For most of Kansas, crabgrass typically begins to germinate around May 1. Therefore, April 15 is a good target date for applying the preventer because it gives the active ingredients time to evenly disperse in the soil before crabgrass germination starts. Since the weather varies from one spring to the next, and with it the timing of crabgrass germination, it is often better to base timing on the bloom of ornamental plants. The eastern redbud tree is a good choice for this purpose. When the trees in your area are approaching full-bloom, apply crabgrass preventer. A follow-up application will be needed about 8 weeks later unless you are using Dimension (dithiopyr) or Barricade (prodiamine), each of which will provide season long control.
Control of crabgrass after it has emerged is more difficult but products containing quinclorac (Fertilome WeedOut with Crabgrass Killer, Ortho Weed-B-Gon Plus Crabgrass Control, BioAdvanced All-in-One Lawn Weed and Crabgrass Killer, Drive, others) are usually effective. Clippings from lawns treated with quinclorac should be returned to the lawn or discarded. Do not use clippings to mulch vegetable or flower gardens as they can harm garden crops.
Information from KState Research and Extension