By Lauren Keiper
BOSTON (Reuters) - Soaring stems topped with clusters of small white flowers some 10 feet in the air and flanked by massive leaves spanning more than three feet wide make a spectacular summer sight.
But the impressive mid-summer bloom of giant hogweed comes with dire warnings from experts -- do not touch this plant.
"It's a horrible weed. It's just a nasty weed," said Lois Stack, a specialist in ornamental horticulture at the University of Maine.
Sap from giant hogweed on skin reacts with sunlight and causes severe skin irritation, painful blistering and permanent scars. Contact with eyes may cause blindness.
"Boy, at this time of year, when you see an eight-foot plant in bloom, it's tempting to want to touch," said Stack.
Heracleum mantegazzianum, commonly called giant hogweed, is listed federally as a noxious weed in part because of the health threat it poses to humans and even animals that come in contact with sap from its stem or leaves.
Popular among specialist gardeners for its stature and interesting appearance, giant hogweed is not native to the United States.
It is native to the Caucasus, a region that spans the borders of Europe and Asia, and it was initially introduced in North America as an ornamental garden plant.
These days it can be found in the Northeast, including known populations throughout New England into New York and Pennsylvania, and on the West Coast in Oregon and Washington.
Although people cultivate it in gardens, giant hogweed thrives in moist soil and partial shade and can sprout along roadsides and riverbanks or in abandoned lots.
"Like other plants in those areas, it grows best where it's ignored, where people don't notice it or where there's no competition," said Vermont state plant pathologist Tim Schmalz.
Giant hogweed likes to steal the show, known to crowd out other plants in the vicinity. Along riverbanks, it causes soil erosion.
The plants can grow as tall as 15 feet, with hollow stems that are two to four inches in diameter. The leaves typically measure about three feet wide and display a unique pattern of veins.
During its June and July blooming season, it boasts an umbrella of tiny white flowers brimming with thousands of seeds.
"When it puts up that flower stalk and is suddenly 10 to 15 feet high with a giant cluster of white flowers at the top, it definitely attracts people's attention," said Jennifer Forman Orth, a plant pest survey coordinator for the Massachusetts Department of Agricultural Resources.
Most states rely on the public to report this harmful plant growing in the area. In Massachusetts, calls from residents led experts last year to a new giant hogweed site in the state where the pesky plant was first tracked in 2002.
In New York, people are urged to report giant hogweed sightings to the state's dedicated hogweed hotline. New York is armed with six giant hogweed crews tasked with controlling the plants at nearly 950 known giant hogweed sites statewide.
But mix-ups can happen. Giant hogweed, a member of the carrot family, resembles cow parsnip and Queen Anne's lace.
Although there are methods to control and even eradicate the plant, its abundant seeds are easily spread through water, soil and compost.
(Editing by Ellen Wulfhorst and Greg McCune)