Plant Assessment Form

Centaurea diffusa

Synonyms: Acosta diffusa (Lam.) Sojak

Common Names: diffuse knapweed

Evaluated on: 11/28/05

List committee review date: 10/01/2006

Re-evaluation date:


Elizabeth Brusati, project manager
California Invasive Plant Council
1442A Walnut St. #462, Berkeley, CA 94709
Joseph M. DiTomaso/Coop. Ext. Specialist
University of California, Davis
Weed Science Program, Robbins Hall

List commitee members

Joe DiTomaso
John Randall
Peter Warner
Jake Sigg

General Comments

No general comments for this species

Table 2. Criteria, Section, and Overall Scores

Overall Score? Moderate
Alert Status? No Alert
Documentation? 3.5 out of 5
Score Documentation
1.1 ?Impact on abiotic ecosystem processes B Reviewed Scientific Publication
Four-part score BABD Total Score
1.2 ?Impact on plant community A. Severe Reviewed Scientific Publication
1.3 ?Impact on higher trophic levels B. Moderate Reviewed Scientific Publication
1.4 ?Impact on genetic integrity D. None Other Published Material
2.1 ?Role of anthropogenic and natural disturbance in establishment B. Moderate Reviewed Scientific Publication
Total Points
13 Total Score B
2.2 ?Local rate of spread with no management B. Increases less rapidly Observational
2.3 ?Recent trend in total area infested within state D. Declining Other Published Material
2.4 ?Innate reproductive potential
(see Worksheet A)
A. High Reviewed Scientific Publication
2.5 ?Potential for human-caused dispersal A. High Reviewed Scientific Publication
2.6 ? Potential for natural long-distance dispersal B. Occasional Other Published Material
2.7 ?Other regions invaded C. Already invaded Other Published Material
3.1 ?Ecological amplitude/Range
(see Worksheet C)
A. Widespread Other Published Material
Total Score B
3.2 ?Distribution/Peak frequency
(see Worksheet C)
D. Very low Observational

Table 3. Documentation

Scores are explained in the "Criteria for Categorizing Invasive Non-Native Plants that Threaten Wildlands".

Section 1: Impact
Question 1.1 Impact on abiotic ecosystem processes? B Reviewed Scientific Publication
Identify ecosystem processes impacted:

Increases erosion along streambanks.

Sources of information:

1. Sheley, R.L, J.S. Jacobs, and M. F. Carpinelli. 1998. Distribution, biology, and management of diffuse knapweed (Centaurea diffusa) and spotted knapweed (Centaurea maculosa). Weed Technology. 12:353-362

Question 1.2 Impact on plant community composition,
structure, and interactions?
A Reviewed Scientific Publication
Identify type of impact or alteration:

Reduces biodiversity (1). Compounds in shoots extracts reduce germination of other species through allelopathy (2,3) and it has a stronger negative effect on North American plants than on plants that evolved with knapweed in Eurasia (2). Can form monocultures, although populations in California are mostly small patches due to eradication efforts (1). Formation of monotypic stands is aided by the fact that seedling emergence is distributed over several weeks, allowing diffuse knapweed to occupy all available safe sites (4).

Sources of information:

1. Sheley et al. 1998
2. Callaway, R., and E. T. Aschehoug. 2000. Invasive plants versus their new and old neighbors: a mechanism for exotic invasion. Science. 290:521-523
3. Muir, A. D., and W. Majak. 1983. Allelopathic potential of diffuse knapweed (Centaurea diffusa) extracts. Canadian Journal of Plant Sciences. 63:989-996
4. Sheley, R. L., and L. L. Larson. 1996. Emergence date effects on resource partitioning between diffuse knapweed seedlings. Journal of Range Management. 49:241-244.

Question 1.3 Impact on higher trophic levels? B Reviewed Scientific Publication
Identify type of impact or alteration:

Reduces forage for livestock and wildlife, but is grazed by deer. Elk foraging was reduced by 98% on plots of C. maculosa (1). Presumably, diffuse knapweed would act the same.
There is anecdotal evidence that diffuse knapweed contains a compound that, when absorbed through the skin through cuts or abrasions, can cause benign tumors in humans, although there is no medical literature on this. Anyone working with diffuse knapweed should wear gloves as a precaution (2).

