So, when should you apply preemergence herbicide in the spring? Where I live, it is typically during early March, but there is a universal way to get the timing right no matter where you live. When dayime temperatures reach 65-70 degrees for four consecutive days, it is time to apply the preemergence herbicide. This will control seed germination of summer annuals.
Here in coastal South Carolina, we are about at the right time for a spring application. With the exception of Thursday, we are in a week-long-plus warm spell that meets the criteria set out above. High temperatures most days will exceed 65 degrees.
What about fall? Here, PE is applied around March 1st, but, for everyone else, it is when nighttime temperatures reach 55-60 degrees for four consecutive days.