Poisonous to Touch Weed Series: Giant Hogweed (Heracleum mantegazzianum)

Giant hogweed (Heracleum mantegazzianum) plant in flower. Photo credit: Robert Vidéki, Doronicum Kft., Bugwood.org

Giant hogweed (Heracleum mantegazzianum) is likely the largest and most dangerous to touch perennial weed in North America. In Ontario it commonly reaches heights of 3-4.5m but in other parts of the world it can reach heights of 5.5m (MacDonald F. and Anderson H. 2012). There are a few other Giant Hogweed species that are a bit smaller in size but Heracleum mantegazzianum is the most common and the subject of this article. All parts of the plant contain a clear, watery, phototoxic sap, particularly the lower part of the hollow stems, the petioles and the coarse hollow hairs. When this sap comes in contact with human skin, it sensitizes it to sunlight, resulting in severe burns. The reaction is called phytophotodermatitis and actually damages your DNA, changing the way your skin protects itself from ultraviolet (UV) light (Goldman R. Updated on Dec. 2018). Initial burns usually appear within 48 hours after contact with the sap. In severe cases the individual may experience large painful blisters and 2nd to 3rd degree burns that leave behind scars and a long term sensitivity to the sun.

Where Does Giant Hogweed Grow?

Giant Hogweed is native to the Caucasus Mountains in Southwest Asia, but it has invaded many regions of the world. It has spread throughout much of Europe, United Kingdom, Australia, United States and Canada. In the U.S. it has been reported in 16 U.S. states in the Northeast, along the Eastern seaboard, Midwest, the Pacific Northwest, and Alaska. In Canada it has a scattered distribution across southern and central Ontario, with limited populations occurring in Quebec, New Brunswick, Nova Scotia, Newfoundland, Labrador and British Columbia (MacDonald F. and Anderson H. 2012).

Giant hogweed grows in a variety of habitats including roadsides, ditches, along river banks, meadows, ravines, vacant lots, along forest edges and streams, pastures, wetlands and open woodlands. It is particularly fond of rich, moist soils in seldom disturbed areas.

Identification of Giant Hogweed

Overall growth habit: Giant hogweed can reach heights of 3-4.5m, with huge leaves that are highly jagged. The plant sends up thick, tall stems which are toped with large, white, compound umbel flowers at the top.

Leaves: The leaves of young plants form a rosette of deeply lobed and pointy leaves. As they mature the become palmately compound dividing into 3 main lobes that are joined together at the stem. The leaves are fairly shiny, deeply lobed, sharply serrated and hairless. Leaves can grow up to 1m wide.

Giant hogweed (Heracleum mantegazzianum in early spring. Photo credit: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
Giant hogweed (Heracleum mantegazzianum). Photo credit: Robert Vidéki, Doronicum Kft., Bugwood.org

Stem: Giant hogweed stems are thick and tall measuring 10-15cm (4-8″) in diameter and 3-4.5m (10-15′) tall. They are green in colour with purple blotching. The blotching tends to be more concentrated lower down on the plant, almost appearing completely purple at times. The stems are covered in stiff hairs which are capable of penetrating clothing, putting skin in contact with the phototoxic sap. The hollow stem bases, petioles and hollow stiff hairs contain the highest concentrations of the sap. Poisonous hemlock also have purple spotting or blotching on their stem but they are smooth and hairless. Cow parsnip may also have a few purple spots but not blotching and the overall plant is smaller.

Giant hogweed (Heracleum mantegazzianum) showing the purple blotching on the stem. . Photo credit: Robert Vidéki, Doronicum Kft., Bugwood.org

Flowers: Large white umbels (umbrella shaped clusters of short-stalked flowers), form on the top of sturdy hollow stems that reach about 3-4.5m in height. Each flower head can measure up to 1m across. Each compound umbel can have 50-150 rays which can lead to a single plant producing well over 50,000 flowers (MacDonald F. and Anderson H. 2012). Giant hogweed flowers in mid-June for about one month (time may vary depending on your region.)

Seeds: Each plant typically produces about 50,000 winged seeds. The seeds are green turning to brown by late summer. They need to undergo 2-3 months of cold in order to break their dormancy and begin to sprout (Grand River Conservation Authority 2010).

Roots: Giant hogweed puts on a taproot that can go down in the soil profile more than a meter.

Life cycle: Seeds may take several years to germinate and are viable in the soil for up to 15 years. Once germination takes place the plant forms a large rosette of leaves up to a meter in height. At the end of the season the hogweed plant dies to the ground and returns from the root the following (early) spring. Over the course of the next 2-5 years the plant continues to enlarge and put on a large tap root and form thick, hollow stems. Once it has established a strong foundation it sends up large flower stocks and blooms mid June. It flowers for about 1 month then forms about 50,000 winged seeds. Once the plant has successfully produced seed it dies.