Sources of information:

1. Sheley et al. 1998
2. Carpenter, A. T., and T. A. Murray. 1998. Element Stewardship Abstract for Centaurea diffusa. The Nature Conservancy, Arlington, VA.

Question 1.4 Impact on genetic integrity? D Other Published Material

None. There are no native Centaurea spp. in California.

Sources of information:

Hickman, J. C. (ed.) 1993. The Jepson Manual, Higher Plants of California. University of California Press. Berkeley, CA

Section 2: Invasiveness
Question 2.1 Role of anthropogenic and natural disturbance
in establishment?
B Observational
Describe role of disturbance:

Invades highly disturbed areas such as roadsides and overgrazed grasslands, but can also invade after minor disturbance such as a hailstorm, rodent activity, or light grazing (1, 2,3). Disturbance greatly increases the the rate and final density of diffuse knapweed and allows it to invade a wider range of habitats (1).

Sources of information:

1. Sheley et al. 1998
2. Myers, J. H., and D. E. Berube. 1983. Diffuse knapweed invasion into rangeland in the dry interior of British Columbia. Canadian Journal of Plant Sciences. 63:981-987.
3. Lacey, J., P. Husby, and G. Handl. 1990. Observations on spotted and diffuse knapweed invasion into ungrazed bunchgrass communities in western Montana. Rangelands. 12: 30-32.

Question 2.2 Local rate of spread with no management? B Observational
Describe rate of spread:

Has spread at very rapid rates in other western states. In California, the rate of spread appears to be a bit slower, probably due to the efforts of CDFA to manage the species.

Sources of information:

DiTomaso, observational.

Question 2.3 Recent trend in total area infested within state? D Other Published Material
Describe trend:

In California, diffuse knapweed primarily occurs as single plants or small patches and is under eradication through biological control in most areas (1). Stable to declining/decreasing statewide because of treatment efforts (2, 3). Once the seed production has been controlled, infestation levels drop rapidly (3).

Sources of information:

1. Joley, D. B., and D. M. Woods. 1996. Biological control of diffuse knapweed, Centaurea diffusa. California Department of Food and Agriculture, Biological Control Program Annual Summary, 1996. Sacramento, CA
2. Carri Pirosko, California Dept. of Food and Agriculture. e-mail 11/29/05
3. Marla Knight, botanist, Klamath National Forest. e-mail 11/29/05

Question 2.4 Innate reproductive potential? A Reviewed Scientific Publication
Describe key reproductive characteristics:

Typically biennial, sometimes annual or short-lived perennial. Obligate outcrosser. Flowers June-September. Seeds can persist in soil for many years. Purple-flowered plants were shown to set significantly more seed than white-flowered plants, event though white-flowered plants out-numbered purple-flowered plants and major insect visitors did not discriminate according to color. Seed production in Washington averaged 11,200 to 48, 100 seeds per square meter.

Sources of information:

DiTomaso, J.M., and E. Healy. 2006. Weeds of California and Other Western States
Harrod, R. J., and R. J. Taylor. 1995. Reproduction and pollination biology of Centaurea and Acroptilon species, with emphasis on C. diffusa. Northwest Science. 69:97-105.

Question 2.5 Potential for human-caused dispersal? A Reviewed Scientific Publication
Identify dispersal mechanisms:

Can be spread by hitchhiking in the frames of vehicles (1). Was originally introduced to the United States in contaminated alfalfa, and spread further in alfalfa and hay (2).

Sources of information:

1. Roche and Roche 1999
2. Sheley et al. 1998

Question 2.6 Potential for natural long-distance dispersal? B Other Published Material
Identify dispersal mechanisms:

Ball-shaped plants can be spread by the wind, similar to tumbleweed. Seeds are lost gradually, allowing them to spread long distances. Plants can also be carried in rivers and irrigation systems.

Sources of information:

1. Roche and Roche 1999

Question 2.7 Other regions invaded? C Other Published Material
Identify other regions:

Native to Eurasia. Occurs in all western states, some central and eastern states, especially Illinois and surrounding states. Listed as a noxious weed in several states (1).

