Submitted by Jim Crawford, Field Specialist in Agricultural Engineering, UM Extension, Atchison County

Crabgrass preventers are another name for pre-emergence 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 active ingredients time to evenly disperse in the soil before crabgrass germination starts. 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 approach full bloom, apply crabgrass preventer. A follow-up application will be needed about eight weeks later unless you are using Dimension or Barricade.

Products containing Dimension and Barricade are the only two 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

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.

For more information on managing your coll season lawn, you can refer to publication G6705 Cool-Season Grasses: Lawn Maintenance Calendar at https://extension.missouri.edu/publications/g6705.