So, when should you apply pre-emergent herbicide in the fall? Where I live, it is typically during the second half of September, but there is a universal way to get the timing right no matter where you live. When nighttime temperatures reach 55-60 degrees for four consecutive days, it is time to apply the pre-emergent herbicide. This will control seed germination of winter annuals - perennial weeds, such as Florida Betony, will not be affected.
What about spring? Here, PE is applied around March 1st, but, for everyone else, it is when high temperatures reach 65-70 degrees for four consecutive days.