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Controlling Canada thistle with Milestone

By Carlyle Holen, IPM Specialist, U of Minnesota Extension

What is the optimum time to treat Canada thistle (Figure 1) in non-cropland with Milestone? Based on field trials at Ada in 2007 the window for application is pretty wide and perhaps a better way to frame the question might be: What is the least effective time to treat Canada thistle?

In the Ada trials, applications were made at two week intervals from June 1 to August 23 (Table 1). We found new shoots are initiated on a nearly continuous basis during the growing season with a large 'flush' of new rosettes in the spring and in late summer/early fall after older plants have finished dispersing seed. During the spring 'flush' new stems are rapidly added by the extensive root system and a single plant may have dozens of individual, interconnected stems.

Figure 2 shows the increase in Canada thistle stem number, from June 1 to August 23 from untreated plots. In the two week period from June 1 to June 15 there was a 44% increase in stem number and from June 15 to June 29 the increase was 18%. The speed of shoot emergence is driven initially by soil temperature and continues at a fairly rapid pace until plants begin to flower. With a continuous emergence of new Canada thistle shoots you will find stems that are blooming next to ones that are just emerging. When staging plants make your assessment on the most advanced plants in the patches.

Milestone provided excellent control at the 5 and 7 oz/a rates on all but two of the application dates (Table 2). On June 1, Milestone applied at 5 oz/a resulted in significantly lower control compared to the other application dates at the same application rate. We speculate that incomplete emergence of rosettes resulted in less total uptake of herbicide by the interconnected plant. Uptake of Milestone by Canada thistle is believed to be less efficient through the soil compared to leaf tissue. Though not statistically significant, the June 1 application timing at 7 oz/a rate was the least effective timing at this herbicide rate. We are not certain if the lower control on the July 23 at 5 oz/a, was related to less translocation of herbicide to the root system during seed shed as mature plants were declining in quality. We believe it is more likely this reduced control was related to very stressful conditions (100 F) at application.

Based on this one year of data, the optimum application timing for Milestone appears to be quite long:
  1. Applications made when the most advance plants at the site are in the bud stage of development through the month of August appear equally effective. However, it makes sense to avoid applications when there is strong environmental stress such as drought and high temperatures.
  2. Milestone applied at the 7 oz/a rate appears to provide more consistent control across all application dates (not statistically significant) and is especially evident during timings when plant stage or environmental stress is less than optimum.

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