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Spread of toxic invasive plant alarms U.S. forest authorities

by Patterson Clark

The Washington Post

Beware the giant hogweed.

Named after the Roman hero 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 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.”

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 also have 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. Workers wearing protective clothing remove the plant and treat the site with herbicides. For almost a decade, the MDA has been monitoring the area around Gunpowder Falls State Park, “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 of Columbia, the Department of the Environment has been fighting one patch of hogweed, near the Embassy of Bahrain, since 2006.

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.