December applications sometimes work but weeds are normally hardened off by then, reducing your odds of success. Spring herbicide applications can be as effective as fall applications in controlling winter annual weeds. However, since weeds are allowed to compete with crops for many more months, gaps left by dying weeds may be filled in by summer annual weeds. Cultural methods of controlling winter annual weeds can also be effective during the fall. Early planting of winter wheat or other winter annual crops can give those crops a head start on weeds when they emerge later on.
Tillage also can control flushes of winter annuals following late summer or fall rains.
However, delaying planting can cut yields and tillage can reduce soil moisture. Biennials, including many species of thistle or common mullein, usually germinate during the summer and are present in the fall as rosettes. Take your time in scouting for biennial weeds in the fall. They may be difficult to see within crops and other desired vegetation. Often, these plants are not noticed until spring when they develop large, vigorous flowering stalks.