The plant’s clear, watery sap works with moisture and sunlight in a reaction called phytophotodermatitis. People coming into contact with the sap develop large, painful blisters that resemble severe sunburn. Some have had to be hospitalized for intravenous antibiotics and cortisone injections and have taken a month or more to heal.
I don't know how I've missed news of this scourge up til now! If you're unaware, too, click through to an informative poster from NY state. (Giant hogweed, Latin name: Heracleum mantegazzianum)
The USDA lists giant hogweed as a "Federal Noxious Weed, and the state of New Hampshire lists it as one of the "Prohibited Invasive Species." See photos here.
Do not touch giant hogweed or try to remove it yourself, but instead, the Extension recommends that
NH residents who suspect they have found giant hogweed should call the Cooperative Extension’s Family, Home & Garden Education Center’s Info Line at 1-877-398-4769, Monday-Friday, 9:00 am -2:00 pm, prepared to describe the plant and its location. People who have already made a positive identification should notify Doug Cygan at 271-3488.
Learn more about plant and other invasive species in these two books:
Baskin, Yvonne. A Plague of Rats and Rubbervines: The Growing Threat of Species Invasions. [577.18 BAS]
Simberloff, Daniel. Invasive Species: What Everyone Needs to Know. [578.62 SIM]
Photo courtesy NYS DEC.