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.
See more from Shane Thomas’s blog here.