'Plant from Hell': Giant Hogweed Can Scar, Burn and Blind

A towering weed with sap that causes blisters and burns is blooming this month across the U.S.
ABC's Good Morning America calls it the "plant from hell," and health and environment authorities from coast to coast are warning people to steer clear of the plant, giant hogweed, because its sap can cause serious blisters, scarring and even blindness.

"If you see this plant, avoid it!" advises the New York State Department of Health.
The sap in giant hogweed is clear and watery, but it contains toxins that cause photo-dermatitis, a skin reaction to ultraviolet rays. As a result, skin that comes in contact with the sap and is then exposed to sunlight will blister and burn — injuries that can lead to long-lasting skin discoloration and scars. Sap in the eyes can cause blindness.
The fearsome plant is chiefly recognizable by its impressive height and its clusters of white flowers. The noxious weeds program of King County, Wash., gives a more technical description:

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. [...] 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 0.25 mm long).

Giant hogweed, or Heracleum mantegazzianum, was originally native to central Asia, but it spread to Europe during the 19th century and has now been in North America for decades. According to the U.S. Department of Agriculture, giant hogweed can be found in the in New England, New York, Pennsylvania, parts of the Midwest and the Pacific Northwest. The plant also grows in Canada in New Brunswick, Quebec, Ontario and British Columbia.
If you see something you suspect may be giant hogweed, do not touch it and don't try to kill the plant, as this may put you in contact with the plant's noxious sap. Instead, notify your local authorities.
If you do come in contact with giant hogweed sap, King County recommends that you wash the contact site thoroughly and keep it covered from sunlight. Be sure to contact a doctor.