New growth regulator herbicide provides excellent control of yellow starthistle

Yellow starthistle is the most widespread invasive weed in California. An exotic that invaded California in the Gold Rush days, starthistle was once a minor annoyance but is now out of control. One reason is that few herbicides are registered for use on California rangelands, pastures and wildlands. Of the registered herbicides, the majority are effective only when applied to the foliage of target plants; season-long control of yellow starthistle is not provided because seedlings that emerge after application of the herbicide escape injury. In a comparison of herbicides, the newly registered herbicide clopyralid provided excellent control at low use rates and worked eaually well whether applied to leaves or to soil to control germinating seedlings. In soil, it showed residual activity throughout the season. Complete yellow starthistle control was achieved with applications made from December through April, but treatments in February maximized desirable forage production, particularly grasses. Late-season applications of glyphosate and clopyralid were effective for control of starthistle plants in late rosette and bolting stages.