Fall is a great time to work on controlling perennials such as Canada thistle and dandelions. As temperatures get cooler, the nutrients start moving down into the roots and if a herbicide can be taken up and translocated with those nutrients, it can equal better control.
A frost event though, can kill some weeds or can damage leaf tissue which will reduce herbicide uptake and therefore reduce the level of weed control. Within the next few days after the frost, you need to assess the target weeds in the areas that you want to obtain control – are the weeds still growing? How much leaf tissue has been damaged?
If the plant leaves are still shiny green with minimal leaf tissue damage (i.e. not blackened/brown or brittle) there still may be the window to control the perennial, biennial and winter annual weeds with glyphosate. If you do spray – spray in the afternoon when temperatures are warm and sunny, this will help with herbicide uptake. Use rates appropriate to the stage and time of year – fall applications of glyphosate are recommended at a higher rate than when controlling pre-harvest.
One last thing – look at the forecast for the next week following the application. If the temperatures look like they are going to continually be below freezing each night, it may be too late to make the application to get the economic control you are looking for.