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Invasive Species Spotlight: Giant Hogweed (Heracleum mantegazzianum)

Invasive Species Spotlight: Giant Hogweed (Heracleum mantegazzianum)

Giant hogweed flowers.
September 14, 2018

Giant hogweed (Heracleum mantegazzianum).
Giant hogweed (Heracleum mantegazzianum). Giant hogweed can grow to reach over 15 feet in height. Photo credit: Robert Vidéki, Doronicum Kft., Bugwood.org
Giant hogweed, Heracleum mantegazzianum, may not be one of the most well-known invasive species in our area, but this supersized plant has recently been in the spotlight for its potentially dangerous side effects. Giant hogweed produces a sap that makes human skin sensitive to sunlight, causing serious burns and blistering. In some cases, contact with eyes may even cause blindness. Despite the potentially hazardous reaction, experts say don’t panic. Giant hogweed is an easy plant to avoid. Its large size is an obvious give away and the populations have become far and few between.

Giant hogweed is native to central and southwest Asia. It was first introduced to the United States in the early 1900s as an ornamental because of its impressive size. It prefers disturbed moist soil, lots of sunlight, and can be found along roadsides, streambanks, railroads, right-of-ways and fallow fields. The enormous size of the plant shades out native species below, starving them from sunlight. Giant hogweed plants thrive along stream banks, creating a problem when the plant dies during the winter, leaving bare ground along the stream open to erosion. Hogweed populations in the United States have been reported in over 15 states within the Northeast, the Great Lakes and Northwest regions. More recently, giant hogweed has been reported as far south as Virginia. Pennsylvania has recorded giant hogweed in 17 counties with over 50% of the 500 sites being in Erie County. Chester County does not currently have any active reported giant hogweed sites.

Giant hogweed stem.
Giant hogweed stem. The stems of the look-alikes found in our area, cow parsnips and poison hemlock, lack the characteristic purple spots of the giant hogweed stem. Photo credit: Robert Vidéki, Doronicum Kft., Bugwood.org
Giant hogweed is a biennial to short-lived perennial and is a member of the carrot family. It has been nicknamed “Queen Anne’s Lace on steroids” for its similar appearance, except one blaringly obvious difference: its size. Giant hogweed can grow to reach over 15 feet in height. The large umbrellaed flower heads have clusters of small white flowers. The fruits of the plant are flat and ellipse shaped, bearing over 100,000 seeds per plant annually. The hollow stems of giant hogweed have distinct purple spots with stiff white hairs. Leaves of the plant are compound and deeply incised, growing up to five feet in width.

The sap of giant hogweed is what has brought this plant into recent headlines. Contact with the sap causes to the skin to become hyper sensitive to sunlight. Within 24 to 48 hours of contact painful burns and blistering may occur. In rare cases, if the sap comes into contact with eyes, temporary or even permanent blindness may occur. If you know that you have encountered the plant, it is recommended to wash the affected area immediately with soap and water, and to avoid direct sunlight for 48 hours.

Listed on both the Pennsylvania and Federal Noxious Weed Control Lists, any sale or transport of giant hogweed is illegal. If you believe you have sighted giant hogweed, it is recommended that you do not attempt to remove the plant on your own. Instead, contact the Pennsylvania Department of Agriculture to report the population and they will take care of it.

Similar Plants Commonly Mistaken for Giant Hogweed

There are several plant species in our area that may be mistaken for giant hogweed. Native common cow parsnip, Heracleum maximum, is also in the carrot family. Like Giant Hogweed, the cow parsnip sap can cause a rash when contact with skin is made. To help identify which is which, there are a few obvious differences between the two. Common cow parsnip is overall smaller in size, reaching only three to seven feet in height. The stems do not have the purple spots and the leaves are not as deeply lobed or serrated. While giant hogweed has umbrella shaped flower heads, cow parsnip flower heads are flat. Non-native poison hemlock, Conium maculatum, is another species often misidentified as giant hogweed. Poison hemlock has fern-like leaves that have a waxy appearance and the flower tops are flat. Poison hemlock is also smaller, typically reaching no more than nine feet.

Giant hogweed foliage.
Giant hogweed foliage. Robert Vidéki, Doronicum Kft., Bugwood.org
Giant hogweed flowers.
Giant hogweed flowers. Photo credit: Robert Vidéki, Doronicum Kft., Bugwood.org

Poison hemlock (Conium maculatum) foliage.
Poison hemlock (Conium maculatum) foliage. Photo credit: Pedro Tenorio-Lezama, Bugwood.org
Poison hemlock (Conium maculatum) flowers.
Poison hemlock (Conium maculatum) flowers. Photo credit: Pedro Tenorio-Lezama, Bugwood.org

Common cowparsnip (Heracleum maximum) foliage.
Common cowparsnip (Heracleum maximum) foliage. Photo credit: Rob Routledge, Sault College, Bugwood.org
Common cowparsnip (Heracleum maximum) flowers.
Common cowparsnip (Heracleum maximum) flowers. Photo credit: William M. Ciesla, Forest Health Management International, Bugwood.org

Giant hogweeds large size, quick growth and harmful sap make this plant a threat. Overall, researchers feel that the population of this invasive species seems to be under control for now. While there may not currently be any active populations in our area, it is still worthwhile to be able to recognize the species and identify the differences between its lookalike counterparts. If giant hogweed is found, the Pennsylvania Department of Agriculture should be contacted to work with landowners on an eradication method.

Learn more here:

Pennsylvania Noxious Weed Alert: Giant hogweed (Heracleum mantegazzianum)

Pennsylvania Department of Agriculture: Giant hogweed guide

Pennsylvania Department of Conservation and Natural Resources: Giant hogweed guide