Heracleum mantegazzianum_Fritz Geller-Grimm Wikimedia
Photo by Fritz Geller-Grimm

Heracleum mantegazzianum Risk Assessment

Common names: giant hogweed

Heracleum mantegazzianum -- California

Primary tabs

Evaluation Summary
Summary: 
General Evaluation Information
Date of Evaluation: 
June 30, 2016
Evaluation Time (hrs): 
Not Recorded
Evaluation Status: 
Completed
Plant Information
Plant Material: 
If the plant is a cultivar, and if the cultivar's behavior differs from its parent's (behavior), explain how: 
Regional Information
Region Name: 
Climate Matching Map
These maps were built using a toolkit created in collaboration between GreenInfo Network, PlantRight, Cal-IPC, and Agricultural Sustainability Institute at UC Davis.
Climate Matching Maps PDF: 
Invasive History and Climate Matching
1. Has the species (or cultivar or variety, if applicable; applies to subsequent "species" questions) become naturalized where it is not native?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
Naturalized in Canada, Europe, and the United States. USDA PLANTS shows it in 12 US states and 4 Canadian provinces. Cal-IPC received a note for our watchlist from the Santa Clara Weed Management Area about this species but Calflora does not have a record of it in California and it's not in the Jepson Manual. The first record of naturalization was in 1828 in England and it has spread in Europe from garden plantings.
Reference(s): 
2. Is the species (or cultivar or variety) noted as being naturalized in the US or world in a similar climate?
Yes or No: 
Yes
Points: 
2
Confidence Level: 
Very High
Answer / Justification: 
USDA PLANTS shows it in Oregon, Washington, and British Columbia, which are similar to California. As of 2005, it was considered an "emerging invader" in Mediterranean France.
3. Is the species (or cultivar or variety) noted as being invasive in the U.S. or world?
Yes or No: 
Yes
Points: 
2
Confidence Level: 
Very High
Answer / Justification: 
An "emerging invader" in Mediterranean France (Brunel and Tison). One of the most problematic invasive species in Europe because it establishes monocultures along streambanks, has toxic sap, and creates stream erosion problems. It spreads along creekbanks as well as disturbed areas such as roads and railroad tracks. Based on GBIF maps, it seems pretty widespread in France.
Reference(s): 
4. Is the species (or cultivar or variety) noted as being invasive in the US or world in a similar climate?
Yes or No: 
Yes
Points: 
3
Confidence Level: 
High
Answer / Justification: 
Becoming invasive in Mediterranean France. Very invasive in central and northern Europe but those areas are not similar to California. It is also listed on the Washington State Noxious Weed List.
5. Are other species of the same genus (or closely related genera) invasive in a similar climate?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
Two other hogweeds, H. sosnowskyi and H. persicum, are also considered invasive along rivers in northern and central Europe, although they are not as widespread as H. mantegazzianum. Their distribution is more northern than the part of Europe that is similar to California, however (Cal-IPC).
6. Is the species (or cultivar or variety) found predominately in a climate matching the region of concern?
Yes or No: 
No
Points: 
0
Confidence Level: 
Very High
Answer / Justification: 
The native range is Georgia (in Europe) and part of Russia. The naturalized range includes much of central Europe, the northeastern and northwestern US and Canada, and parts of Australia and New Zealand. Given the wide range, it does not grow predominantly in areas like California.
Impact on Native Plants and Animals
7. Does this plant displace native plants and dominate (overtop or smother) the plant community in areas where it has established?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
One of the most problematic invasive species in Europe because it establishes monocultures along streambanks, has toxic sap, and creates stream erosion problems. It reduces the diversity of native flora where it establishes. Many references cited within Global Invasive Species Database page related to this impact.
Reference(s): 
8. Is the plant noted as promoting fire and/or changing fire regimes?
Yes or No: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
No mention of this.
Reference(s): 
9. Is the plant a health risk to humans or animals/fish? Has the species been noted as impacting grazing systems?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
This species has toxic sap that make the skin over-sensitive to sunlight, resulting in blisters. This reaction can occur after only 15 minutes of exposure to the sap but the exposed person does not feel anything until the reaction begins a day or two later. The resulting inflammation causes hyperpigmentation that can last as long as six years. The toxicity was only described in relation to humans; nothing on toxicity to animals. Hogweed lowers the value of recreational land because of its dangerous properties.
Reference(s): 
10. Does the plant produce impenetrable thickets, blocking or slowing movement of animals, livestock, or humans?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
It produces dense stands, especially in riparian areas, and is a tall herb growing up to 4m, so could create thickets that would be at least difficult to walk through.
Reference(s): 
Reproductive Strategies
11. Does this species (or cultivar or variety) reproduce and spread vegetatively?
Yes or No: 
No
Points: 
0
Confidence Level: 
Very High
Answer / Justification: 
Reproduction is only by seed.
Reference(s): 
12. If naturally detached fragments from this plant are capable of producing new plants, is this a common method of reproduction for the plant?
Yes or No: 
No
Points: 
0
Confidence Level: 
Very High
Answer / Justification: 
Reproduction is only by seed.
Reference(s): 
13. Does the species (or cultivar or variety) commonly produce viable seed?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
Produces seeds that germinate in early spring.
Reference(s): 
14. Does this plant produce copious viable seeds each year (> 1000)?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
One plant can produce 5000 to 100,000 which remain viable for up to seven years.
Reference(s): 
15. Is there significant germination (>25%) of seeds the next growing season, with no requirement of an infrequent environmental condition for seeds to germinate (i.e. fire) or long dormancy period?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
Hogweed requires cold winter dormancy for seed germination but presumably that means seeds are germinating in the next growing season. It produces many seeds and a dense carpet of seedlings.
Reference(s): 
16. Does this plant produce viable seed within the first three years (for an herbaceous species) to five years (for a woody species) after germination?
Yes or No: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
Typically hogweed grows in a rosette stage for three to four years, then flowers once before dying.
Reference(s): 
17. Does this plant continuously produce seed for >3 months each year or does seed production occur more than once a year?
Yes or No: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
In central Europe, plants flower from June to mid-July and seeds are released from late August to October.
Reference(s): 
Dispersal
18. Are the plant’s propagules frequently dispersed long distance (>100 m) by mammals or birds or via domestic animals?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Between 60-90% of seeds fall within 4m of the parent plant. Seeds have a resinous coating and can stick to sheep or cattle.
Reference(s): 
19. Are the plant’s propagules frequently dispersed long distance (>100 m) by wind or water?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Hogweed often grows in riparian areas and seeds can be carried by water, especially during floods. Wind causes dispersal at shorter distances, especially during the winter when seeds are blown across frozen ground.
Reference(s): 
20. Are the plant’s propagules frequently dispersed via contaminated seed (agriculture or wildflower packets), equipment, vehicles, boats or clothing/shoes?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Seeds have a resinous coating and can stick to tires or clothing.
Reference(s): 
Evaluation Notes

