Pre-emergent herbicides may fail for several reasons. If you do not sufficiently water in the herbicide as stated on the label, some of the active ingredient may be lost to the atmosphere, reducing its effectiveness. This is more of a problem with liquid formulations than granular formulations. Excessive rainfall can also shorten the time period that the herbicide is effective. It is also important that you apply the product at the correct rate, uniformly across the lawn. Pre-emergent herbicides are most effective at preventing crabgrass if you have a dense stand of turf. If you had severe crabgrass problems in the past, you may want to make two applications of pre-emergent herbicide. Make the first application in late winter/early spring, and a second application six to eight weeks later.

Check with your local extension office for chemical recommendations and timing.