A shot of glyphosate post-harvest and just before winter is the ideal way to gain control of perennials and winter annuals. Controlling them in the fall is effective because perennial weeds, such as Canada thistle and dandelions, are beginning to shut down for winter and are actively trans-locating sugars from the leaves to the roots to over-winter. Glyphosate gets trans-located efficiently with the sugars, killing the growing points of the plants. Winter annuals are smallest and easiest to kill in the fall. A fall-applied burn-off is the most effective kill for these types of weeds; a pre-seed burn-off is still likely required to kill spring annuals.
That said, temperature fluctuations and frost in the forecast can make fall spray timing a more difficult task, so it is best to follow some simple guidelines when it comes spraying at this time of year.
Glyphosate, like most non-residual actives, can be touchy if sprayed at a time when it is not readily absorbed and trans-located within the plant. Glyphosate’s effectiveness wanes when applied in the late evening or early morning and conditions before and after frost. This leads some farmers to believe that a killing frost ends the post-harvest glyphosate window but that isn’t necessarily the case. How hard a frost and the re-growth conditions after a frost dictate when the spray window closes.
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 weeds are still remaining over 50% to 60% green, and 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. It’s 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.
It’s best to spray glyphosate in the middle of the day, and the warmer and sunnier the better. If possible, go in the day after a light rain as you’ll see better efficacy once plants perk up from the drink. 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 some residual (glyphosate does not). 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 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.
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