giant hogweed

Heracleum mantegazzianum

   

Family: Apiaceae

Other common names: giant cow parsley

Weed Class: A

Year Listed: 1991

Native to: The Caucasus mountains and southwestern Asia

Toxic: humans

Other Legal Listings: WA Quarantine list, WAC 16-750

additional photos

 

   

Why is it a noxious weed?

It forms dense canopies outcompeting native species and increasing soil erosion. It exudes a clear watery sap which sensitizes the skin to ultraviolet radiation, resulting in severe burns to the affected area causing blistering and painful dermatitis.

How would I identify it?

General Description: Giant hogweed is a perennial distinguished by its large size, capable of growing to a height of 15-20 feet. It has stout dark reddish-purple spotted stems and leaf stalks. Stalks and stem produce sturdy pustulate bristles.

Flower Description: Giant hogweed has broad, flat-topped flower clusters (umbels) of many small white flowers. Each flower cluster may grow to a diameter of 2.5 feet.

Leaf Description: The compound leaves of giant hogweed may expand to five feet in breadth. Each leaflet is deeply cut.

Stem Description: The stem and stalks are hollow and vary 2 to 4 inches in diameter.

Fruit/Seed Description: The flowers produce large elliptic dry fruits marked with brown swollen resin canals.

May be confused with: Cow Parsnip, Heracleum lanatum, is a native plant in Washington and except for its size, has a similar appearance to giant hogweed. Your county noxious weed coordinator can also help with identification.

How does it reproduce?

Reproduction is by seed and perennating buds formed on the crown and tuberous root stalk. Abundant seed production, a persistent root stalk, and reproduction from perennating buds are cited as reasons for its capability to colonize rapidly.
 

Where does it grow?

Giant hogweed may colonize a wide variety of habitats but is most common along roadsides, other rights-of-way, vacant lots, streams and rivers. Please click here to see a county level distribution map of giant hogweed in Washington.
 

How do I control it?

General Control Strategy

The key strategy for this plant is to get rid of it before it sets seed. When controlling giant hogweed, make sure to wear protective clothing to avoid contact with its toxic sap.

Mechanical Control

It’s possible to hand-pull seedlings and young plants but it is important to avoid getting any of the sap on your skin. The toxic sap that causes the burning is contained in all portions of the plant. Always wear protective clothing and gloves. Large plants can be dug up, but be sure to get all of the roots to prevent re-sprouting. Be sure to discard at least the flowerheads into the regular trash to prevent seed spread. Mowing serves only to stimulate budding on the main root stalk, but it might be successful if done consistently and persistently enough to starve the rootstalk and may be effective for large infestations.

Biological Control

Cattle and pigs are cited as possible biocontrol agents. Both eat giant hogweed without apparent harm.

Herbicide Control

Please refer to the PNW Weed Management Handbook, or contact your county noxious weed coordinator.


 

For more information

 
See our brochure on giant hogweed.
See our postcard for early detection information about giant hogweed.
See our Written Findings for more information about giant hogweed (Heracleum mantegazzianum).

Additional Photos


 
 

 


Giant hogweed infestation

 


Giant hogweed leaf

 


Giant hogweed inflorescence

 


Giant hogweed seed

 


Scars caused by Giant Hogweed

 


Left,native coltsfoot. Right, giant hogweed

 


flower detail

 


giant hogweed inflorescence

 


flowering plant

 


leaves

 


plants growing in field

    

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