What is pre-emergent and why do you need it? Pre-emergent is used to prevent summertime annual weeds. You want to apply herbicides when the soil temperature is around 55 degrees or above for at least 36-72 hours. If you are using granule herbicides, you want to water that into the soil to release the herbicide that is trapped in each granule. Just beware, this application does nothing for the current weeds that are present. This is to aide in having a weed free lawn in the summer.