Centaurea diffusa - Lam.
Diffuse Knapweed
Other English Common Names: Bushy Star-thistle, White Knapweed
Other Common Names: diffuse knapweed
Taxonomic Status: Accepted
Related ITIS Name(s): Centaurea diffusa Lam. (TSN 36958)
Unique Identifier: ELEMENT_GLOBAL.2.137343
Element Code: PDAST1Y060
Informal Taxonomy: Plants, Vascular - Flowering Plants - Aster Family
 
Kingdom Phylum Class Order Family Genus
Plantae Anthophyta Dicotyledoneae Asterales Asteraceae Centaurea
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Concept Reference
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Concept Reference: Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.
Concept Reference Code: B94KAR01HQUS
Name Used in Concept Reference: Centaurea diffusa
Conservation Status
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NatureServe Status

Global Status: GNR
Global Status Last Changed: 22Mar1994
Rounded Global Status: GNR - Not Yet Ranked
Nation: United States
National Status: NNA
Nation: Canada
National Status: NNA (31Jul2016)

U.S. & Canada State/Province Status
United States California (SNA), Colorado (SNA), Connecticut (SNA), Idaho (SNA), Illinois (SNA), Indiana (SNA), Iowa (SNA), Kentucky (SNA), Massachusetts (SNR), Michigan (SNA), Missouri (SNA), Montana (SNA), Nevada (SNA), New Jersey (SNA), Oregon (SNA), Utah (SNA), Washington (SNA), Wyoming (SNA)
Canada Alberta (SNA), British Columbia (SNA), Manitoba (SNA), Ontario (SNA), Quebec (SNA), Saskatchewan (SNA)

Other Statuses

NatureServe Global Conservation Status Factors

Other NatureServe Conservation Status Information

Distribution
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U.S. States and Canadian Provinces
Color legend for Distribution Map
NOTE: The distribution shown may be incomplete, particularly for some rapidly spreading exotic species.

U.S. & Canada State/Province Distribution
United States CAexotic, COexotic, CTexotic, IAexotic, IDexotic, ILexotic, INexotic, KYexotic, MA, MIexotic, MOexotic, MTexotic, NJexotic, NVexotic, ORexotic, UTexotic, WAexotic, WYexotic
Canada ABexotic, BCexotic, MBexotic, ONexotic, QCexotic, SKexotic

Range Map
No map available.


U.S. Distribution by County Help
State County Name (FIPS Code)
NV Churchill (32001), Clark (32003), Elko (32007), Humboldt (32013), Lincoln (32017), Lyon (32019), Pershing (32027), Washoe (32031), White Pine (32033)
* Extirpated/possibly extirpated
U.S. Distribution by Watershed Help
Watershed Region Help Watershed Name (Watershed Code)
15 Muddy (15010012)+, Meadow Valley Wash (15010013)+
16 Hamlin-Snake Valleys (16020301)+, Upper Humboldt (16040101)+, Lower Humboldt (16040108)+, Little Humboldt (16040109)+, Truckee (16050102)+, Granite Springs Valley (16050104)+, East Walker (16050301)+, West Walker (16050302)+, Long-Ruby Valleys (16060007)+, Spring-Steptoe Valleys (16060008)+, Dry Lake Valley (16060009)+
18 Honey-Eagle Lakes (18080003)+
+ Natural heritage record(s) exist for this watershed
* Extirpated/possibly extirpated
Ecology & Life History Not yet assessed
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Economic Attributes Not yet assessed
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Management Summary Not yet assessed
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Population/Occurrence Delineation Not yet assessed
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Population/Occurrence Viability
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U.S. Invasive Species Impact Rank (I-Rank)
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Disclaimer: While I-Rank information is available over NatureServe Explorer, NatureServe is not actively developing or maintaining these data. Species with I-RANKs do not represent a random sample of species exotic in the United States; available assessments may be biased toward those species with higher-than-average impact.

I-Rank: High/Medium
Rounded I-Rank: High
I-Rank Reasons Summary: Centaurea diffusa occurs in every state west of the Rocky Mountains and is particularly widespread in the Pacific Northwest; it also occurs in Nebraska and is scattered in several eastern states. The worst infestations are reported to be in Idaho, Montana, Oregon, and Washington. Centaurea diffusa is very aggressive and can quickly dominate disturbed habitats. Centaurea diffusa produces an allelopathic chemical, that inhibits the root growth of other species and prevents them from competing for soil moisture and nutrients. Centaurea diffusa is most abundant in disturbed and overgrazed lands but it can also invade undisturbed grasslands, shrublands, and riparian communities. A single plant can produce up to 18,000 seeds, some of which remain dormant in the soil for at least a few years. Management is difficult, and usually requires a combination of methods over more than 5 years.
Subrank I - Ecological Impact: High/Medium
Subrank II - Current Distribution/Abundance: High/Medium
Subrank III - Trend in Distribution/Abundance: High/Medium
Subrank IV - Management Difficulty: High/Medium
I-Rank Review Date: 25Aug2004
Evaluator: Tomaino, A.
Native anywhere in the U.S?
Native Range: Native to eastern Europe and western Asia (GRIN 2001).

