Plant Cousins Unwelcome Garden Guests

Giant hogweed, among others, plagues backyard gardeners with blisters and burns
Lee Van Wychen, Weed Science Society of America,
June 20, 2008
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LAWRENCE, KANSAS — With spring in full bloom, lawns regain their lustrous green color and glisten with morning dew, while tulips and daffodils beautify the landscape. If you’re tempted to get outside and prepare your flowerbeds for the onset of summer, make certain you’re prepared. You may run into weeds that can irritate or blister your skin.

Well-known garden plants such as carrots, parsnip and dill have cousins with some nasty characteristics. Giant hogweed (Heracleum mantegazzianum), common hogweed (Heracleum sphondylium) and their hybrids were introduced into the United States from Europe in the early 1900s as showy garden ornamentals. But they can pack a painful punch.

Another invasive weed that causes watery blisters and burns is wild parsnip (Pastinaca sativa). Our native cow parsnip (H. lanatum) and Angelica species also irritate skin if exposed directly to the sap, but to a much lesser degree than their giant cousins from Europe. All are members of the carrot family. These showy plants contain a clear, watery sap that can produce irritation or blisters on the skin when triggered by sunlight. Giant hogweed is the worst offender, though, because of its sheer size and the quantity of sap produced.

Melissa Bravo, manager of the Pennsylvania Department of Agriculture’s Giant Hogweed Program and a member of the Weed Science Society of America, warns that protection is the best policy.

“Contact with giant hogweed leaves or stems can easily ‘burn’ skin and lead to painful blisters that form within 48 hours if exposed to direct sunlight,” Bravo says. “As the blisters heal, they often become darkly pigmented purple blotches or scars that can last a few months or, in the worst cases, can be permanent. You really have to wear gloves, long-sleeved shirts and long pants when working around giant hogweed.”

Giant hogweed is a perennial or long-lived biennial that grows in a rosette until a flower stalk bolts. Multiple leaves as large as three- to five-feet long are deeply divided and lobed. Both leaves and stems are covered in fine prickly hairs, and stems are often tinged with purple blotches. Flowering plants can attain heights of 15 feet in fertile flowerbeds, with umbrella-like clusters of flowers that look much like Queen Anne’s lace and measure up to two-feet across.

In addition to being dangerous, giant hogweed also is highly invasive, especially along waterways. It was declared a federal noxious weed in 1983, and several states have begun eradication programs to assist property owners with infestations in flowerbeds, yards, stream banks, roadsides and forest edges.

Fortunately, giant hogweed has limited distribution in the United States, and local efforts to eradicate this invasive plant are achieving success. Currently 16 states have wild populations of giant hogweed: Connecticut, Illinois, Indiana, Maine, Maryland, Massachusetts, Michigan, New Hampshire, New York, New Jersey, Ohio, Oregon, Pennsylvania, Washington, Wisconsin and Vermont.

For more information on giant hogweed, contact your state department of agriculture or the USDA’s program manager for Animal and Plant Health Inspection Services in your area. Information about giant hogweed and other invasive plants is also available from Lee Van Wychen, director of science policy for the Weed Science Society of America, at 202-746-4686.

The Weed Science Society of America, a nonprofit professional society, was founded in 1956 to encourage and promote the development of knowledge concerning weeds and their impact on the environment. The Weed Science Society of America promotes research, education and extension outreach activities related to weeds, provides science-based information to the public and policy makers, and fosters awareness of weeds and their impacts on managed and natural ecosystems. For more information, visit

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