Canada thistle (Cirsium arvense) is a widespread noxious weed found throughout the northern half of the United States. Field studies were established on Canada thistle in several states in the upper Midwest to measure effectiveness of herbicides applied at various application times including mid-summer and fall. Milestone® herbicide at 5 and 7 fluid ounces per acre (fl oz/A) was applied at various growth stages from May through early November (Figure 1).
Plot design was a randomized complete block with three replications of each treatment. Evaluations of herbicide treatments were made one year following treatment. Control was determined from visual estimates of Canada thistle shoot growth reduction relative to the non-treated control.
Milestone® herbicide at either 5 or 7 fl oz/A applied in June/ July (bud to flower growth stage) and throughout summer and early fall provided significantly better control one year after treatment than applications made in May (rosette to early bolt). Control declined slightly with applications made at the seed dispersal/dormant growth stage compared to bud/flower-stage, but was greater than 80 percent with Milestone at the 7 fl oz/A rate (Figure 2).
Milestone applied in fall resulted in greater control when applied in September compared to late October or November, indicating that new basal growth in September may be the optimal time for fall applications (Figure 3).
Results of these studies indicate that the optimal time to control Canada thistle plants and stop seed production is to apply Milestone herbicide when Canada thistle plants are at bud to early flower growth stage. However, good control of Canada thistle plants can be achieved at later growth stages. Apply the higher rate of Milestone at 7 fl oz/A at late flower to early seed dispersal to achieve greater than 80 percent control. Fall herbicide applications are effective if green growth is present on Canada thistle stems and basal rosettes. Milestone at 7 fl oz/A was more consistent than lower rates of Milestone at seed dispersal and late fall application timings.
Almquist TL, RG Lym. 2010. Effect of aminopyralid on Canada thistle (Cirsium arvense) and the native plant community in a restored tall grass prairie. Invasive Plant Science Management 3:155-168
Burch PL, RG Wilson, ES Hagood, RA Masters, and VF Carrithers. 2006. Canada thistle management with Milestone herbicide. Weed Science Soc. of Am. 46th Conf. In Proceedings. New York, New York.
Deneke D, M Moechnig, D Vos, J Alms. 2012. Optimal herbicide application timing for Canada Thistle Control. In Proceedings, Western Society of Weed Science. p. 22-23.
Dow AgroSciences LLC. Internal field data.
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