Post harvest glyphosate applications are a very effective method to gain control of perennials and winter annuals before heading into the winter. Controlling them in the fall is effective because weeds such as Canada thistle, dandelions etc are beginning to shut down for winter and therefore actively translocating sugars to their roots to over winter, the glyphosate gets translocated efficiently down as well, killing the plants at their roots. This leaves a much cleaner field come spring to seed into and reduces the headache of trying to get rid of these difficult to control weeds (Note: fields should still be considered for pre burn applications in the spring).
With all of this said, the temperature fluctuations and potential frost threats in the fall make spray timing a more difficult task, so it is best to follow some simple guidelines when it comes spraying at this time of the year.
Glyphosate (like most non-residual actives) is a product that can be touchy if sprayed at a time when it is not readily absorbed and translocated within the plant. This means to watch time of day spraying (glyphosate is notorious for being less effective sprayed in the late evening or early morning) and conditions before and after the frost, on top of the weed species you are targeting and the severity and length of the frost. Lots of growers believe that if a killing frost has already hit, their post harvest spray season has ended, but that isn’t necessarily the case.
If you have a hard frost of (-4 degrees C or colder) then you must wait before going into spray any weeds. It is best to wait anywhere from 48-72 hours before going into assess the severity of the frost damage. If you go out and see the plants are still remaining over 50-60% green, there are signs of active growth and the weather conditions have begun to be more favourable, then aiming to spray the next day once temperature have risen to above 10 degrees C for over 2 hours would be your next opportunity to hit that field. Remember, there shouldn’t be frost in the forecast for the next couple days either.
If there was simply a light frost (0 to -3 degrees C), this only slightly affects perennial and winter annual weed control. With that said, time below freezing can come into play and it is still good to go out the next day and assess the plants. Typically, if it was a light frost like this and you go out and spray the following day once temperatures have reached 10 degrees for a couple of hours (and no frost in the forecast) then you should be safe. Giving the plants an extra 24 hours after a light frost can be an effective strategy though.
It is best to spray glyphosate in the middle of the day, especially late in the season like this. If you can catch a warmer, sunnier day, then it is even better. To further it even more, if you get a nice shower, go in the next day and you will see an increase in efficacy as the fall moisture gives the plants a big perk and from what I have seen significantly helps fall glyphosate effectiveness. As for rates, I personally don’t like to see less than a 1L equivalent of glyphosate used when trying to take down any perennials or winter annuals.
I always recommend putting a second chemical in with the glyphosate such as 2,4-D, Express, Dicamba, Florasulam etc. as having a second mode of action is effective in decreasing the potential of glyphosate resistance and also because these products all have a residual in some capacity. I am not going to touch on these to much as the date they are used, rates, soil texture, soil moisture, soil temperature, organic matter and more come into play when determining what can be seeded there next year, if you have any specific questions feel free to ask or talk to your local agronomist as they will be aware of what products will be effective for your situation in your area. I will note that florasulam and Express (tribenuron) typically have strong activity on asteraceae family weeds which is what family our thistles fall into.
Dandelions are generally considered to be the most frost hardy, while Canada thistle, sow thistles and perennial/winter annual grasses are thought to be less frost tolerant. Also, keep in mind if you were late harvesting you must assess the plants regrowth and you should see new leaves being put out by the plant before spraying.
Spraying in the fall is the most effective way to clean up the perrenniels/winter annuals in your field so if you can take advantage of that you’ll be happy you did come the next season.