Giant Hogweed | Heracleum mantegazzianum

California Pest Rating for
Giant Hogweed | Heracleum mantegazzianum
Family : Apiaceae
Pest Rating : A  |  Proposed Seed Rating: P

Initiating Event:  

Giant hogweed is a federal listed noxious weed.

History & Status:

Background: Heracleum mantegazzianum, commonly called giant hogweed or cartwheel flower, is an herbaceous biennial or short-lived perennial that is noted for producing rapid and prodigious growth.Giant hogweed has over time escaped gardens and naturalized in roadside ditches, stream banks, fields, unused farmland,railroad track right-of-ways and along fences in a number of areas in North America, primarily the northeastern and northwestern parts of Canada. In the first year, this plant produces a large mound of coarse, compound, basal leaves (each to as much as 3′ long) with deeply cut leaflets. In the second year, the plant will rise up to 10-16′ tall topped by a gigantic, flat-topped, umbrella-like, compound umbel (2-4′ wide). Each umbel contains thousands of tiny white flowers. Plants bloom in late June. After flowering and fruiting, the plant dies. Each large flower umbel produce 20,000 seeds. Green stems are hollow and distinguished by white hairs and purple blotches. Giant hogweed is in the same genus as the cow parsnip (Heracleum maximum) which is native to North America3. It was  brought to the U.S. in 1917 to an ornamental garden in New York. Giant hogweed is a USDA federally listed noxious weed and it is especially invasive in riparian  and urban sites5&6.

Large plants can restrict the access to rivers, trails, and paths; studies indicate that giant hogweed can negatively impact soil dynamics, fisheries, and other species in its nonnative habitats. Contact with plant sap can burn, blister, and scar to exposed skin. In severe cases the irritation leads to blistering that result in painful dark purple scars4.

Official Control: Giant hog weed is under official control in Colombia, Korea and in the Republic of Mexico7.

California Distribution: Giant hogweed has not yet been detected in California.

California Interceptions: None.

United StatesGiant hogweed has been identified in 16 states since its inclusion on the Federal Noxious Weed list.These state Includes  New York, Connecticut, the District of Columbia, Illinois, Maine, Maryland, Massachusetts, Michigan, Ohio, Oregon, Pennsylvania, Washington and  Wisconsin4.

Worldwide Distribution: Giant hogweed is native to the Caucasus region of Eurasia and was introduced into Europe in the 1800s. It is reported in Austria, Czechoslovakia, Denmark, Finland, France, Germany, Georgia, Hungary, Ireland, Italy, the Netherlands, Norway, Sweden, Switzerland and in the United Kingdom. In  Asia it is found in Central  Russia and Iran. It is introduced into Australia and Canada as well1&3.

Consequences of Introduction: 

1) Climate/Host Interaction: Giant hogweed is well established in Oregon and Washington  and is spreading there. These States have  similar habitat to Northern   Therefore, there is a high risk that it will establish in California. Giant hogweed receives a High(3) in this category.

Evaluate if the pest would have suitable hosts and climate to establish in California.  Score(3)

-Low (1) not likely to establish in California; or likely to establish in very limited areas

-Medium (2) may be able to establish in a larger but limited part of California

High (3) likely to establish a widespread distribution in California

2) Pest Host Range: Giant hogweeds do not require any one  host, but grow wherever ecological conditions are favorable. It receives a High(3) in this category.

Evaluate the host range of the pest:  Score:(3)

-Low (1) has a very limited host range

-Medium (2) has a moderate host range

-High (3) has a wide host range

3) Pest Dispersal Potential: Giant hogweed can produce up to 20,000 seeds, allowing it to spread quickly and  form dense canopies, outcompeting native plants3.  It receives a High(3) in this category.

