RMD-13-04: Consolidated Pest Risk Management Document for pest plants regulated by Canada
Appendix 4A: Pest Risk Assessment Summary for Centaurea solstitialis (yellow star-thistle)

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Identity of Organism

Name: Centaurea solstitialis (L.) (Asteraceae) (USDA-ARS, 2009)

Synonyms: Leucantha solstitialis (L.) A. Löve & D. Löve (USDA-NRCS, 2009)

English Common Names: Yellow star-thistle (USDA-NRCS, 2009), Centaurea solstitialis, golden star-thistle, St. Barnaby's thistle, yellow centaury, yellow cockspur (USDA-ARS, 2009)

French Common Names: Centaurée du solstice, chardon-doré, auriole

Description: Centaurea solstitialis is a winter annual, herbaceous plant, rarely a biennial or short-lived perennial. Populations in the United States are variable in their plant characteristics. The stems are stiff and erect, 15-200 cm in height. Centaurea solstitialis produces rosette leaves that lie close to the ground in the first fall. The inflorescences are borne in solitary flowerheads on stem tips, although vigorous plants may produce flowerheads in branch axils. The involucre is about 1.2-1.8 cm long. The involucral bracts have one long central spine 1.0-2.5 cm long and two or more pairs of short lateral spines and are covered with hairs. The achenes (fruits) are of two types, both glabrous and about 2 to 3 mm long. The disc florets produce most of the achenes (75-90%). These disc achenes are shiny and have a short (2-5 mm), stiff pappus. Achenes produced by the outer (ray) florets are duller, darker in color and have no pappus. Centaurea solstitialis has a large taproot that grows to soil depths of 1 m or more, allowing access to deep soil moisture during dry summer and fall months. The dead stalks are persistent and usually remain standing through the winter (Zouhar 2002).

Organism Status

It is known historically from a few locations in Canada, but those populations have not established. No evidence was found that this species is cultivated in Canada (CNLA 2009). Based on this information, it is considered absent from Canada.

Current Regulatory Status

Federally, it is regulated by the CFIA as Class 1 prohibited noxious weed in the Weed Seeds Order under the Seeds Act. Provincially, it is regulated as a "Noxious Weed" in British Columbia, as a "Prohibited Noxious Weed" in Alberta, and as a "Prohibited Weed" in Saskatchewan under their respective Weed Control Acts and Regulations.

It is not regulated as a federal noxious weed in the U.S. but is regulated in the following states: AZ, CA, CO, HI, ID, MO, MT, ND, NM, NV, OR, SD, UT, WA, WV and WY.

Probability of Entry

It is an annual that spreads only by seed. Seeds have been found as contaminants in alfalfa and grain crops, and contaminated seeds are thought to have been the pathway for introduction of the species outside the native range (CAB International 2007).

Table 1: Summary of pathways for Centaurea solstitialis (yellow star-thistle)
Type of pathway Specific pathways
Natural dispersalSeeds can disperse 1.5 m by wind and are dispersed over longer distances by animal activity. (CAB International 2007). The pappus bristles are covered with stiff, microscopic barbs that readily adhere to hair (Zouhar 2002).
Intentional introductionNone identified.
Unintentional introductionLong-distance dispersal of seed is often directly related to human activities and occurs by movement of livestock, vehicles and equipment (Zouhar 2002). This is a likely pathway for entry into Canada. Seeds have been found as contaminants in alfalfa and grain crops, and contaminated seeds are thought to have been the pathway for introduction of the species outside the native range. There have been multiple introductions of Centaurea solstitialis into California, from Europe and Chile, usually as a contaminant in alfalfa seed (CAB International 2007). This would be the most likely pathway for entry into Canada.

Probability of Establishment

It is native to Eurasia, including Armenia, Azerbaijan, Georgia, Iran, Iraq, Lebanon, Syria, Tajikistan, Turkey, Turkmenistan, Ukraine, Algeria, Tunisia, Albania, Bulgaria, France, Greece, Italy, Spain and former Yugoslavia (USDA-ARS 2009). It is now found throughout Europe, in North and South America and Africa. It has spread to Central Asia, but does not persist in cold northerly areas. The earliest specimens in the U.S. were found in California in 1869 (CAB International 2007).

