How late in the fall can one go when applying a postemergent herbicide for broadleaf weeds? As long as the broadleaf weeds are photosynthesizing and appear green, a postemergent product can be applied. These products work faster when the plants are actively growing, these herbicides will still work. The chances are that you may not even see any effects on the weeds this year but over the winter they will die. If applying a postemergent herbicide try applying on dry sunny day when temperatures are warm (> 50 F). If you have problems with winter annual broadleaf weeds like common chickweed, a late application of a broadleaf herbicide. The justification for this application is that these winter annuals are normally late germinators in the fall.