Proper Timing for Crabgrass Preventers

Crabgrass preventers are another name for preemergence herbicides that prevent crabgrass seeds from developing into mature plants. Many people have a somewhat foggy idea of how they work. They don’t actually keep the seed from germinating; rather, the very young germinating plant is killed. Crabgrass preventers are just that – preventers. With few exceptions they will have no effect on existing crabgrass plants. Therefore, preventers have to be applied before crabgrass germinates. Additionally, they don’t last forever once applied to the soil. Microorganisms and natural processes begin to gradually break them 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. See below.)

For our area, crabgrass typically begins to germinate around May 1, or a little later. 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. The April 15 target works well for most of the state, however for parts of Missouri & Kansas April 1 is more appropriate, and for northwest Kansas May 1 is best. Additionally, weather varies from one spring to the next, and with it the timing of crabgrass germination. For this reason 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 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.

Though Dimension cannot be applied as early as Barricade, it is the herbicide of choice if you must apply later than recommended. It is the exception to the rule that preemergence herbicides do not kill existing weeds. Dimension can kill crabgrass as long as it is young (2 to 3 leaf stage). Dimension is also the best choice if treating a lawn that was planted late last fall. Normally we recommend not using a preemergence herbicide unless the lawn has been mowed 2 to 4 times. However, 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 for Barricade is prodiamine. Remember, when using any pesticide, read the label and follow instructions carefully.


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