Download "An Invasive Species Assessment Protocol: Evaluating Non-Native Plants for their Impact on Biodiversity". (PDF, 1.03MB)
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Screening Questions

S-1. Established outside cultivation as a non-native? YES
Comments: Established outside cultivation in the U.S. (Kartesz 1999).

S-2. Present in conservation areas or other native species habitat? Yes
Comments: Centaurea diffusa occurs in riparian areas, sandy river shores, gravel banks, rock outcrops, rangelands, pastures, roadsides, and waste areas (Prather et al. 2002).

Subrank I - Ecological Impact: High/Medium

1. Impact on Ecosystem Processes and System-wide Parameters:High/Low significance
Comments: Centaurea diffusa produces an allelopathic chemical, cnicin, that inhibits the root growth of other species and destroys their ability to compete for limited soil moisture and nutrients (Fletcher and Renney 1963 in Carpenter and Murray 1998). Residual allelopathic chemicals in the soil may hinder the restablishment of other species even after Centaurea diffusa is removed (Carpenter and Murray 1998). More study is needed.

2. Impact on Ecological Community Structure:High/Moderate significance
Comments: A winter-hardy biennial or short-lived perennial, growing 15 to 60 cm in height (Wilson and Randall 2003). Diffuse knapweed is highly competitive and agressive; it forms dense colonies (Tu 2001). Centauria diffusa generally forms large dense infestations (Watson and Renney 1974). Diffuse knapweed is very aggressive and can quickly dominate rangelands (Butterfield et al. 1996).

3. Impact on Ecological Community Composition:High significance
Comments: Centaurea diffusa produces an allelopathic chemical, cnicin, that inhibits the root growth of other species and destroys their ability to compete for limited soil moisture and nutrients (Fletcher and Renney 1963 in Carpenter and Murray 1998). Centaurea diffusa can displace native vegetation in undisturbed areas (Wilson and Randall 2003). Diffuse knapweed is very aggressive and can quickly dominate rangelands (Butterfield et al. 1996).

4. Impact on Individual Native Plant or Animal Species:Unknown
Comments: In north central Oregon, Centauria diffusa, threatened populations of several rare plants such (Tu 2001). Although this particular infestation was successfully controlled, presumeably, Centaurea diffusa threatens other rare plants.

5. Conservation Significance of the Communities and Native Species Threatened:Moderate significance
Comments: Centaurea diffusa can invade undisturbed grasslands, shrublands, and riparian communnities (Zimmerman 1997 in Carpenter and Murray 1998). Presumeably, some of these communities are of conservation conern. In north central Oregon, Centauria diffusa, threatened populations of several rare plants such (Tu 2001). Although this particular infestation was successfully controlled, presumeably, Centaurea diffusa threatens other rare plant populations.

Subrank II. Current Distribution and Abundance: High/Medium

6. Current Range Size in Nation:High/Moderate significance
Comments: Centaurea diffusa occurs in every state west of the Rocky Mountains and is particularly widespread in the Pacific Northwest; it also occurs in Nebraska, and is scattered in several eastern states. See the distribution data in these sources: Kartesz (1999), Rice (2004), Baldwin et al. (2004), Weber et al. (2004), Wisconsin State Herbarium (2004), Iverson et al. (1999), Voss (1996), and University of Tennessee Herbarium (2002).

7. Proportion of Current Range Where the Species is Negatively Impacting Biodiversity:High/Moderate significance
Comments: In the western United States, it infests an estimated area of 5000 square miles (1,264,000 hectares) (Zimmerman 1997 in Carpenter and Murray 1998). The worst infestations are reported to be in Idaho, Montana, Oregon, and Washington (Zimmerman 1997 in Carpenter and Murray 1998). In Michigan, it is often abundant over great areas of roadside and dry fields (Voss 1996). In Flagstaff Arizona, it dominates the landscape in open disturbed fields in several areas (Moser, not dated).

