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.
This species is also on the Washington quarantine list (known as the prohibited plants list) (external link) and it is prohibited to transport, buy, sell, offer for sale, or to distribute plants or plant parts, seeds in packets, blends or "wildflower mixes" of this species, into or within the state of Washington.
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).
Additional information on giant hogweed
What to do if you find this plant in King County, Washington
Please notify us if you see giant hogweed growing in King County. Our program staff can provide the property owner or appropriate public agency with site-specific advice on how best to remove it. We map all known locations of regulated noxious weeds such as giant hogweed in order to help us and others locate new infestations in time to control them.
Giant hogweed photos - click thumbnail for larger image