light availability is impacted in dense stands of squarrose knapweed- this is the only plant in stands that reach monotypic levels
It is suspected that available water in the rangeland profile is severely reduced by dense stands of squarrose knapweed, based on findings with yellow starthistle in similar rangeland settings. The success of Centaurea species in dominating grasslands is explained by their ability to compete successfully for the new limiting resource or resources (K.D. LeJeune et al.)
Wildfires create the ideal conditions for rapid expansion. Squarrose knapweed's fire and drought tolerance, excellent seed dispersal, and rapid response to soil resources released by fire nearly guarantee spread into burned closed-canopy juniper sites with little understory.

C. Pirosko, personal observation.
LeJeune, Katherine D., Seastedt, Timothy R. 2001. Centaurea species: The forb that won the west. Conservation Biology. 15(6). December: 1568-1574.
Sheley and Petroff. Biology and Management of Noxious Rangeland Weeds.

Dense stands impact bird and wildlife, as well as range animal use of the system.
Highly competative, dense stands can exclude desirable vegetation and wildlife in natural areas

J. DiTomaso and E. Healy, Weeds of California, as yet unpublished.
Sheley, R. L, and J. K. Petroff. 1999. Biology and Management of Noxious Rangeland Weeds. Oregon State University Press, Corvallis, OR

Largely invades disturbed sites.
Largely dependant on human disturbances such as: over grazing, logging/replanting activities, roadside maintenance/scraping, roads and trails.
Dispersal by vehicles and trains appears increasingly important, judging by the expansion of squarrose knapweed along roads, railroads, and off-road vehicles.
Often grows on degraded rangeland soils and is more adaptable to drought and cold temperatures than other knapweeds.

J. DiTomaso and E. Healy, Weeds of California, as yet unpublished.
Sheley and Petroff. Biology and Management of Noxious Rangeland Weeds.

Not widely distributed throughout the State. Primarily has been isolated to the Great Basin area in northeastern California.
Rate of spread is slow and steady. Spread in the Big Valley area of Lassen County is the best documentation of spread. Since 1950 when the plant was first officially documented (infestation likely had been there since 1930-40's), the infestation has slowly crept along roadways, from ranch to ranch on equipment and sheep. So that today extensions of the original infestation spread up into the forest behind the Kramer Ranch in Big Valley, up into Lookout, Modoc County, and down all the way into Pittville area to the southeast of Big Valley.

personal observation, C. Pirosko California Department of Food and Agriculture.
Internal Document (typed document from 1973, author unknown), California Department of Food and Agriculture, Integrated Pest Control Branch, Noxious Weed Program, Redding Field Office.

Primarily Human caused. Long distance dispersal is primarily through sheep. The recurved spines of squarrose knapweed are ideally suited to transport by sheep's wool. Historically, most of the movement of squarrose knapweed has been associated with sheep. No reference has been found regarding the initial introduction of squarrose knapweed into the western U.S., but it is possible that seed was carried in wool, either of sheep or wool products, yarn, blankets, or clothing.
Often grows on degraded rangeland soils and is more adaptable to drought and cold temperatures than other knapweeds.
Roadside equipment, scraping of road shoulders by CalTrans and County Road departments is also a mode of long distance dispersal along roadways.

Bellue, M. K. 1952. Virgate Star Thistle, Centaurea virgata var. squarrosa (Willd.) Boiss. in California. Bulletin, California Dept. of Agriculture. Vol. 41(2): 61-63.
J. DiTomaso and E. Healy, Weeds of California, as yet unpublished.
C. Pirosko, personal observation.

