Dimension is unique in that it can be applied after the crabgrass has germinated, as long as the plants are still small. And for all you procrastinators, Dimension can be applied through early May with very good results. Both products have longer residual activity than other crabgrass preventers. Most years, April 1-15 is a good time to apply the preventer. This allows sufficient time for the herbicide to evenly spread and form an effective barrier in the soil before crabgrass germination starts.
Weather varies from one spring to the next and affects when crabgrass begins to germinate.
For this reason, application timing for crabgrass preventers is sometimes based on the bloom of ornamental plants. If there aren't redbuds in your area, or you are unsure about using this method, a soil thermometer is the best way to predict crabgrass germination. Crabgrass germinates when soil temperatures reach 55 degrees. Soil thermometers can be purchased for minimal cost at most garden stores and are also useful for timing garden plantings. Remember, when using pesticides, read the label and follow instructions carefully.