It has been reported from at least 40 U.S. states and four Canadian provinces. However, in eastern North America, infestations are sporadic and localized, and populations have failed to become established, in spite of repeated introductions. In California over 6.9 million hectares are infested, and the species is spreading in Idaho, Oregon and Washington (CAB International 2007).

In spite of historical reports of Centaurea solstitialis from Alberta, Saskatchewan, Manitoba and Ontario, there are no established populations anywhere in Canada.

Based on the results of the North Carolina State APHIS Plant Pest Forecasting (NAPPFAST) System, it appears that Centaurea solstitialis would survive to global plant hardiness zone 5 (Figure 11). Data from the pacific northwest states indicates that the species is common in warmer micro-climates on south and southwest facing slopes (Shafi et al. 2003; Zouhar 2002), therefore it is likely that it will be these micro-climates that will be vulnerable within zone 5 in Canada.

Figure 1: The potential North American range forCentaurea solstitialis (yellow star-thistle) as predicted by the NAPPFAST system (Red shading corresponds to areas at risk or global plant hardiness zones 5-9)

Figure 1. Description follows.
Description of Figure 1:

This image is a map of Canada and the Central United States with areas colored in red. The read areas are showing the establishment of the Yellow Star-thistle. Thy include the Canadian West Coat and some of the Mid-west States as well as the States south of the Great Lakes and the Canadian and U.S. East coast.

Probability of Spread

Seeds can disperse 1.5 m by wind, and seeds are dispersed over longer distances by animal or human activity. In California, it is thought that human activity accounts for most of the seed dispersal (CAB International 2007). The pappus bristles are covered with stiff, microscopic barbs that readily adhere to clothing and hair. Long-distance dispersal is often directly related to human activities and occurs by movement of livestock, vehicles, equipment, and contaminated hay and crop seed (Zouhar 2002).

Potential Economic Consequences

Centaurea solstitialis is toxic in large amounts to horses. The muscles of the lips, face and tongue become stiff and swollen, giving the horse a fixed expression. Poisoning can result in permanent brain damage, and severely affected animals eventually die of thirst and starvation (CAB International 2007).

Losses result from interference with livestock grazing and forage harvesting procedures, and lower yield and forage quality of rangelands. Livestock and wildlife avoid grazing in heavily infested areas resulting in slower weight gain and reduced quality of meat, milk, wool and hides (CAB International 2007).

Centaurea solstitialis is the most important roadside weed problem in much of central and northern California and causes occasional problems in cereals, orchards, vineyards, cultivated crops and wastelands (CAB International 2007).

Potential Environmental and Social Consequences

Centaurea solstitialis can reduce wildlife habitat and forage, displace native plants, and decrease native plant and animal diversity. Dense infestations threaten natural ecosystems by fragmenting plant and animal habitats (CAB International 2007). Infestations can also limit access to recreational areas, and reduce land value. Several rare and sensitive plant species in Oregon, Nevada and Idaho are thought to be threatened by C.solstitialis. Large populations of Centaurea solstitialis can alter the water cycle in annual grassland and foothill woodland ecosystems in California, by using more water to a greater soil depth than native vegetation (Zouhar 2002).


There is some doubt about its potential range in Canada. It is very likely that the potential range is less than that shown in Figure 11, based on NAPPFAST zone 5. However, it is also very likely that there are suitable environments for persistent populations in southern British Columbia.


Based on the outcome of the CFIA's pest risk assessment, Centaurea solstitialis is likely to establish and become invasive in parts of Canada, including southern British Columbia if introduced. This plant should be considered for regulation under the Plant Protection Act (note that it is regulated by the CFIA as a prohibited noxious weed seed under the Seeds Act).

Technical Issues for Consideration

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