Utah, Oregon, Washington, Nevada, Michigan
Primarily invades range and pasture lands, as well as along roadsides.
Often grows on degraded rangeland soils and is more adaptable to drought and cold temperatures than other knapweeds.
Native to Eastern Europe, Southern Russia, and Western Asia--- The climate of these native ranges are very similar to that of the Great Basin found in northeastern California, Eastern Oregon, and parts of Utah.
Squarrose knapweed is native to Bulgaria, Lebanon, Anti-Lebanon, Transcaucasia, northern Iraq, Iran, Afghanistan, and Turkestan. In the Flora of Turkey, the U.S. specimens key to subspecies squarrosa of Centaurea virgata, which is found mainly in Inner Anatolia; much of the plateau of Inner Anatolia lies between 2,500 and 3,300 feet elevation, falling to a large salt lake in the center. The climate in that region is harsh, characterized by erratic precipitation, temperature extremes, wind and devastating hail storms. Precipitation primarily falls as snow in the winter and spring; summers are dry with very low humidity; Winter temperatures are lower than Mediterranean climates and in summer temperatures soar during the day and drop drastically at night. (NOTE: A PERFECT DESCRIPTION OF BIG VALLY, LASSEN COUNTY, CA, USA as well!) Squarrose knapweed is a threat to similar ecological types already invaded; similar great basin-like habitat found in eastern portions of the state; Could still invade range and pasture lands south of Lassen County, into regions along the California/Nevada Border.
STILL VAST OPPORTUNITY to keep spreading in areas already invaded, expansion of current distribution.
PLUS lots of range/pasture in the GREAT BASIN is still susceptible to invasion which would have significant impacts on productivity of these lands. Squarrose knapweed simply hasn't been introduced in these suceptible areas- if given the opportunity, squarrose knapweed could do more damage to northeastern rangelands.

Internal Document (typed document from 1973, author unknown), California Department of Food and Agriculture, Integrated Pest Control Branch, Noxious Weed Program, Redding Field Office. J. DiTomaso and E. Healy, Weeds of California, as yet unpublished.
Sheley and Petroff. Biology and Management of Noxious Rangeland Weeds.

In northern California, squarrose knapweed grows on dry rocky sites of degraded juniper-shrub savanna with scattered western juniper and poderosa pine and chaparral-type understory (Sheley and Petroff)
In the Great Basin and intermountain foothills, the sagebrush and juniper range types appear to be the most susceptible to invasion by squarrose knapweed.
Often grows on degraded rangeland soils and is more adaptable to drought and cold temperatures than other knapweeds.
First official record was in 1950; this species was collected at the Kramer Ranch in Big Valley, Lassen County by Loring White and identified by J.T. Howell of the Academy of Sciences, and subsequently identified by Dr. S.F. Blake of the USDA. Prior to 1910 there is no information about this ranch on which the sample was collected. After 1910 records indicate that the ranch was rented out as sheep pasture and under lease to neighboring rancher, James Summers. This site was then used as an overnight campground by sheepmen when they trailed their bands of sheep from the Sacramento Valley to higher ranges in the summer and returned them in the fall. In 1935 Mr. Summers reported to have commented on a weed taking over his leased pasture. About 1944 Mr. Rupert Lyons purchased the property and found a heavy infestation of the weed. Mr Lyons sold the land to Mr. O.E. Brown, whose son collected the specimen identified in 1950. Squarrose knapweed has largely Squarrose knapweed is a threat to similar ecological types already invaded; similar great basin-like habitat found in eastern portions of the state; Could still invade range and pasture lands south of Lassen County, into regions along the California/Nevada Border.
STILL VAST OPPORTUNITY to keep spreading in areas already invaded, expansion of current distribution.
PLUS lots of range/pasture in the GREAT BASIN is still susceptible to invasion which would have significant impacts on productivity of these lands. Squarrose knapweed simply hasn't been introduced in these suceptible areas- if given the opportunity, squarrose knapweed could do more damage to northeastern rangelands.

Internal Document (typed document from 1973, author unknown), California Department of Food and Agriculture, Integrated Pest Control Branch, Noxious Weed Program, Redding Field Office; C. Pirosko personal observation.
J. DiTomaso and E. Healy, Weeds of California, as yet unpublished.
Sheley and Petroff. Biology and Management of Noxious Rangeland Weeds.