There is still time this fall to apply herbicides for control of winter annuals and marestail.
Regarding the question of how late fields can be treated, we have applied well into December under cold conditions and still maintained control with effective treatments.
Applying under cold conditions slows the rate of kill but this is not an issue for fall treatments anyway, since the goal is to have fields free of weeds by early spring. We have had some cold nights but no extended periods of freezing weather, and it appears to us that the winter weeds that are targets of fall applications are mostly green and healthy.
A few other reminders:
There is no need to include herbicides with residual activity at this point, since emergence of additional plants is largely finished for the fall. None of the residual herbicides except products that contain chlorimuron (Canopy, Cloak) provide control of weeds that emerge in the spring. So if you were adding metribuzin or other residuals for control of weeds that could emerge later in fall, these can be omitted. Note: metribuzin plus 2,4-D does control most emerged winter annuals so that is still a viable option even where residual is not desired.
We have heard a lot about the use of 2,4-D/dicamba premix products this fall, possibly due to availability or price. This broad-spectrum mix can be weak on a few weeds, and dicamba has not been an especially effective cold-weather herbicide in our research. We suggest the addition of some glyphosate, metribuzin, or other herbicide to 2,4-D/dicamba treatments, especially for late fall applications in cold weather.
- A reminder that fall is the most effective time of application for control of wild carrot and poison hemlock. These are biennial weeds and the low-growing first-year plants present at this time are much more susceptible to herbicides compared with taller plants next spring. Take advantage of this opportunity to apply herbicide to field borders, roadsides, fencerows, etc for control of these weeds. We suggest using a mixture of glyphosate plus 2,4-D and/or dicamba. Where the goal is not to kill grass, options include a 2,4-D/dicamba mix and Crossbow.