Early Detection and Control Stops Purple Starthistle Spread in Idaho

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Purple starthistle (Centaurea calcitrapa) is established in at least 14 states in the U.S., but recently expanded its range to Twin Falls County, Idaho. A quick response from the land owner, field inspector, county weed coordinator, and Idaho State Department of Agriculture is stopping the weed in its tracks. 

Kali Van Leeuwen-Sherrill, Twin Falls County Weed Coordinator is the lead for the eradication effort. “In June 2014 a private field inspector contacted our office and reported purple starthistle in a pasture near Castleford,” explains Kali. “The weed had spread from a few plants found by the landowner in 2013 to infest over 20 acres in 2014—it’s a very aggressive plant.” 

The county developed a strategy that included surveying all properties within several miles of the known infestation, including neighboring fields, roadsides, and federal land. “We didn’t find purple starthistle in any other fields, pastures, or rangeland,” says Kali. “However, we did find five plants along the roadside near the infestation and believe that the weed was introduced by a vehicle and spread from the roadside into the field.”

Idaho State Department of Agriculture declared an emergency listing of both purple starthistle and closely related Iberian star thistle (Centaurea iberica) in June 2014. The noxious weed designation gave the county weed district authority to control the weed until the state legislature moves to make the designation law. 

The infested pasture was treated with Milestone® herbicide at 6 fluid ounces per acre (fl oz/A) in June 2014. County weed district field crews monitored the pasture through the summer of 2014. In August of that year, Twin Falls County had a record rainfall of more than four inches in three days. By September, many new purple starthistle rosettes established. A second application of Milestone was made at the same rate in fall 2014. 

Field and roadside surveys were conducted in spring and summer of 2015. To date, there are no purple starthistle rosettes or seedlings that have been found within or outside of the treated infestation. 

“We plan to continue to monitor the roadside and 60-acre pasture for at least five more years,” explains Kali “Keeping Idaho free of invasive plants like purple starthistle requires a long term commitment from everyone to find and eradicate newly invading weeds.” 


  • Pitcairn MJ, JA Young, CD Clements, and J Balciunas. 2002. Purple starthistle (Centaurea calcitrapa) seed germination. Weed Technology 16(2):452-456. 

  • Randall J. M. 2000. Centaurea calcitrapa. In C. C. Bossard, J. M. Randall, and M. C. Hosbovsky, eds. Invasive Plants of California Wildlands. Berkeley, CA: University of California Press. pp. 94–97. 

  • Roche CT and BF Roche Jr. 1990. Distribution and amount of four knapweed (Centaurea L.) species in eastern Washington. Northwest Sci 62:242–253.           

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