If you’re planning to apply pre-emergent products on your lawn to prevent annual summer weeds, late February-early March is normally the best time to do so. City Parks and Recreation staff say that soil temperatures will be quickly rising soon and weed seeds will start to germinate, so it’s important to have the pre-emergent in the top portion of the soil profile so that any new weed will emerge through the product, then wilt and die. Depending on what type of pre-emergent is used, watering in the product will help the chemical reach and saturate the soil.  

Texas A&M AgriLife Extension recommends that herbicide treatments should always be applied per manufacturer’s labeled instructions and only for the types of weeds specific to your lawn. Use caution when applying chemicals around trees, and avoid “over applying,” as this can cause increased pollution flowing into storm drains and waterways. 
More information: A Homeowner’s Guide to Herbicide Selection for Warm-Season Turfgrass Lawns – Texas A&M Agrilife Extension Service (tamu.edu)