Santa Clara University

Invasive yellow starthistle (Centaurea solstitialis) is one of California’s most pernicious invasive species, causing millions of dollars in damages annually to ranchers. The thistle is unpalatable to cattle and toxic to horses, so its spread is a serious threat to grazing land in the state. Most ranchers use herbicides to control the plant, but this strategy raises the possibility of herbicide resistance in starthistle populations.

New research from Dr. Virginia Matzek of Santa Clara’s Department of Environmental Studies and Sciences shows that mowing the plant in its flowering stage controls yellow starthistle and wipes out its soil seedbank, with no negative effects on annual or perennial forage species. Starthistle biomass was reduced 91-95% over three years in various mowing treatments, while the size of seedbank (which regenerates starthistle from year to year) was reduced by as much as 100% by mowing. The research, recently published in Rangeland Ecology and Management, recommends that ranchers and rangeland managers consider late-season mowing as an alternative to herbicides for yellow starthistle control.

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