Sources of information:

1. DiTomaso and Healy 2006

Section 3: Distribution
Question 3.1 Ecological amplitude/Range? A Other Published Material

Occurs in the North Coast, North Coast Ranges (Del Norte, Humboldt, Mendocino Cos.), Klamath Ranges (Trinity Co.), Cascade Ranges (Siskiyou, Shasta Cos.), north and central Sierra Nevada (Plumas, Nevada, sc Placer, e El Dorado, e Amador Cos), northern Sacramento Valley (c&e Tehama, sc Glenn, s Sutter, n Sacramento Cos), Modoc Plateau (Modoc, Lassen Cos), southern San Francisco Bay region (ne Santa Clara Co.), South Coast Ranges (se Monterey Co.), South Coast (Los Angeles, San Diego Cos), to 2300m. A population that has been eradicated occurred in the central area of the border between Mariposa and Madera counties (1).
Habitats invaded (based on eradication projects listed in NRPI database): oak woodlands, blue oak-foothill pine, pasture, mixed evergreen forest, Great Basin scrub, coastal prairie, north coast coniferous forest, riparian forest and woodland, valley and foothill grassland (2)
Mainly invades roadsides, waste areas, and rangelands. Found in some dry/seasonal creek beds in Siskiyou County. Mostly dry areas rather than true riparian areas (3).

Sources of information:

1. DiTomaso and Healy 2006
2. Natural Resources Projects Inventory. Database of conservation and monitoring projects in California. Available: Accessed 11/28/05
3. Carri Pirosko, California Dept. of Food and Agriculture. e-mail 11/29/05.

Question 3.2 Distribution/Peak frequency? D Observational
Describe distribution:

Mostly small infestations/patches; many single to small plants/patches that are eradicated.

Sources of information:

Carri Pirosko, California Dept. of Food and Agriculture. e-mail 11/29/05

Worksheet A - Innate reproductive potential

Reaches reproductive maturity in 2 years or less Yes
Dense infestations produce >1,000 viable seed per square meter Yes
Populations of this species produce seeds every year. Yes
Seed production sustained over 3 or more months within a population annually Yes
Seeds remain viable in soil for three or more years Yes
Viable seed produced with both self-pollination and cross-pollination No
Has quickly spreading vegetative structures (rhizomes, roots, etc.) that may root at nodes No
Fragments easily and fragments can become established elsewhere Yes
Resprouts readily when cut, grazed, or burned Unknown
Total points: 9
Total unknowns: 1
Total score: A?

Related traits:

Worksheet B - Arizona Ecological Types is not included here

Worksheet C - California Ecological Types

(sensu Holland 1986)
Major Ecological Types Minor Ecological Types Code?
Marine Systemsmarine systems
Freshwater and Estuarine lakes, ponds, reservoirs
Aquatic Systemsrivers, streams, canals
Scrub and Chaparralcoastal bluff scrub
coastal scrub
Sonoran desert scrub
Mojavean desert scrub (incl. Joshua tree woodland)
Great Basin scrubD, < 5%
chenopod scrub
montane dwarf scrub
Upper Sonoran subshrub scrub
Grasslands, Vernal Pools, Meadows, and other Herb Communitiescoastal prairieD, < 5%
valley and foothill grassland
Great Basin grassland
vernal pool
meadow and seep
alkali playa
pebble plain
Bog and Marshbog and fen
marsh and swamp
Riparian and Bottomland habitatriparian forestD, < 5%
riparian woodlandD, < 5%
riparian scrub (incl.desert washes)
Woodlandcismontane woodlandD, < 5%
piñon and juniper woodland
Sonoran thorn woodland
Forestbroadleaved upland forest
North Coast coniferous forestD, < 5%
closed cone coniferous forest
lower montane coniferous forest
upper montane coniferous forest
subalpine coniferous forest
Alpine Habitatsalpine boulder and rock field
alpine dwarf scrub
Amplitude (breadth): A
Distribution (highest score): D

Infested Jepson Regions

Click here for a map of Jepson regions

  • Cascade Range
  • Central West
  • Great Basin Province
  • Great Valley
  • Modoc Plateau
  • Northwest
  • Sierra Nevada
  • Sierra Nevada East
  • Southwest