Giant Hogweed Giant Hogweed is originally from Asia and was introduced as an ornamental. It is similar in appearance to the 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

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 river or stream banks.