when is the ideal time to put on preventer in this area?
Moody County South Dakota
Taken from iGrow article: http://igrow.org/gardens/gardening/americas-most-weeded-crabgrass/ This should answer your question:
Control of crabgrass in lawns is of primary concern for most people. Homeowners spend lots of money each year trying to prevent crabgrass from growing in their lawns, usually using a weed-and-feed type product that is primarily a fertilizer that also contains an herbicide to kill crabgrass seedlings as they are just beginning to germinate. There are several active ingredients that are effective in crabgrass prevention, but one should check the label to be sure the weed-and-feed product they are applying is in fact for crabgrass prevention and not for the control of broadleaf weeds like dandelions. Those products will generally have no effect in controlling crabgrass. Commercial lawn care companies may also be able to apply the crabgrass preventer product alone, not in combination with a fertilizer but those products are usually not available to home owners.
Crabgrass preventer products are usually quite effective if they are applied at the right time. People often refer to the time when lilacs or Forsythia are blooming as an indicator as to the proper time to apply the product to their lawn. If you apply it too early, it may lose its effectiveness before the end of the summer, allowing for a late flush of crabgrass to emerge and grow. But, if you wait too long in the spring to apply it, you might miss the first flush of seedling germination and still have a major crabgrass problem. Once the seedlings are up and growing these products will not kill the seedlings. One other word of caution – crabgrass preventers will also prevent other kinds of weed seeds from germinating, as well as preventing the grass seed that you might apply that spring or summer to fill in bare areas or to thicken and existing stand of grass. If you plan to do some seeding, do it late in the fall to avoid problems with spring applied crabgrass preventers or simply do not use them at all in those areas.