Crabgrass preventers are another name for preemergence herbicides that prevent crabgrass seeds from developing into mature plants. Many people have a somewhat vague idea of how they work. They do not keep the seed from germinating. They form a thin barrier near the surface and intercept the young germinating plants.
Crabgrass preventers are just that – preventers. With few exceptions, they have no effect on existing crabgrass plants, so they must be applied before germination. Additionally, preventers do not last forever once applied to the soil. Microorganisms and natural processes begin to gradually break them down soon after they are applied. 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, but there is considerable variation among products.
For Northwest Missouri, crabgrass typically begins to germinate around April 25 or a little later. April 15 is a good target date for applying preventer because it gives the active ingredients time to evenly disperse in the soil before crabgrass germination starts. Additionally, the 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 approach 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 give season-long control of crabgrass from a single application. In fact, they can be applied much earlier than April 1 and still 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 that have many customers to service in the spring.
Though Dimension cannot be applied as early as Barricade, it is the herbicide of choice if it must be applied 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 (two- to three-leaf stage). Dimension is also the best choice if treating a lawn that was planted late last fall. Normally a preemergence herbicide is not recommended 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. Lawns established in the fall can be safely treated with Dimension the following spring even if they have not been mowed.
Note that products containing 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.
Jim Crawford is a field specialist in agricultural engineering for the University of Missouri Extension in Atchison County.