Herbicides play an important role in integrated management of yellow starthistle and can be used alone or in combination with other techniques such as timely mowing, grazing, burning, or use of biological control insects. The following information includes guidelines on herbicide rate and time of application to effectively control yellow starthistle.
About Yellow Starthistle
Yellow starthistle (Centaurea solstitialis) is one of the most widespread invasive broadleaf weeds on rangeland and natural areas in the United States. It is a taprooted, winter annual, or rarely a biennial or short-lived perennial plant, that spreads by seed. Individual plants can produce more than 100,000 seeds, and dense infestations can yield 50 to 100 million seeds per acre. Plants produce one to many solitary, spiny, yellow flower heads beginning in late June and continuing through September and sometimes much later. Yellow starthistle displaces desirable vegetation in part through competition for available soil moisture. The weed is toxic to horses, and its spiny flower heads can reduce livestock carrying capacity.
Treatment recommendations and data summary
Field trials conducted in California, Idaho, Washington and Oregon show that Milestone® and Transline® provide effective control of yellow starthistle. Milestone at 3 to 5 fl oz/A (fluid ounces per acre) and Transline at 8 to 10 fl oz/A can be applied from early rosette to bolt growth stage when soil moisture is present and plants are actively growing (Figure 1). The higher application rate should be applied at the bolt growth stage.
Results of field trials conducted by the University of California-Davis indicate that the ideal time to apply Milestone® to control yellow starthistle in the Central Valley of California is within a three-month period from December through February (Figure 2).
Additional studies conducted near Lewiston, Idaho (Figure 3) showed that Milestone at 4 to 6 fl oz/A applied to yellow starthistle at fall rosette (November), spring rosette (April), and late spring bolting (May) growth stage effectively controlled yellow starthistle. Although the bolt growth stage treatment stopped seed production when evaluated at the end of the growing season in Idaho, there is a greater risk of viable seed production at this and later application timings.
An additional benefit to controlling yellow starthistle at the early rosette growth stage is an increase in annual grass forage production compared to later application timings. Field studies conducted at University of California-Davis show significantly greater annual grass production when yellow starthistle was controlled at the early rosette growth stage (Table 1). These data are supported by similar field studies conducted by University of Idaho (Wallace and Prather 2007).
Summary of herbicide treatments for yellow starthistle control
Both Milestone® and Transline provide excellent control of yellow starthistle.
- Milestone at lower rates of active ingredient provides the same level of yellow starthistle control as Transline.
- Transline® may be more effective on yellow starthistle at later growth stages such as bolt to early bud, compared to Milestone.
- The optimum time to apply Milestone to control yellow starthistle AND maximize annual grass production is when yellow starthistle is at the seedling to early spring rosette growth stage, which is an earlier timing window compared to applying Transline.
DiTomaso J, V Carrithers, E Flora, W Belles. 2005. Control of yellow starthistle (Centaurea solstitialis) with aminopyralid. In Proceedings Western Society of Weed Science. P 61.
Wallace J, and T Prather. 2009. Yellow starthistle control with aminopyralid on Idaho rangeland. WSWS Research Progress Report. Pg. 32-33.