When should I apply a preemergence herbicide to my lawn to control crabgrass?

FAQ
Question: 

When should I apply a preemergence herbicide to my lawn to control crabgrass?

Answer: 

The keys to successful control of crabgrass in lawns are correct timing of the preemergence herbicide application and proper application of the material.  Preemergence herbicides must be applied before the crabgrass seeds germinate.  If the material is applied too early, crabgrass seeds that germinate late in the season will not be controlled.  If applied too late, some crabgrass seeds will have already germinated.  Preemergence herbicides should normally be applied in early to mid-April in southern Iowa, mid-April to May 1 in central Iowa, and late April to early May in northern areas of the state.  Weather often varies considerable from year to year in Iowa.  Accordingly, gardeners should make minor adjustments in the timing of the preemergence herbicide application.  If the weather in March and April is consistently warmer than normal, apply the preemergence herbicide early in the normal time period.  Apply the herbicide late in the recommended time period if Iowa is experiencing a cold early spring.  If you’re still uncertain as to when to apply the preemergence herbicide, Mother Nature provides some helpful (colorful) clues.  Preemergence herbicides should be applied when the forsythia blossoms start dropping or when redbud trees begin to bloom.  Crabgrass seed germination typically begins after these events. 

To insure the herbicide is applied properly, carefully read and follow the label directions on the package.  Also, make sure the spreader has been correctly calibrated and is working properly.