Centauria diluta_C031-31
Photo courtesy UC Davis Weeds of California

Centaurea diluta Risk Assessment

Common names: spotted knapweed

Centaurea diluta -- California

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Evaluation Summary
Summary: 
General Evaluation Information
Date of Evaluation: 
April 11, 2016
Evaluation Time (hrs): 
2 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: 
Is naturalized or occurs as a casual in some countries of Europe (Great Britain, Germany and Switzerland) (Centaurea diluta Aiton, new to the Italian flora). Naturalized in Maltese Islands (Introduced species in the Maltese Islands).
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: 
Medium
Answer / Justification: 
Is naturalized or occurs as a casual in some countries of Europe (Great Britain, Germany and Switzerland) (Centaurea diluta Aiton, new to the Italian flora). These countries do not overlap by more than 50% with California's climate zones (Cal-IPC global map of climate areas matching California). Per Reviewer Lynn Sweet: The problem with evaluating this species is that it is listed as "noxious" on the FAO list for "agricultural areas" in Spain, where it is "native." The only areas that I can figure out that it is possibly documented to be non-native and invasive are a) Hungary http://www.cabi.org/isc/abstract/19922318884 and Malta (cited source, Schembri and Lanfranco). ITIS mostly shows this to be a casual alien in many places, with vague "weed" status in the US http://www.hear.org/gcw/species/centaurea_diluta/ . Otherwise it seems to be just in disturbed areas or in agricultural areas (which are important for this question, but whether this is happening in its non-native range seems to be a question) or very rare casual in Great Britain https://www.brc.ac.uk/plantatlas/index.php?q=plant/centaurea-diluta. Screener response: Q2 is related to naturalization; Q4 is related to invasiveness (causing economic damage, etc.). Naturalization does not confer automatic invasiveness. That said, something causing major economic damage as an agricultural weed, as this is, would be considered invasive, regardless of whether or not naturalization has occurred. That said, Q2 should still be a "Yes", because new protocols regarding climate matching indicate that ANY match in climate with California makes this a yes; as Switzerland and Germany overlap with California's climate zones, this becomes a "yes". More information from Lynn Sweet: The text is misleading, sounding as if this species has naturalized primarily in temperate climates. The species is noted as native to Southwestern Europe and North Africa. It has become naturalized, according to the source listed, as in Great Britain, and "central Europe" (the cities listed in the one source are in Germany and Switzerland) as well as in Italy, Malta and Hungary. Looking at the "Cal-IPC global map of climate areas matching California" this distribution differs slightly from areas matching CA, with some of these naturalized occurences seeming to be in more temperate areas. On this, I suspect a) propagule pressure since it is a noted bird seed contaminant https://www.brc.ac.uk/plantatlas/index.php?q=plant/centaurea-diluta, b) microclimates such as southern-facing slopes or exposure-buffered areas or c) misidentifications with the similar C. maculosa or C. diffusa, which is noted in Flora of Missouri entry: http://www.tropicos.org/name/2702146?projectid=23). On this issue, while other sources agree on taxonomy it seems, GBIF has a really confusing taxonomy for this species, so it's hard to know which observations to use from that source. So I'd suggest add Italy and Malta to the text. I just think there is some ambiguity as to whether it is extensively naturalized in these temperate areas at all, and it does seem to be naturalized in areas that are similar to California, including 53 specimen records from coastal California. Screener response: this remains as a "Yes".
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: 
Noxious weed in Spain (FAO).
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: 
Noxious weed in Spain (FAO). This matches California's climate (Cal-IPC). Per reviewer Lynn Sweet: The problem with evaluating this species is that it is listed as "noxious" on the FAO list for "agricultural areas" in Spain, where it is "native." The only areas that I can figure out that it is possibly documented to be non-native and invasive are a) Hungary http://www.cabi.org/isc/abstract/19922318884 and Malta (cited source, Schembri and Lanfranco). ITIS mostly shows this to be a casual alien in many places, with vague "weed" status in the US http://www.hear.org/gcw/species/centaurea_diluta/ . Otherwise it seems to be just in disturbed areas or in agricultural areas (which are important for this question, but whether this is happening in its non-native range seems to be a question) or very rare casual in Great Britain https://www.brc.ac.uk/plantatlas/index.php?q=plant/centaurea-diluta. Screener response: something causing major economic damage as an agricultural weed, as this is, would be considered invasive, regardless of whether or not naturalization has occurred. Moreover, this plant is considered invasive in areas that also overlap with California (Hungary, Malta, as indicated by the reviewer), and this should remain a "yes".
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: 
Many species of Centaurea are highly invasive in California and areas throughout the world with similar climates (UC IPM; Cal-IPC).
Reference(s): 
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: 
Found primarily in Europe (Spain), northern Africa (iNaturalist; CABI). This matches much of California's climates (Cal-IPC global map of climate areas matching California).
Reference(s): 
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: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
An exhaustive Google/Google Scholar search did not reveal any evidence of displacement of native plants, and this defaults to a "no" answer.
Reference(s): 
8. Is the plant noted as promoting fire and/or changing fire regimes?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
An exhaustive Google/Google Scholar search did not reveal any evidence of fire promotion or changes to fire regimes, and this defaults to a "no" answer.
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: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
An exhaustive Google/Google Scholar search did not reveal any evidence of health risks or impacts to grazing systems, and this defaults to a "no" answer.
Reference(s): 
