While the best weed control in any lawn is a thick and healthy stand of turf, there are products that can help keep lawns as weed free as possible. Preemergent (crabgrass preventers) are one of those products. With soil temperatures gradually warming in early April here in Central Kansas, homeowners often wonder when to apply preemergent herbicides to the lawn to help reduce summer annual weeds like crabgrass, goosegrass, foxtail, sandbur, and others. Here are some tips that will help.
How it works
Crabgrass preventers are another name for preemergent herbicides that prevent crabgrass seeds from developing into mature plants. Many people have a somewhat foggy idea of how they work. Preemergent herbicides do not keep the seed from germinating but instead kill the young germinating plant. Crabgrass preventers are just that – preventers. With few exceptions they have no effect on existing crabgrass plants, so they must be applied before crabgrass seeds germinate.
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. (Dimension and Barricade last longer.)
When to apply
For most of Kansas, crabgrass typically begins to germinate around May 1. Applying preemergent by or before April 15th is a good target because it gives active ingredients time to evenly disperse in the soil before crabgrass germination starts.
For an even better indicator of when to apply preemergent herbicide, watch for flowers on certain 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 7-8 weeks later unless you are using Dimension or Barricade. Products that do require a follow-up application include pendimethalin (Scotts Halts) and Team (Hi-Yield Crabgrass Control).
What to use
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 earlier than April 15 and still have sufficient residual strength to last the season. Barricade can even be applied in fall and still provide some early crabgrass control the following 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. The common chemical name for Dimension is dithiopyr and Barricade is prodiamine.
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 pre-emergent 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 pre-emergent herbicide is not recommended unless a new lawn has been mowed two to four times. But Dimension is kind to young tall fescue, perennial ryegrass, and Kentucky bluegrass seedlings and some formulations can be applied as early as two weeks after the first sign of germination. Remember, when using any herbicide, read the label and follow label directions carefully.