Evaluate the dispersal potential of the pest: Score(3)

-Low (1) does not have high reproductive or dispersal potential

-Medium (2) has either high reproductive or dispersal potential

-High (3) has both high reproduction and dispersal potential

4) Economic Impact: Giant hogweed can  regrow very rapidly and often invades fallow fields, especially in riparian zones. The plant leaves and stem exude a clear, watery sap that photosensitizes skin, causing a condition known as photodermatitis. Blisters and burns may result on subsequent exposure to the sun. It could effect the skins of small farm animals, especially dogs and Cats2. Giant hogweed receives a High(3) in this category.

Evaluate the economic impact of the pest to California using these criteria.

Economic Impact: A, B, D,F

A.  The pest could lower crop yield.

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural practices.

E. The pest can vector, or is vectored, by another pestiferous organism.

F. The organism is injurious or poisonous to agriculturally important animals.

G. The organism can interfere with the delivery or supply of water for agricultural uses.

Economic Impact:   Score (3)

-Low (1) causes 0 or 1 of these impacts

-Medium (2) causes 2 of these impacts

High (3) causes 3 or more of these impacts

5) Environmental Impact: Giant hogweed can become quite dense, owing to the plant’s prolific seed production and rapid growth rate. Such dense stands crowd out slower growing plants, the thick hogweed canopy displacing native state enlisted endangered  plant  such as Humboldt milk –vetch (Astragalus agnicidus) that need direct sunlight to grow. The decreased abundance of beneficial native plants can reduce the utility of the area for wildlife habitat. When riparian plants are displaced, stream bank erosion can increase and stream beds can be covered with silt. Giant hogweed receives a High(3) in this category.

Evaluate the environmental impact of the pest on California using the following criteria.

Environmental Impact: A,B,C,D.

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes

B. The pest could directly affect threatened or endangered species

C. The pest could impact threatened or endangered species by disrupting critical habitats

D. The pest could trigger additional official or private treatment programs

E. Significantly impacting cultural practices, home/urban gardening or ornamental plantings.

Environmental Impact:   Score(3)

-Low (1) causes none of the above to occur

-Medium (2) causes one of the above to occur

High (3) causes two or more of the above to occur

Consequences of Introduction to California for Giant hog weed: 

Rating (Score): Add up the total score and include it here:

-Low = 5-8 points

-Medium = 9-12 points

-High = 13-15 points

Total points based on above criteria, which does not take into account the pathogen’s already wide distribution in California: High (15).

6) Post Entry Distribution and Survey Information: Giant hogweed has never been found in California and receives a Not Established (0) in this category

Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included.

Not established (0) Pest never detected in California, or known only from incursions.

Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Final Score:

The final score is the consequences of introduction score minus the post entry distribution and survey information score: (15)


Although this plant is not yet established in California, its invasive behaviour in Washington and Oregon, areas similar to NW California, means that it is high likely to prove invasive in NW CA if it establishes there.

Conclusion and Rating Justification:

Conclusions of the harm associated with this pest to California using all of the evidence presented above: Proposed Rating: Based on the score listed above, Giant hogweed is a High risk. Given its ability to spread widely and displace native plants, an “A” rating is proposed.

  1. Giant Hogweed (Heracleum mantegazzianum). Written Findings of the State Noxious Weed Control Board – Class A Weed. State of Washington. 4 pp.
  1. Linda Cole; CANIDAE® Pet Foods,San Luis Obispo. Accessed  11-15-2016.
  1. Missouri Botanical Garden.  Accessed  11-15-2016
  1. New York Invasive Species Information, Giant Hogweed. Accessed:11-15-2016
  1. Oregon Department of Agriculture. 2008. Toxic plant alert!: Giant hogweed–Heracleum mantegazzianum. Salem OR: Oregon Department of Agriculture, Plant Division, Noxious Weed Control Program. Accessed: 11-15-2016
  1. Ohio State University Extension. Accessed: 11-15-2016
  1. USDA Phytosanitary Certificate Issuance & Tracking System (PCIT) Phytosanitary Export Database (PExD). Accessed: 11-15-2016
Responsible Party:

Javaid Iqbal,  California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 403-6695;[@]

Pest Rating : A  |  Proposed Seed Rating: P

Posted by ls