Comment by Ron Vanderhoff: My only comment is that the suitable range in California for this species appears to be somewhat restricted by to its mesic requirements and its need for some level of dormancy or winter vernalization. If the species were accepted it would seem to be a strongly regional invasive. This does not exclude it from consideration, but does limit its potential CA impact.

USDA GRIN: https://npgsweb.ars-grin.gov/gringlobal/taxonomydetail.aspx?id=18896

USDA PLANTS: http://plants.usda.gov/core/profile?symbol=HEMA17

GBIF: http://www.gbif.org/species/3034825

Reviewed by Ron Vanderhoff and Lynn Sweet.

Global Invasive Species Database: http://www.iucngisd.org/gisd/species.php?sc=418

Total PRE Score

  • < 13 : accept (low risk of invasiveness)
  • 13 - 15 : evaluate further
  • > 15 : reject (high risk of invasiveness)

PRE Score: 
17
Number of questions answered: 
20
Screener Confidence (%): 
89.0
PRE Content Access and Privacy
Evaluation visibility: 
Public - accessible to all site users

Review this Evaluation

1692

Click on the button below to mark this evaluation as "Reviewed". Once you click the button, please wait a second, and the site will return to this Evaluation and your name will be on the "Reviewers List" in the right hand column (below the Evaluation Summary). For more information, please see the help page on How to Review an Evaluation?

Vertical Tabs