Crabgrass preventers are herbicides. The germinating seed takes up some of the herbicide and is killed. These herbicides work well on the target crabgrass seed, but the same herbicide works on almost any other seed that is germinating during the chemicals’ effective life. You cannot apply crabgrass preventer and overseed your lawn in the same season.
But crabgrass preventers don’t last forever once applied to the soil. Micro-organisms and natural processes begin to gradually break herbicides down soon after they are applied. Therefore, if some products are applied too early, they may have lost much of their strength by the time they are needed.
Most crabgrass preventers are fairly ineffective after about 60 days, although there is considerable variation among products. Dimension and Barricade last longer.
For most of Kansas, crabgrass typically begins to germinate around May 1, or a little later. Therefore, April 15 is a good target date for applying the preventer. It gives the active ingredients time to evenly disperse in the soil. Weather varies from one spring to the next, and with it the timing of crabgrass germination.
Base your application timing on the bloom of ornamental plants. When the Eastern redbud tree trees in your area are approaching full bloom, apply crabgrass preventer.
A follow-up application will be needed about eight weeks later unless you are using Dimension or Barricade.
Dimension and Barricade are the only two products that will give season-long control of crabgrass from a single application. In fact, they can be applied much earlier than April 15 and still will have sufficient residual strength to last the season. Barricade can even be applied in the fall for crabgrass control the next season.
Dimension can be applied as early as March 1. Because of the added flexibility in timing, these products are favorites of lawn care companies who have many customers to service in the spring.
Dimension is also the best choice if treating a lawn that was planted late last fall. Normally we recommend not using a pre-emergence herbicide unless the lawn has been mowed two to four times. But Dimension is kind to young, tall fescue, perennial ryegrass and Kentucky bluegrass seedlings and can be applied as early as two weeks after the first sign of germination.
Therefore, lawns established in the fall can be safely treated with Dimension the following spring even if they have not been mowed.
Note that products that contain Dimension and Barricade may use the common name rather than the trade name. The common chemical name for Dimension is dithiopyr and Barricade’s is prodiamine.
Remember, when using any pesticide, read the label and follow instructions carefully.