Look Alike Plants

  • Cow Parsnip (Heracleum maximum) NATIVE WEED
  • Wild Parsnip (Pastinaca sativa) NOXIOUS WEED
  • Poison Hemlock, (Conium maculatum) SECONDARY NOXIOUS WEED
  • Spotted Water Hemlock (Cicuta maculata) NOXIOUS WEED
  • Angelica Angelica atropurpurea NATIVE WEED
  • Queen-Anne’s Lace, Daucus carota NOXIOUS WEED…but not poisonous nor dangerous

How Does Giant Hogweed Spread?

Giant Hogweed primarily spreads by seed. Most of the seeds drop or are carried on the wind within 10 m of the mother plant. However hogweed plants growing near water can disperse their seeds much further. Seeds floating down stream can remain viable for up to 3 days before dying (MacDonald F. and Anderson H. 2012). Seeds can also be blown around on the snow and travel great distances.

Symptoms of Giant Hogweed Reaction

All parts of the giant hogweed plant contain a phototoxic sap. When an individual gets the sap on their skin it causes a reaction called phytophotodermatitis. This is not an allergic reaction as with poison ivy rather it is a chemical reaction that occurs when the organic chemicals called furanocoumarins, contained in the sap, become activated by ultraviolet radiation. The activation of this chemical damages your DNA and changes the way your skin protects itself from ultraviolet (UV) light (Goldman R. Updated on Dec. 2018). This results in severe burns when exposed to the sun. This chemical reaction can happen as quickly as 15 minutes after getting the sap on your skin.

The initial burns usually appear within 48 hours after contact with the sap. Just how severe these burns are depends on how long the sap remained on the skin and how long the skin was in the sun after exposure. In mild cases there may only be a bit of discolouration occur on the skin. In severe cases the individual may experience large painful blisters and 2nd to 3rd degree burns that leave behind scars and a long term sensitivity to the sun. Severe burns could require surgery to graft new skin over the damaged skin.(Goldman R. Updated on Dec. 2018).

Typical symptoms:

  • Redness and painful burn blisters that form within 48 hours.
  • The affected skin becomes dark and pigmented.
  • Scars that last for a few months to six years.
  • Long-term sensitivity to sunlight is common.
  • If the sap gets in your eyes it can cause temporary or permanent blindness.
  • Breathing in sap particles from the air can cause respiratory problems.

Other plants with similar phototoxic properties include Wild Parsnip (Pastinaca sativa) and Cow Parsnip (Heracleum maximum) but reactions to these 2 plants are much less severe.

Giant hogweed (Heracleum mantegazzianum) burn injury to an arm. Photo credit: USDA APHIS PPQ – Oxford, North Carolina , USDA APHIS PPQ, Bugwood.org

What To Do If You Come Into Contact With Giant Hogweed?

  1. Wash the affected area thoroughly with soap and COLD water as soon as possible.
  2. Keep exposed area away from sunlight for 48 hours.
  3. If a reaction occurs, seek medical attention.
  4. Topical steroids applied early can reduce the severity of the reaction and ease discomfort.
  5. If sap gets in your eyes, rinse them with cold water, wear sunglasses and seek medical help.
  6. If a reaction has occurred, the area of skin may be sensitive to sunlight for a few years. Keeping the area covered or wearing sunblock should help.
  7. Immediately launder any clothing that has come into contact with Giant Hogweed.
  8. Wash down the surfaces of anything that has come in contact with the plant using gloves and protective clothing.
  9. Report the sighting of the plant to Invading Species Hotline (1-800-563-7711) or online to www.invasivetrackingsystem.ca.

How To Get Rid of Giant Hogweed Plants?

If the giant hogweed plants are on public property report the sighting. If they are located on your private property it is advised to hire a professional to remove the plant(s), due to the danger of their removal.

If you decide to remove them yourself wear the proper safety equipment:

  • Wear full-length protective clothing and waterproof gloves taped at the wrists. The coarse hairs on the stems are able to penetrate thin fabric so be sure to wear long, canvas-style pants and long sleeves, or a rain suit or overalls. There are disposable “spray suit” coveralls that are available, they are ideal.
  • Closed-toed shoes and long socks (not ankle socks). Work boots preferably.
  • Protect face and eyes with a safety helmet with face shield.
  • Immediately launder clothing that has come into contact with Giant Hogweed.
  • Use soap and water to wash down equipment that has made contact with plant.

