Giant hogweed
Heracleum mantegazzianum

Giant hogweed is a Class A Noxious Weed in Washington State due to its limited distribution in the state and the potential for significant impact to state resources. Public and private landowners are required to control this plant when it occurs on their land. Because of the risk of injury when handling this plant and the difficulty of distinguishing it from the native plant cow parsnip, we recommend contacting the noxious weed program for a positive identification and advice on control methods before removing.

History and impact

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.

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 description

A member of the parsley family, its most impressive characteristic is its massive size. Giant hogweed 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 raised spots and stiff bristle-like hairs. Coarse white hairs are also at the base of the leaf stalk. 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, elliptical to oval dry fruits that have a broadly rounded base, and broad marginal winged ridges. Giant hogweed is similar in appearance to our native cow parsnip, only it is much larger, the purplish blotches are more raised and bumpy, and the hairs on the under surface of the leaf are shorter (about .25 mm long). Cow parsnip seeds also tend to be wider at the base whereas giant hogweed seeds are more often elliptical, the same width at the base and seed tip (click here for a detailed description of giant hogweed seeds).

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.