When to apply weed prevention
You did not mention what weeds you are trying to control. Take a look at our publication on Turfgrass Maintenance and look at Spring tips for weed control.
If nitrogen fertilizer was applied in the fall, a spring application should not be necessary. http://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/HG112_Turf...