Products containing corn gluten meal are available at garden centers and mailorder companies. 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 germinates. If the material is applied too early, the crabgrass that germinates late in the season will not be controlled. If applied too late, some crabgrass will have already germinated.
Weather often varies considerable from year to year in Iowa.
Accordingly, gardeners should make 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 does supply some helpful clues. Crabgrass seed germination usually coincides with the time forsythia blossoms start dropping or when redbud trees reach full bloom.