Beware the giant hogweed.

Named after Hercules, Heracleum mantegazzianum captures attention with its impressive foliage: huge, sharply lobed leaves; hollow, hairy, purple-splotched stems, four inches in diameter; and towering umbrella-shaped white flowers that bloom in late spring.

But an informed person will resist hogweed's allure, giving the plant a wide berth — and alerting the authorities.

Brushing up against this relative of carrots and parsnips can trigger the release of its toxic sap, which causes the skin to become hypersensitive to the sun's ultraviolet rays. If not quickly washed with soap and water and shielded from the sun, affected skin can develop phytophotodermatitis, an itchy, burning inflammation with blisters, scarring and discoloration, which might take months to heal. Skin can remain sensitive to sun exposure for years.

The U.S. Forest Service warns that "sap in the eyes can cause temporary or possibly permanent blindness — although no cases seem to be recorded in medical literature.

The offending toxins are furanocoumarins, which, with the aid of ultraviolet light, bind to a skin cell's nuclear DNA, causing the cell to self-destruct.

Caterpillars that chew on plants containing those compounds have developed several defenses against the toxins, said May R. Berenbaum, an entomologist at the University of Illinois at Urbana-Champaign. For example, some caterpillars accumulate the yellow pigment lutein, which absorbs ultraviolet light, reducing photoactivation of the hogweed compounds.

Could humans use similar tactics?

"I doubt that people want to ingest enough lutein to turn yellow," Berenbaum said. "That's what happens to the caterpillars."

"Other caterpillars roll leaves around themselves as they feed to reduce exposure to activating wavelengths of ultraviolet light," she said. "The human equivalent is to cover up exposed skin, which actually does work but can get uncomfortable in hot weather."

But most caterpillars detoxify the chemicals "with very efficient gut enzymes," Berenbaum said. "Ours are puny by comparison."

SOURCES: Vanessa Orlando, Maryland Department of Agriculture; New Zealand Medical Journal; University of Maryland Extension; New York Sea Grant; U.S. Forest Service; Encyclopaedia Iranica; Canadian Journal of Plant Science; New York Department of Environmental Conservation.

Giant Hogweed, Heracleum mantazzegium, illustration by Patterson Clark

Noxious and invasive

Originally from the mountains of Central Asia, giant hogweed made its debut in New York about 100 years ago as a dramatic ornamental plant. It's been spreading ever since, becoming a serious pest in the Northeast and the Pacific Northwest, and it's making inroads into the Mid-Atlantic.

Hogweed's winged seeds use several methods of dispersal. They can float downstream for three days and germinate in sunny, damp soils along waterways. Wind, animals and misguided gardeners have also disseminated seeds, which can remain viable in the soil for 10 years.

Extermination of the toxic, aggressive weed is a priority for the Maryland Department of Agriculture, which reports 21 known sites of the plant, mostly in the western reaches of the state but also in Harford and Baltimore counties. Workers wearing gloves, goggles and protective clothing remove the plant and treat the site with herbicides. Hogweed usually succumbs after three years. For almost a decade, the MDA has been monitoring Gunpowder Falls, "making sure no new plants are coming up" from seeds washed down from treated sites, said Dick Bean, weed management program manager for the MDA.

Virginia's agriculture department is not aware of any giant hogweed infestations, but in the District, the Department of the Environment has been fighting one patch of hogweed, on Tilden Street near the Embassy of Bahrain, since 2006.

"The plant poses minimal risk to human health since it is mostly located behind a fence and off of the walking path," said biologist Laura Washington of the District's environmental department, but "nearby pedestrians can potentially transport the seed to other areas" if the seeds get caught in the tread of their shoes. The weed hasn't yet been found in Rock Creek Park, but the National Park Service is aware of a patch on private property near Glover-Archbold Park, according to Ana Chuquin, a Park Service biological technician.

Not every culture is an enemy of Heracleum. Iranians pickle young shoots of the plant and grind the nontoxic seeds of several species of hogweed to make golpar, a mildly bitter spice used in salads, stews and soups. In Iran, the aromatic seeds were once placed in coffins to help mask the stench of death.