8. Proportion of Nation's Biogeographic Units Invaded:Moderate significance
Comments: Approximately 38% of units, inferred from TNC (2001), Kartesz (1999), Rice (2004), Baldwin et al. (2004), Weber et al. (2004), Wisconsin State Herbarium (2004), Iverson et al. (1999), Voss (1996), and University of Tennessee Herbarium (2002).

9. Diversity of Habitats or Ecological Systems Invaded in Nation:Medium/Low significance
Comments: Centaurea diffusa occurs in pastures, over-grazed rangelands, riparian communities, grasslands, and shrublands (Zimmerman 1997 in Carpenter and Murray 1998). In Idaho, it occurs in riparian areas, sandy river shores, gravel banks, rock outcrops, rangelands, pastures, roadsides, and waste areas (Prather et al. 2002). In Michigan, it occurs in roadsides, dry fields, railroads, sand dunes, vacant lots, dumps, gravel pits, parking areas, and other waste places (Voss 1996). In California, it occurs in fields and roadsides (Baldwin et al. 2004).

Subrank III. Trend in Distribution and Abundance: High/Medium

10. Current Trend in Total Range within Nation:High/Moderate significance
Comments: In the western United States, the area infested is increasing an estimated 18 percent per year (Zimmerman 1997 in Carpenter and Murray 1998).

11. Proportion of Potential Range Currently Occupied:Low significance
Comments: Inferred from USDA (1990) and Kartesz (1999), 30-90% of its potential generalized range in the U.S. is currently occupied.

12. Long-distance Dispersal Potential within Nation:High/Moderate significance
Comments: Seed disperal is mainly by wind; mature plants break off at ground level and become tumbleweeds or become attached to vehicles (Watson and Renney 1974). Seeds are also dispersed by animals and people (Wilson and Randall 2003). Centaurea diffusa has the ability to spread seeds over relatively long distances as a tumbleweed (Carpenter and Murray 1998).

13. Local Range Expansion or Change in Abundance:High/Moderate significance
Comments: Diffuse knapweed spreads rapidly along rights-of-way and farm roads and can invade undisturbed grassland, shrubland, and riparian communities (Tu 2001).

14. Inherent Ability to Invade Conservation Areas and Other Native Species Habitats:Moderate significance
Comments: Centaurea diffusa can displace native vegetation in undisturbed areas (Wilson and Randall 2003). Centaurea diffusa is most abundant in disturbed and overgrazed lands but it can also invade undisturbed grasslands, shrublands, and riparian communnities (Zimmerman 1997 in Carpenter and Murray 1998).

15. Similar Habitats Invaded Elsewhere:Low significance
Comments: In southern British Columbia, Centaurea diffusa is common on semiarid rangeland, highway and railroad right-of-ways and waste places (Watson and Renney 1974).

16. Reproductive Characteristics:Medium/Low significance
Comments: A single plant can produce up to 18,000 seeds (Harris and Cranston 1979 in Carpenter and Murray 1998). Reproduces mainly by seeds (Watson and Renney 1974; APRS Implementation Team 2001). Seeds may remain dormant but viable in the soil for several years (Carpenter and Murray 1998). Centaurea diffusa's rosette growth form in the first year resists mowing and grazing (Butterfield et al. 1996).

Subrank IV. General Management Difficulty: High/Medium

17. General Management Difficulty:High significance
Comments: Lasting control will require a combination of proper land management, biological control, physical control, chemical control, and suppression by desirable vegetation; this cumulative stress method will keep the plant constantly under stress (Carpenter and Murray 1998). A large infestation of Centaurea diffusa at a TNC Preserve in northeast Oregon was successfully controlled by hand-pulling three times a year for 6 years; a large pool of volunteers were available (Tu 2001). In the first year of the control effort, 360 person hours were logged; after 11 years only a few hours a year are required to maintain control (Tu 2001).

18. Minimum Time Commitment:Moderate significance
Comments: A single plant can produce up to 18,000 seeds (Harris and Cranston 1979 in Carpenter and Murray 1998). Seeds remain viable in the soil for 1 to 5 years (APRS Implementation Team 2001). A large infestation of Centaurea diffusa at a TNC Preserve in northeast Oregon was successfully controlled by hand-pulling three times a year for 6 years; a large pool of volunteers were available (Tu 2001). In the first year of the control effort, 360 person hours were logged; after 11 years only a few hours a year are required to maintain control (Tu 2001).

