No need. If there are existing weeds, they will die or go dormant. Preen is not what you need for these anyway. As far as new growth coming up between now and freezing, it will be few and far between. That stuff has to have a couple of weeks to really form a barrier, anyway, so there just isn't much sense. Waith until early spring, and apply it when the soil temps are preparing to reach 55 degrees (50ish) and higher.