Physical Control:

Ideally dig up the young plants in spring before they get to large and while the digging is easier. Place all plant debris in sealed heavy black garbage bags and label them poisonous plant material.

Chemical Control

Glyphosate, a systemic herbicide, is most effective when applied in the spring to the smaller plants, followed up by a summer application for missed plants or those that may have re-grown. Mulching the area 10-14 days after application will help to manage seedling germination (MacDonald F. and Anderson H. 2012).

“Monitor the same location in following years (spring and summer). Seeds in the soil can be viable for 7 to 15 years, so continued vigilance in monitoring the area for newly germinating plants is important” (Nottawasaga Valley Conservation Authority n.d.).


Giant Hogweed, while quite spectacular in it’s own right, is a force to be reckoned with. The chemical compounds contained within the phototoxic sap, when activated by ultraviolet radiation, cause severe burning and painful blisters on any skin area that has come into contact with the sap. The shear size of the plant, it’s highly invasive nature and the danger involved in it’s removal has made this plant challenging to eradicate. Report any sightings to the Invading Species Hotline (1-800-563-7711) or online at www.invasivetrackingsystem.ca.

Poisonous to Touch Weeds Covered in This Series Include:

  1. Poison Ivy (Toxicodendron radicans and Toxicodendron Rydbergii)
  2. Giant Hogweed (Heracleum mantegazzianum)
  3. Cow Parsnip (Heracleum maximum)
  4. Wild Parsnip (Pastinaca sativa)
  5. Poison Sumac (Toxicodendron vernix)
  6. Poison Oak (Toxicodendron diversilobum) and (Toxicodendron pubescens)

Other Poisonous to Touch Plants Not Covered in This Series Include:

  1. Poke Weed (Phytolacca decandra): besides being mildly to highly toxic this plant can cause a rash in sensitive individuals.
  2. Stinging Nettle (Urtica dioica): the leaves and young stems of this herbaceous plant are fitted with stinging hairs tipped with formic acid and other irritants. If touched, these needle-like hairs inject the stinging acid into the skin, triggering a burning, tingling sensation and an itchy rash. Thankfully the symptoms usually do not persist longer than 24 hours.
  3. Monkshood (Aconitum napellus): This is a highly toxic plant. The neurotoxins, aconitine and mesaconitine can be absorbed through the skin and cause severe respiratory and cardiac problems. So do not pick or handle this plant without gloves, especially by the root.
  4. Oleander (Nerium oleander): This is a highly toxic plant. The gummy sap can cause skin irritation or possibly dermatitis in those who are sensitive. If you are growing this plant as an ornamental in your garden be sure to were gloves, long sleeves, long pants, closed shoes and protective eye wear before pruning this plant.
  5. Bloodroot (Sanguinaria canadensis): the flower of this plant produces a reddish toxic sap that looks very similar to blood and contains the toxin sanguinarine. If this sap comes in contact with bare skin it can cause burning. If you get a high concentration of sap on your skin it can even eat away the flesh. If ingested, it is deadly to animals.
  6. Wood Nettle (Laportea canadensis): like stinging nettle have stinging hairs along its stem.

References and Resources

Department of Environmental Conservation New York State n.d., Retrieved on Feb. 24, 2021 from: Health Hazards & Safety Instructions for Giant Hogweed (with graphic photos) – NYS Dept. of Environmental Conservation

Goldman R. Updated on Dec. 2018, Medically reviewed by Shilpa Amin, M.D., CAQ, FAAFP , Health Line, Retrieved on Feb. 24, 2021 from: Hogweed Burns: How to Avoid Giant Hogweed and Treat Burns (healthline.com)

Grand River Conservation Authority 2010, Giant Hogweed (Heracleum mantegazzianum) Fact Sheet, Retrieved on Feb. 24, 2021 from: Microsoft Word – Giant Hogweed Fact Sheet _2_.doc (brantford.ca)

MacDonald F. and Anderson H. 2012, Giant Hogweed (Heracleum mantegazzianum): Best
Management Practices in Ontario. Ontario Invasive Plant Council, Peterborough, ON., Retrieved on Feb 24, 2021 from: Giant-Hogweed-OFAH.pdf (invasivespeciescentre.ca)

Ministry of Natural Resources and Forestry 2018, Updated: Dec. 2019, Retrieved on Feb. 24, 2021, Giant hogweed | Ontario.ca

Nottawasaga Valley Conservation Authority n.d., Invasive Species Fact Sheet – Giant Hogweed, Retrieved on Feb. 24, 2021 from: Giant Hogweed Fact Sheet.pdf (nvca.on.ca)

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