A key to the successful control of annual grasses (such as crabgrass) in established turf is the correct application timing of preemergence herbicides. Preemergence herbicides should be applied by May 1st in central Iowa. Dr. Christians has noticed that this date does not vary much from year to year, after monitoring germination dates for the last 34 years.
In addition to timing: application uniformity, using recommended product rates, and the requirement of (1/2 inch) irrigation within 3-5 days of application can play a vital role in crabgrass control.
Several products are available for effective annual grass emergence control. These products vary slightly in mode of action, length of control, specific weed efficacy, desired turfgrass seed inhibition, and early postemergence control. Benefin, benefin + trifluralin,bensulide, oxadiazon, siduron, pendimethalin, mesotrione, prodiamine, isoxaben, and dithiopyr are preemergence products available in the market today.
Please note that some products are not labeled for certain turfgrass species. For example, oxadiazon is not recommended for use in fine fescue; however, oxadiazon provides excellent goosegrass control in Kentucky bluegrass. Always read thoroughly and follow the label directions. Remember, the label is the law.
Dithiopyr and prodiamine have the longest window of effectiveness and can control weeds for up to 16 weeks. Dithiopyr and mesotrione offer early postemergence control when applications are made following weed emergence. Siduron and mesotrione have a unique property that allows herbicide application to seeded areas. Siduron selectively controls weedy annual grasses such as crabgrass, foxtail, and barnyardgrass, while allowing the desired turfgrasses to grow. Mesotrione is only labeled for preemergent use on newly seeded Kentucky bluegrass lawns. All of the other preemergent herbicides kill the seeds of the cool-season grasses and cannot be used at the time of seeding.
Fertilizer-herbicide combinations are sold at most retail stores. This allows homeowners to combine the two operations into one application. A disadvantage of the combination is that the proper time for weed control often does not coincide with the optimum time to fertilize. Combinations with preemergence herbicides are generally effective in controlling annual grass weeds as long as applications are made at the appropriate time and recommended amount.
In addition to annual grassy weeds, a spring application of a preemergence herbicide will control annual broadleaf weeds, such as prostrate knotweed and spurge. A second application at a reduced rate may be necessary for season-long control.
Paying attention to herbicide timing, application uniformity, product and rate, and ensuring (1/2 inch) irrigation within 3-5 days of application will help prevent annual grass (crabgrass) invasion. Below you will see two pictures of crabgrass in an early leaf-stages.
The last picture is a general guide of preemergence application dates via Quali-Pro's Prodiamine label: