Onopordum illyricum_C056-05
Photo courtesy UC Davis Weeds of California

Onopordum illyricum Risk Assessment

Common names: Illyerian thistle

Onopordum illyricum -- California

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Evaluation Summary
Summary: 
General Evaluation Information
Date of Evaluation: 
August 8, 2016
Evaluation Time (hrs): 
3 Hours
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: 
High
Answer / Justification: 
Naturalized in California. Present in 2 counties according to Calflora. Also naturalized in Australia.
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: 
High
Answer / Justification: 
Naturalized in California. Present in 2 counties according to Calflora. Also naturalized in Australia in areas which match California's climate.
Reference(s): 
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: 
High
Answer / Justification: 
Considered invasive in Australia, details included in an Invasiveness Assessment and Impact Assessment prepared by the Victoria State Government.
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: 
Considered invasive in Australia, details included in an Invasiveness Assessment and Impact Assessment prepared by the Victoria State Government. Is present in New South Wales in areas with climates which match California.
Reference(s): 
5. Are other species of the same genus (or closely related genera) invasive in a similar climate?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Onopordum acanthium is also considered invasive in Australia in areas which match California's climate.
6. Is the species (or cultivar or variety) found predominately in a climate matching the region of concern?
Yes or No: 
Yes
Points: 
2
Confidence Level: 
High
Answer / Justification: 
O. illyricum points in GBIF for its native distribution in southern Europe as well as some of the points for Australia match California's climate.
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: 
Medium
Answer / Justification: 
“Even when plants die, they usually remain standing for a long period of time and, by keeping the area bare of other vegetation, can maintain a niche for recruitment of the next generation.”
Reference(s): 
8. Is the plant noted as promoting fire and/or changing fire regimes?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Medium
Answer / Justification: 
Stems can persist into the next season with spiny phyllaries and receptacles attached. In dense patches, this dry matter may introduce a minor change to the frequency of fire risk.
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: 
Medium
Answer / Justification: 
O. illyricum impacts grazing systems because it is spiny. Illyrian thistle tends not to be grazed by stock because of its dense spines. However, if the weed is grazed, the spines can cause damage, particularly around the mouths and eyes of animals.
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: 
“Dense patches pose a physical barrier to humans.” The spiny nature of the plant is an annoyance to humans.
Reference(s): 
Reproductive Strategies
11. Does this species (or cultivar or variety) reproduce and spread vegetatively?
Yes or No: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
Illyrian thistle's only method of dispersal is by seed which is equipped with a pappus of fine hairs and is spread by both wind and water.
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: 
High
Answer / Justification: 
Illyrian thistle's only method of dispersal is by seed which is equipped with a pappus of fine hairs and is spread by both wind and water.
Reference(s): 
13. Does the species (or cultivar or variety) commonly produce viable seed?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Illyrian thistle's only method of dispersal is by seed which is equipped with a pappus of fine hairs and is spread by both wind and water.
Reference(s): 
14. Does this plant produce copious viable seeds each year (> 1000)?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Medium
Answer / Justification: 
“A heavy seeder” 5 flower heads per plant x 200 seeds per flower = 1,000 seeds per plant (Estimate using pic. & illustrations)
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: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
Illyrian thistle seeds have an initial short-term innate dormancy, following which the majority of seeds acquire induced dormancy, rendering them incapable of immediate germination.
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: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Some O. illyricum plants flower in the first year, others in the second year.
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: 
Medium
Answer / Justification: 
In Austraila, Illyrian thistle flowers for three months and produces seed for 3 months.
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: 
The pappus of the seed attaches to wool, vehicles, clothing, etc. or it can be a contaminant in hay.
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: 
Illyrian thistle's only method of dispersal is by seed which is equipped with a pappus of fine hairs and is spread by both wind and water.
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: 
The pappus of the seed attaches to wool, vehicles, clothing, etc or it can be a contaminant in hay.
Reference(s): 
Evaluation Notes
Total PRE Score

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

PRE Score: 
21
Number of questions answered: 
20
Screener Confidence (%): 
75.0
PRE Content Access and Privacy
Evaluation visibility: 
Public - accessible to all site users

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