Giant Hogweed
Heracleum mantegazzianum

History and Impact

Giant Hogweed is originally fromAsia and was introduced as an ornamental. It is similar in appearance to our native cow parsnip, only it is much larger and the hairs on the under surface of the leaf are shorter (about .25 mm long). A public health hazard, hogweed's clear, watery sap has toxins that cause photo-dermatitis. Skin contact followed by exposure to sunlight produces painful, burning blisters that may develop into purplish or blackened scars. Click here for photos and more information on hogweed burns.

Biology and Morphology

A member of the parsley family, its most impressive characteristic is its massive size. It reaches a height of 10 to 15 feet when in flower and has hollow stems, 2 to 4 inches in diameter with dark reddish-purple spots and bristles. Coarse white hairs at the base of the leaf stalk are also purplish, and each purple spot surrounds a blister-based hair. The deeply incised compound leaves grow up to 5 feet in width. Giant hogweed flowers 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. Hogweed prefers moist soil and can quickly dominate ravines and stream banks.

Giant hogweed photos - click thumbnail for larger image

If you find giant hogweed in King County, please notify us through our online infestation form.

To find out where we have records of this weed in King County, use our interactive noxious weed map and search for giant hogweed.

For information on giant hogweed identification and control, please download our giant hogweed weed alert (1.38 MB Acrobat file).

For more detailed information on controlling giant hogweed in King County, please read the giant hogweed best management practices (142 KB Acrobat file, 20 seconds on 56K modem).

For additional background on giant hogweed, please see the written findings of the WA State Noxious Weed Control Board.

Read news articles related to giant hogweed.

Program offices are located at 201 S. Jackson St., Suite 600, Seattle, WA 98104. To contact a member of the King County Noxious Weed Control Program, or to report a noxious weed site, please call 206-296-0290 or reach them by e-mail.