Crabgrass preventers are a family of pre-emergence herbicides that prevent young crabgrass plants from emerging from of the ground. Sometimes we, falsely, believe that these herbicides keep the seed from germinating.

Rather, the germinating plant encounters the herbicide in the soil as it tries to emerge from the ground and is killed. For this reason we need to have that chemical barrier complete on the top of our soil before the crabgrass plants start to come up. This is actually more beneficial than if these herbicides prevented germination because a germinated seed is no longer viable and can’t cause future problems.

Once the crabgrass gets up, it is too late. With few exceptions, crabgrass preventers have no effect on existing crabgrass plants. We also know that crabgrass preventers have a limited “shelf life” once applied to the soil. Microorganisms and warm temperatures in the soil gradually begin to break crabgrass preventers down soon after they are applied. In fact, many crabgrass preventers are ineffective after about 60 days (although length of control varies among products).

For most of Kansas, crabgrass typically begins to germinate around May 1. Applying crabgrass preventer with a short shelf life too early can cause it to lose some of its potency by the time it is needed. With many materials, a follow-up application may be needed six to eight weeks later. This will depend on which product you use, so check the label to be sure.

Barricade and Dimension are two products that can be used earlier in the year and will give season-long control.

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. The Eastern redbud tree has been a traditional model to use. When the redbuds in our area are approaching full-bloom, apply crabgrass preventer. If you aren’t sure about this, a soil thermometer is the best way to predict crabgrass germination. Crabgrass germinates when soil temperatures reach 55 degrees.

Sy Nyhart is county extension agent for agriculture.