In central Ohio, for example, it is typically recommended to apply an herbicide by the application deadline of April 15, or April 22 if the homeowner uses a product that contains the active ingredient pendimethalin, which also has some early post-emergent activity. The deadline in southern Ohio is about April 1, and in northern Ohio, about April 30.
There are several pre-emergent products on the market, including an organic product that contains corn gluten meal. This product is available through specialty garden catalogs and is more expensive than the synthetic products.
Though a few crabgrass seeds germinate by the April 15 deadline date, the vast majority of seeds germinate in May. Germination begins when the temperature in the top inch of soil reaches 52-54 degrees or more for at least 5-7 consecutive days; soil moisture must also be present. Seeds also need light to germinate, so those thin areas in the lawn are likely places for crabgrass to become established.
If you have seeded the lawn this spring and still want to prevent crabgrass, there is only one pre-emergent herbicide to use that won't kill the lawn grass seed. Look for siduron or Tupersan and to be aware that this product is more difficult to find and more costly than other products.
As with other pests, it is recommended that treatment be targeted. If crabgrass was a problem in the lawn last season, and it was allowed to go to seed, then you will probably want to apply a pre-emergent herbicide. If you are cutting new ornamental beds in the lawn, if areas along the driveway or sidewalks died out last season, or if the lawn is thin from grubs or disease injury, you might want to treat those areas with a crabgrass preventer. If you maintain a dense lawn and have not seen any crabgrass in the past two or three years, you probably don't need to apply a herbicide.