With fall just around the corner it’s a great time to consider treating biennial and perennial thistles. Research has shown the fall timing to be an effective time to treat these as long as foliage isn’t damaged by frost. Research conducted last year found excellent control of Canada thistle with fall applications of Milestone at 7 fl oz./a when applied just prior to the first frost (October 9^th), just after the first frost (October 15^th), and after several frosts (November 8^th). Note that a week prior to the November 8^th treatment the temperature dropped below 32 F for 5 consecutive days with temperatures below 23F for 3 consecutive days. Despite this, foliage appeared relatively healthy at the time of application.   Glyphosate was also applied during the first two timings as a comparison, with good control the following spring. Results are summarized in Table 1. Realize that Milestone will persist through the winter, so may be providing additional suppression the following spring that glyphosate will not. 

 

Table 1. Environmental conditions and cover of Canada thistle at various fall timings. Conducted in 2007 at Madison, WI.

Name	rate	Unit	DATE APPLIED	Max/Min Air temp day before app.	Max/Min Air temp day of application	Max/Min Air temp 1 day after app.	Canada Thistle % cover 5/23/07
Milestone	7	fl oz/a	PRE FROST 10/9/06	73/47 F	64/ 46 F	54/47 F	0.0 c
Milestone	7	fl oz/a	POST FROST 10/15/06	45/34 F	54/31 F	56/47 F	0.8 c
Milestone	7	fl oz/a	POST FREEZE 11/8/06	56/ 43 F	69/46 F	65/42 F	0.8 c
Glyphosate	0.75	Lb ae/a	PRE FROST 10/9/06	73/47 F	64/ 46 F	54/47 F	15.5 b
Glyphosate	0.75	Lb ae/a	POST FREEZE 10/15/06	45/34 F	54/31 F	56/47 F	13.8 b
Untreated Control			NA	-	-	-	40.0 a

Our extended fall with unseasonably warm temperatures may have contributed to the lack of differences and I would not expect the same result every fall, especially with the post freeze timing. For example my predecessor Jerry Doll performed a similar trial in 1997 with Stinger and found that applications applied pre frost and post frost at 0.5 – 1.0 pt/A provided > 90% control the following spring, but applications post freeze had poor results (57 and 3 % control). While we are not sure why control varied between experiments, average air temperatures were below freezing the day before, day of, and day after application during the 1997 study, while conditions were much warmer in the 2006 study. Also many of the leaves showed visible damage from the freeze. Both of these factors are likely involved in the loss of control. So when trying to decide whether to make an application or not, check the air temperature and leaf tissue of plants, as this may be a good indicator of future success/failure. 

 

Current research is evaluating the timing of several of the common pasture herbicides used for biennial thistle management. We will be applying treatments in late September, October, and November and record temperature data at the site. It is our hope that this information will provide us with more detailed information as to how late you can treat thistles and still observe effective control the following spring. 

I have recently updated the Agronomy Advice fact-sheet (Thistles in Pastures and Beyond: Biology and Management) that provides specific information with regards to the biology and management of these weeds. This fact-sheet also has a specific section that discusses fall thistle management options.

See the attached document  >>>   Thistles_2007.pdf