Pre-emergent is applied to stop crabgrass in spring. It does need to be watered in with about 1/2" of rain or irrigation, but what happens if you get a massive downpour right after you throw down? A real gully-washer? Is your prodiamine application ruined and washed away? Should the prodiamine be applied again?
Prodiamine has been used for many, many years by lawn lovers and professional lawn services alike with great success and continues to be used today. In those many years, rain has been thrown down from above after an application, and sometimes nature wins, but sometimes you also need to have a little faith that the Prodiamine will do its job.
As far as reapplication, you do not want to reapply your pre-emergent. You don't know for sure if this product has been washed out a little, a lot, or not at all. So, you don't reapply because you don't know and you don't want to be irresponsible by doing that.
Variables such as rain are also the reason that we recommend what Allyn calls the "split application strategy" for pre-emergent. This strategy calls for an application of pre-emergent as soil temperatures cross 50 heading to 55 in the spring and a second application as temperatures cross from 60/65 into 70. The second application is your backup and will give you peace of mind in the midst of torrential rain landing in the lap of your first application.
For more information on this, please see the video below:
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