10. Does the plant produce impenetrable thickets, blocking or slowing movement of animals, livestock, or humans?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
An exhaustive Google/Google Scholar search did not reveal any evidence of thicker formation or blockage of movement, and this defaults to a "no" answer.
Reference(s): 
Reproductive Strategies
11. Does this species (or cultivar or variety) reproduce and spread vegetatively?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
An exhaustive Google/Google Scholar search did not reveal any information about asexual reproduction, and this defaults to a "no" answer.
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: 
Medium
Answer / Justification: 
An exhaustive Google/Google Scholar search did not reveal any information about asexual reproduction or production of new plants from fragments, and is this is a terrestrial species that is unlikely to fragment naturally, this defaults to a "no" answer.
Reference(s): 
13. Does the species (or cultivar or variety) commonly produce viable seed?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Medium
Answer / Justification: 
In a stand of canary millet (Phalaris canariensis) in Egerszalok in 1989 a Centaurea weed appeared in high individual numbers that had been unknown until then in the Hungarian flora. Its presence was accompanied by a lower number of other unknown or rare weeds like Ononis alopecuroides L., Bupleurum lancifolium Hornem., Scorpiurus muricatus L. ssp. subvillosus (L.) Thell., Chrysanthemum segetum L, Malope trifida Cav., Trifolium alexandrinum L., Trigonella foenum graecum L., Helminthia echiodes (L.) Gaertn., Sherardia arvensis L., Lathyrus aphace L. These plant species occur primarily in the Mediterranean area. According to glasshouse tests the seeds of Centaurea diluta and Scorpiurus muricatus germinated readily. The population of Centaurea diluta increased in 1990 in winter wheat plots, as compared to the previous year. Many centaurea plant grew in 1991 in sunflower stands as well. Centaurea diluta germinated throughout the whole summer and its frequency gradually decreased in wheat stands in 1992-93. It occurred again in higher numbers on squash and maize plots, but its numbers did not attain again the ones of the first three years. On fallow lands only some individuals were found in places not colonized by other weeds ( The appearance of Centaurea diluta ait. in department Heves (Hungary)).
Reference(s): 
14. Does this plant produce copious viable seeds each year (> 1000)?
Yes or No: 
Points: 
Confidence Level: 
Low
Answer / Justification: 
An exhaustive Google/Google Scholar search did not reveal any information about amount of seed production, and this remains unanswered at this time.
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: 
Medium
Answer / Justification: 
In a stand of canary millet (Phalaris canariensis) in Egerszalok in 1989 a Centaurea weed appeared in high individual numbers that had been unknown until then in the Hungarian flora. Its presence was accompanied by a lower number of other unknown or rare weeds like Ononis alopecuroides L., Bupleurum lancifolium Hornem., Scorpiurus muricatus L. ssp. subvillosus (L.) Thell., Chrysanthemum segetum L, Malope trifida Cav., Trifolium alexandrinum L., Trigonella foenum graecum L., Helminthia echiodes (L.) Gaertn., Sherardia arvensis L., Lathyrus aphace L. These plant species occur primarily in the Mediterranean area. According to glasshouse tests the seeds of Centaurea diluta and Scorpiurus muricatus germinated readily. The population of Centaurea diluta increased in 1990 in winter wheat plots, as compared to the previous year. Many centaurea plant grew in 1991 in sunflower stands as well. Centaurea diluta germinated throughout the whole summer and its frequency gradually decreased in wheat stands in 1992-93. It occurred again in higher numbers on squash and maize plots, but its numbers did not attain again the ones of the first three years. On fallow lands only some individuals were found in places not colonized by other weeds ( The appearance of Centaurea diluta ait. in department Heves (Hungary)).
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: 
Medium
Answer / Justification: 
Acts as an annual (CalFlora; Jepson).
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: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Flowers April - December in California (CalFlora; Jepson). Per reviewer Lynn Sweet: I am skeptical of Calflora's and Jepson's Apr-Dec bloom period. I am not sure where that came from. The CCH specimens are concentrated in June, implying flowering period, with one collection in December. No telling if that plant was flowering. Flora N Amer lists flowering period Apr-June, so 3 months. Sounds like with this information, answer could still be yes but I don't see any evidence that this sp has an extended period of setting seed compared with other plants, but I haven't done many of these evals yet. http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=242416252. Screener response: I cannot guarantee the validity of Jepson or Calflora's bloom period, but they are specific to California, and as such, I rely on them more on Flora of N. America, which gives a general flowering period across a vast geographic area, which may or may not apply to specific regions, like California. As such, this remains a "yes" until further evidence is provided to the contrary.
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: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
An exhaustive Google/Google Scholar search did not reveal any information about seed dispersal by animals, and this defaults to a "no" answer.
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: 
Medium
Answer / Justification: 
Seeds have a hairy pappus for wind dispersal (Jepson; Calflora).
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: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
Stace (1991) states that it was introduced into Great Britain as seeds inside bird food (Centaurea diluta Aiton, new to the Italian flora; Introduced species in the Maltese Islands). However, no other information was found about regular dispersal via this mechanism in an exhaustive Google/Google Scholar search, and as there are no mechanisms for attachment, this defaults to a "no" answer.
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: 
16
Number of questions answered: 
19
Screener Confidence (%): 
66.3
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Evaluation visibility: 
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