Timing is critical, and truly watching the forecast diligently is important. The optimal time to apply pre-emergent herbicides is about two weeks before the crabgrass germinates for the year, which will occur when soil temperatures reach about 55 degrees Fahrenheit. Soil temperatures can be checked with a soil thermometer or even a kitchen meat thermometer. Typically these soil temperatures are reached around mid to late May, so pre-emergent would be applied between early to mid-May; however each year varies. Each yard and even separate areas within your yard might obtain these conditions at different times, especially along sidewalks or driveways. Applying herbicide across the entire yard may not be necessary, instead target areas where crabgrass has been an issue in the past. Overall this will result in the use of less chemicals and cost savings.