It’s always amazing to see apparently still-thriving winter annual weeds underneath the snow or following some really cold weather. Our best advice at this point on fall spraying is that once fields dry or freeze up enough to allow traffic again, there is still considerable benefit to applying herbicides for control of marestail and other weeds that persist through winter. We expect the rate of herbicide activity to slow considerably compared to the application made a month ago when it was warm.
We probably have not applied herbicides following a period of weather exactly like the one we just experienced, but we have in the past applied into late December during or following cold weather, and the herbicides still seem to eventually work. Keep in mind that not treating fields with a history of marestail problems this fall can make for a more challenging situation next spring, and more variability in control.
This can be adjusted for by using a more aggressive combination of burndown herbicides next spring, or applying earlier in spring and beefing up the residual herbicide component, or using two spring pre-plant treatments (early and at-plant).
The goal of fall treatments is to ensure that the spring herbicide program has to address only small plants that start emerging in late spring, which allows more flexibility in herbicide choice and application timing.