Giant hogweed (Heracleum mantegazzianum) is an invasive plant. It is a native of Caucasus and was first detected in North America in the early 1900's.
Also known as Cow Parsley, this weed grows to 6 to 16 feet high and has a dark red stem up to 3 inches in diameter. It lives for 5 to 7 years and flowers in its final year, producing umbrella shaped clusters of small white flowers up to 31 inches in diameter. Contact to bare skin can cause photodermatitis, resulting in blisters and scars.