the remaining vegetation and leave on-site to decompose. It will be necessary to return to the site repeatedly over the next couple of years to maintain a hogweed-free site.

Download giant hogweed control in BC document

Identification

A member of the parsley family, its most impressive characteristic is its massive size. Giant hogweed reaches a height of 6 to 16 feet when in flower and has hollow stems, 2 to 4 inches in diameter and stiff bristle-like hairs. The sharply incised compound leaves grow up to 5 feet in width. Giant hogweed blooms from mid-May through July, with numerous white flowers clustered in an umbrella-shaped head that is up to 2.5 feet in diameter across its flat top. The plant produces flattened, 3/8-inch long, oval dry fruits that have a broadly rounded base, and broad marginal ridges. 

In the Sea to Sky region giant hogweed has only been found to date on a handfull of sites in the Squamish area and south, including Brittania Beach and Lions Bay. See video below for a story on hogweed but be aware that there is a native look-alike called cow parsnip. Native cow parsnip is common in the Sea to Sky and is a smaller version of hogweed that is generally a maximum of 5 feet when in flower. In comparison, hogweed in flower will always be at least 6 feet tall or larger. Some tips on identification below:

Native cow parsnip

Cow_parsnip.jpg

 

Invasive giant hogweed

Giant_hogweed.jpg

 

Giant hogweed vs. cow parsnip

 

Feature

Cow parsnip*
Heracleum lanatum

Giant hogweed Heracleum mantegazzianum

Height of flowering stem

3 - 6 feet

6 - 16 feet

1 – 2 m

2 – 5 m

Mature stem diameter

1 – 2 ”

2 – 4”

2.5 – 5 cm

5-10 cm

Stem hairs (not as reliable)

Fuzzy hairs

Stiff bristles

# Rays in umbel (not as reliable)

15 to 30

50 to 150

Stem colouring & leaf shape

Not diagnostic

Not diagnostic

Giant hogweed health hazard:Leaves and stems contain a clear, watery, highly toxic sap that can cause hypersensitivity to sunlight resulting in burns, blisters and scarring when coming into contact with skin.

*Cowparsnip:Possesses similar chemicals to Giant hogweed, capable of causing photosensitivity when in contact with skin (to a lesser extent but noteworthy).

 

For news story on giant hogweed in the Greater Vancouver area click here.

More hogweed photos below:

Giant hogweed new growth - click for larger image Giant hogweed new leaf - click for larger image Giant hogweed patch - click for larger image Giant hogweed stem closeup - click for larger image Giant hogweed flower and root - click for larger image  
Giant hogweed young plant - click for larger image Giant hogweed jungle - click for larger image Cutting down giant hogweed - click for larger image Young giant hogweed stem - click for larger image Giang hogweed Lake Union view - click for larger image  
Giant hogweed early season plants in an alley - click for larger image Giant hogweed plants - click for larger image Giant hogweed in flower - click for larger image Giant hogweed infestation - click for larger image Giant hogweed flowering plants - click for larger image  
       

 

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