RMD-13-04: Consolidated Pest Risk Management Document for pest plants regulated by Canada
Appendix 3A: Pest Risk Assessment Summary for Centaurea iberica (Iberian starthistle)

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Identity of the organism

Name: Centaurea iberica Trevir. ex Spreng (family Asteraceae) (USDA-ARS 2009)

Synonyms: Calcitrapa iberica (Trevir. ex Spreng.) Schur, Leucantha iberica (Trevir. ex Spreng.) Á. Löve & D. Löve (Tropicos 2008)

English common names: Iberian star-thistle, Iberian star thistle, pale starthistle, Spanish centaury-thistle, Iberian knapweed, Spanish thistle.

French common name: None found.

Description: Centaurea iberica is a biennial herbaceous plant, but may behave as an annual or short-lived perennial in some environments (Graham and Johnson, 2003). In Oregon, a rosette forms in May and June and the plants bolt and bloom from midsummer to fall. The plants grow from 30 to 200 cm tall. The leaves are divided into narrow linear segments. The rosettes have spines in the centre. The flower heads are white, pink, or pale purple, and 15–20 mm long with straw-colored, spine-tipped bracts. The spines are more than 2.5 cm long. The achenes (cypselae) are white- or brown-streaked, 3–4 mm long, and have a white pappus which is 1–2.5 mm long (FNA Editorial Committee 1993+, ODA 2007).

Organism Status

Centaurea iberica is not reported to be naturalized in Canada, and no evidence was found that it is cultivated in Canada (CFIA 2008). Based on this information, the species is considered to be absent from the PRA area.

Current Regulatory Status

Centaurea iberica is not regulated in Canada.
United States:
Centaurea iberica is not regulated as a Federal Noxious Weed in the U.S. but it is regulated in Arizona, California, Nevada and Oregon (USDA-NRCS 2009). No person may move Centaurea iberica or commodities found to contain this species or parts of it into or through these states (ADA 2006; CDFA 2003; State of Nevada 2003; ODA 2006).

Probability of Entry

No information could be found on the probable pathway for entry of Centaurea iberica into North America (Table 8).

Table 8: Summary of Pathways for Centaurea iberica (Iberian star-thistle)
Type of pathway Specific pathways
Natural dispersal
  • Centaurea iberica is a biennial that spreads only by seed.
  • Natural seed dispersal is an unlikely pathway for entry into Canada, given the current range.
Intentional introduction
  • None identified.
Unintentional introduction
  • Centaurea iberica could occur as a contaminant in seed lots. Some Centaurea species are found as seed contaminants in Turkey (Uygur 2001), so seed imported from the native range is one possible source. This is known to be the pathway for other Centaurea species, such as yellow star-thistle (Centaurea solstitialis) (Zouhar 2002). There are no records of Centaurea iberica being found in imported seed lots in Canada in the past 10 years, based on CFIA Seed Laboratory data, but two samples of imported clover seed from the U.S. were contaminated with "starthistle" seeds that were not identified to species.
  • It is possible that livestock being imported from the U.S. could transport the spiny flower heads in their hair or wool (Graham and Johnson 2003).
  • Centaurea iberica could potentially be transported in hay, but, as a biennial plant, it is not reported from cultivated fields so this pathway is unlikely.

Probability of Establishment

In Europe, Centaurea iberica is native in Bulgaria, Greece, former Yugoslavia, Romania and Ukraine (Crimea) (Pankhurst, 1998). It is also native in Asia (Afghanistan, Cyprus, Iran, Iraq, Israel, Jordan, Lebanon, Syria, Turkey, Armenia, Azerbaijan, Georgia, Russia (Ciscaucasia and Dagestan), southeastern Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, Uzbekistan, China (Xinjiang), northern India and Pakistan) (USDA-ARS 2009). Centaurea iberica has been introduced in North America in California, Kansas, Oregon, Washington and Wyoming (FNA Editorial Committee, 1993+) (see Figure 1). It spread abundantly in some counties of California during the early 1950's (Graham and Johnson 2003). Centaurea iberica is extremely competitive along roadsides and in range and pastures (ODA 2007).

Figure 1: Range of Centaurea iberica (Iberian star-thistle) in North America

Figure 1. description follows
Description for - Figure 1

This map shows the range of Centaurea iberica in North America indicating its presence through the use of the colour green. The five states covered in green are Washington, Oregon, California, Wyoming and Nebraska.

Figure 2: Source (NAPPFAST zones 6-9)

Figure 2, description follows
Description for - Figure 2

This image shows the potential range of Centaurea iberica in Canada and northern America through the use of a map. Red is used to indicate the regions in which Centaurea iberica could survive according to the Canadian Plant Hardiness Zones map, in this case NAPPFAST Hardiness Zone 6. In Canada, this includes coastal and southernmost BC, extreme southwestern Ontario and small areas on the coasts of the Maritime Provinces, including Newfoundland. In the U.S., it includes parts of the Eastern and Western coast as well as areas located near the Great Lakes.

Source: (USDA-NRCS 2009)

Probability of Spread

Centaurea iberica is a biennial plant that spreads by seed. The achenes have a pappus which suggests that they are dispersed by wind. The spiny involucres could be trapped in hair or clothing and be transported by animals or people.

Potential Economic Consequences

Centaurea iberica is considered a common weed in Lebanon (Holm et al., 1991). In Oregon, Centaurea iberica displaces valuable forage species in pastures and rangelands. The sharp spines deter grazing animals, restricting access for livestock and reducing the value of hay (ODA, 2007).

Potential Environmental and Social Consequences

Centaurea iberica seems to be confined to disturbed areas, including over-grazed rangelands (Uygur, 2001). The sharp spines mean that infestations can impede recreational use and restrict access for wildlife (ODA, 2007).

No other potential social consequences were identified.


There are a number of areas of uncertainty in this risk assessment. This species is not well-covered in weed books or websites in North America. Basic information on self-compatibility, potential for apomixis, seed productivity and seed bank persistence are lacking. However, better information in these poorly-documented characteristics would not change the outcome from "reject" as the species scored very high in the assessment.


Based on the outcome of this pest risk assessment, Centaurea iberica is likely to establish and become weedy or invasive in some parts of southern and coastal British Columbia, southwestern Ontario and the east coast of Canada if it is introduced to these areas.

Technical Issues for Consideration

The plants of Centaurea iberica are very similar to those of purple star-thistle (Centaurea calcitrapa) as both have purple flower heads. New infestations have been misidentified in the past due to observers being familiar with purple star-thistle but unaware of Centaurea iberica. The plants of Centaurea iberica are more robust on average (FNA Editorial Committee 1993+).

However, the plants are best identified on the basis of the more elongated the flower heads (more rounded in purple star-thistle) and the paler color of the corollas (Graham and Johnson 2003). The achenes can be easily separated by the presence of a pappus and the shinier pericarp in Centaurea iberica (ODA 2007).

In Canada, Centaurea calcitrapa L. (purple star-thistle) has been previously reported from southwestern BC and southern Ontario (Scoggan 1979). As it is not listed in the Illustrated Flora of British Columbia (Douglas et al. 1998), it seems that no persistent populations have become established there.

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