Giant Hogweed
(Heracleum mantegazzianum)
Identification Gallery
Giant Hogweed Profile
Common Name(s):
giant hogweed
cartwheel flower
giant bearclaw
giant cow parsnip
wild rhubarb
Scientific Name:
Heracleum mantegazzianum
Native Range:
Sub alpine region of the western Caucasus Mountains of Georgia, Azerbaijan, and southern Russia
Non-native Range US:
CT, DC, IL, IN, MA, MD, ME, MI,
NC, NJ, NM, NY, OH, OR,
PA, WI, WA, VT
Non-native Range Canada:
BC, NB, NF, NS, ON, PE, QC
USDA Symbol:
HEMA17
Rash Information:
Toxic chemicals in the clear, watery sap called furocoumarins cause blisters and burns to human skin that has come in contact with it while exposed to sunlight. This reaction is called phytotophotodermatitis. Blisters often give way to black or purple scars that persist for many months or years. Contact with the sap may cause temporary or potentially permanent blindness if it gets into the eyes. The sap is most toxic when plants are in flower. Symptoms typically occur within approximately 48 hours after contact.
What to do if You Come in Contact with the Sap:
• rinse with soap (if available) and water immediately after coming in contact with giant hogweed sap
• stay out of sunlight
• contact a physician if symptoms appear
Growth Habit/Form:
• biennial or monocarpic perennial herb
Habitat:
• floodplains
• streambanks
• roadsides
• railway embankments
• fallow fields
• prefers rich, moist soils
• disturbed soils
Plant Height:
10-20 feet (3.5-6.5 meters) tall
Stem Description:
• hollow and fleshy
• covered in coarse trichomes (stiff hairs)
• green with few or numerous purple splotches
• may be deep purple
• 2-4 inches (1-2 centimeters) in diameter
Leaf Description:
• palmately compound (3-part compound)
• 3 deeply incised leaf blades
• lower leaves are larger and can reach up to 10 feet (3 meters) long and 5 feet (1.5 meters) wide
• basal leaves are produced the first year
• spotted petioles (leaf stalks)
• petioles may have trichomes like stem
• at maturity, plants have 4-6 stem leaves and 3-4 basal leaves
• the first true leaves that emerge are simple, round or kidney shaped, with a serrated margin
• rosettes have three to four leaves
Leaf Arrangement:
• alternate
Leaf Margin:
• serrate
Leaf Surface:
• underside has dense soft hairs
Flowering Period:
• late June through July
Flower Description:
• small white (occasionally pinkish) flowers cluster to form a loose, flat umbel
• inflorescence forms compound umbels with the terminal umbel flowering first and the outer or umbellets flowering after
• the terminal umbel grows larger and is a hermaphrodite surrounded by up to 8 satellite umbels on elongated curved stalks raising them above the the terminal umbel
• can be up to 3 feet (1 meter) wide
• produce flowers in 2-5 years
• flowers once in a plant's lifetime (hybrids may flower multiple years)
• flower stalks are thick and fuzzy, with lateral lines
• flower stalks are green and not usually splotched like stems and leaf stalks
• umbels have 50-15- unequal hairy rays
• rays are shorter and thicker at the center of the umbel and get longer toward the outer edge
• the flowers have an unpleasant urine-like odor
• each flower produces two single seeded fruits
Flower Color:
• white
• occasionally pinkish
Seed Description:
• small flattened oval winged seeds with ridged margins
• 0.5 inches (1.25 centimeters) long
• green until they mature then turn brown
• mature plants can produce between 10,000 and 100,000 seeds
• seeds are viable in the soil for up to 15 years
Similar Species:
• cow parsnip (Heracleum maxim)
• poison hemlock (Conimum maculatum)
• queen Anne's lace (Daucus carota)
• angelica (Angelica atropurpurea)
Threats from this Species:
• its large size enables it to outcompete native species
• causes erosion along streambanks because roots don't secure soil
How it Arrived in North America:
• North American ornamental gardeners planted it due to its uniqueness. Gardeners cultivated it until 1917
• Many seeds have come to North America with foreign visitors who cook with the seeds
Benefits of this Species:
• due to its large flower, it is beneficial for bees
How it Spreads and Why it is so Successful:
• spreads by winged seeds carried off by animals, humans (machinery) surface runoff of rain, or wind
• spreads by seeds being transferred from one site to another due to construction activities and soil removal
• can survive in water up to three days and be carried by wind up to 30 feet (10 meters)
• is successful due to the large number of seeds (up to 100,00), establishing new colonies is likely if this plant is allowed to go to seed
• is successful because it can inhabit disturbed areas and outcompete other plants due to its large size
• is successful possibly because it has not natural enemies to keep it in check
Other Information:
• giant hogweed is a member of the carrot or parsley family
• the genus of giant hogweed is named after the mythical god, Hercules, due to its impressive size
• the species is named after the Italian dermatologist Paulo Mantegazza
• designated a noxious weed by the federal government due to its poisonous sap
• due to its preference for moist soils, this plant is also considered an aquatic invasive species
• has a taproot when growing in a nutrient rich substrate or numerous fibrous roots when growing in shallow soils where the highly branched root system spreads out to obtain nutrients
• after die-off, they leave behind large woody stalks
• do not burn or compost any part of this plant
For More Information About This Species Please Visit:
USDA National Invasive Species Information Center
New York Department of Environmental Conservation
Invasive Plant Atlas of the United States
Ontario's Invading Species Awareness Program
GB Non-native Species Secretariat
Texts:
Invasive Plants of the Upper Midwest: an Illustrated Guide to Their Identification and Control, by Elizabeth J. Czarapata, University of Wisconsin Press, 2005, pp. 136–137.
Invasive Plants: Guide to Identification and the Impacts and Control of Common North American Species, by Sylvan Ramsey Kaufman, Stackpole Books, 2012, pp. 303–304.