19. Impacts of Management on Native Species:Medium/Low significance
Comments: Control methods may harm other plants and result in a disturbance that will favor reinvasion by diffuse knapweed or other exotic species (Butterfiled et al. 1996). Although hand pulling is labor intensive, it is very specific and may be the best method when controlling Centaurea diffusa among native plants (Tu 2001). Hand-pulling alone may not be effective in some situations (Carpenter and Murray 1998).

20. Accessibility of Invaded Areas:Low significance/Insignificant
Comments: Unpalatable to livestock and its spines may cause injury to grazing animals (Tu 2001). Classified as a noxious weed in 10 western states (Kartesz 1999) and infestations are not in extreme or remote habitats (Carpenter and Murray 1998); assumption is accessibility problems are not high or moderate.
Authors/Contributors
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Botanical data developed by NatureServe and its network of natural heritage programs (see Local Programs), The North Carolina Botanical Garden, and other contributors and cooperators (see Sources).

References
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  • Alien plants ranking system (APRS) Implementation Team. 2001a. Alien plants ranking system version 7.1. Southwest Exotic Plant Information Clearinghouse, Flagstaff, AZ. Online. Available: http://www.usgs.nau.edu/swepic/ (accessed 2004).

  • Baldwin, B.G., S. Boyd, B.J. Ertter, D.J. Keil, R.W. Patterson, T.J. Rosatti and D.H. Wilken. 2004.
    Jepson Flora Project, Jepson Online Interchange for California Floristics. Regents of the University of California, Berkeley. Online. Available: http://ucjeps.berkeley.edu/jepson_flora_project.html (Accessed 2004).

  • Blondeau, Marcel. 2008. Le Centaurea diffusa Lamarck, une espèce adventice nouvelle pour le Québec. Ludoviciana 31: 47-54.

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  • Butterfield, C., J. Stubbendieck, and J. Stumpf. 1996. Species abstracts of highly disruptive exotic plants. Northern Prairie Wildlife Research Center, Jamestown, ND. Home Page: http://www.greatplains.org/npresource/othrdata/exoticab/exoticab.htm.

  • Carpenter, A. T., and T. A. Murray. 1998. Element Stewardship Abstract for Centaurea diffusa Lamarck (synonym Acosta diffusa [Lam.] Sojak) diffuse knapweed. Edited by J. M. Randall and M. Tu. The Nature Conservancy. Online. Available: http://tncweeds.ucdavis.edu/esadocs/centdiff.html.

  • Cavers, P.B., ed. 1995. The Biology of Canadian Weeds. The Agricultural Institute of Canada, Ottawa.

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  • Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.

  • Kartesz, J.T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. First edition. In: Kartesz, J.T., and C.A. Meacham. Synthesis of the North American Flora, Version 1.0. North Carolina Botanical Garden, Chapel Hill, N.C.

  • Moser, L., and D. Crisp. No date. San Francisco Peaks Weed Management Area fact sheet on Centaurea diffusa. Coconino National Forest. Available: http://www.usgs.nau.edu/SWEPIC/factsheets/cedi3sf_info.pdf (accessed 18 August 2004).

  • Moser, L., and D. Crisp. No date. San Francisco Peaks Weed Management Area fact sheet on Knapweed. Coconino National Forest. Available: http://www.usgs.nau.edu/SWEPIC/factsheets/knapweedssf_info.pdf. (Accessed 2004).

  • Piper, Gary L. 1986. Biological control of noxious Centaurea species in Washington. Douglasia. 10(3):1-2.

  • Prather, T. S., S. S. Robins, D. W. Morishita, L. W. Lass, R. H. Callihan, and T. W. Miller. 2002. Idaho's Noxious Weeds. University of Idaho Extension, Moscow. 76 pp.

  • Rice, P.M. 2004. February 19 last update. Invaders Database System. Division of Biological Sciences, University of Montana, Missoula. Online. Available: http://invader.dbs.umt.edu (accessed 2004).

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  • USDA Agricultural Research Service. 1990. USDA Plants Hardiness Zone Map. Misc. Publ. Number 1475.

  • USDA, ARS, National Genetic Resources Program. 2001. Germplasm Resources Information Network - (GRIN). [Online Database]. National Germplasm Resources Laboratory, Beltsville, Maryland.URL: http://www.ars-grin.gov/var/apache/cgi-bin/npgs/html/taxon.pl?6438. (Accessed 2004)

  • University of Tennessee Herbarium and Austin Peay State University. 2002. Database of Tennessee Vascular Plants. Department of Botany, Knoxville. Online. Available: http://tenn.bio.utk.edu/vascular/vascular.html (accessed 2004).

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