Yellow Starthistle has become a common and troublesome pest in many areas. It readily forms dense infestations that rapidly deplete soil moisture, crowd out annual legumes and grasses, and create a spiny barrier for many activities. An effective long-term management plan for yellow Starthistle must use an integrated approach. The best approaches are use of biological agents, mowing, properly timed grazing, and the judicious use of herbicides.

Historically, post emergence herbicides used to control yellow Starthistle were limited in their effectiveness because of the weed’s ability to germinate continuously throughout Winter, Spring, and into Summer whenever moisture is available. Clopyralid (Transline) is a growth regulator herbicide that stops development of the growing points of plants. It became available in California during the Winter of 1998 for use on non-crop areas including pastures, rangeland, and wildlands. It is not expected to impact biological control agents and has been demonstrated to be very safe on grasses. Unlike other postemergence products, clopyralid also has excellent preemergence activity and is effective at very low rates. In this experiment, we compared timing of application for yellow Starthistle control from before emergence through the bolting stage.

We established field trials at the Calaveras County airport November 1998 to compare effectiveness of clopyralid when applied at the recommend label rate of 4 ounces per acre throughout the growing season and under variable weather conditions. Bivert was the surfactant used and was applied at a rate of 8 ounces per acre. Each plot was ten feet by ten feet and all treatments were applied using a hand held pump type sprayer delivering approximately 40 gallons per acre at 10-15 pounds pressure. All treatments were replicated three times. Evaluations of all plots were made on June 8, 1999.

A total of 51 plots were treated on 17 separate and randomly selected dates between November and June. In the treated plots, only one yellow Starthistle plant was found. In the untreated control plots, an average of 112 yellow Starthistle plants were found per plot. No difference could be detected in control based upon timing of application.

There are several advantages to using clopyralid for yellow Starthistle control over other postemergence herbicides registered for rangeland, pastures, and wild areas. It has both pre- and postemergence activity at very low use rates, a low toxicology profile and no grazing restrictions. Glyphosphate (Roundup) is a more desirable material to use for late season control provided that all grasses and legumes have already gone to seed and turned brown provided the yellow Starthistle plants are not drought stressed. Continued clopyralid use over many years may have a detrimental effect on legume populations. Consequently, other control options should be rotated in the overall management program.

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