Because of the dramatic swings in air temperature that we’ve experienced already this spring, we’ve received a few questions about the effect of air temperatures on our spring burndown applications.
As a general rule, when air temperatures fall below 40 F for an extended period of time after a burndown herbicide application has been made, weed control will most likely be reduced.
Specifically, most people are asking, ‘When is it too cold to apply a burndown herbicide?’ and ‘What are the conditions that lead to poor weed control following a burndown herbicide application?’
As a general rule, when air temperatures fall below 40 F for an extended period of time after a burndown herbicide application has been made, weed control will most likely be reduced. This is especially the case with any burndown application that includes glyphosate (Roundup, Touchdown, etc.), which is a systemic herbicide and needs time to penetrate the leaf cuticles and move throughout the plant in order to have optimum activity. Weed control will likely be even poorer if you have made a burndown application and there is an extended period of cool, cloudy conditions following that initial drop below 40 F.
To make this even more complicated, all herbicides are not going to respond the same way to applications made at high and low air temperatures, and all weeds are not going to respond the same way to different temperatures regimes at the time of application. Although there have been very few studies published on this topic in the weed science literature, weed scientists in Illinois did an experiment on this a few years ago and ultimately found that lower temperatures (<60 F or so) at application had a significant impact on glyphosate activity on henbit, but had very little influence on common chickweed control with glyphosate. In this same study, they reported that glyphosate seemed to be more sensitive to low air temperatures at application than paraquat (Gramoxone).
So what can you do about all this? The simple answer is to watch your forecasts closely and to wait for more favorable temperatures to arrive before you make your burndown herbicide application. We realize that this might not always be possible and that this decision must also be balanced by the size of the weeds at the time of the application — you don’t want to wait so long that your weeds have exceeded the optimum size for control, as can easily occur with horseweed and giant ragweed at this time of year.
If there is no other alternative other than to spray and you know cool conditions are going to persist after application, you may want to increase the rate of glyphosate or whatever burndown herbicide you are using and consider at least one other tank-mix partner to ensure the best chance of burndown success.