Crabgrass preventers are another name for pre-emergence herbicides 
that prevent crabgrass seeds from developing into mature plants. Many 
people have a somewhat foggy idea of how they work and assume they kill 
the weed seed.

Such is not the case. They do not kill the seed or even 
keep the seed from germinating but rather kill the young plant after it 
germinates.  Therefore, they do not prevent germination but prevent 
emergence.

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. (Dimension and Barricade last longer. See below.)

For most of Kansas, crabgrass typically begins to germinate around 
May 1 or a little later. April 15 is normally a good target date for 
applying preventer because it gives active ingredients time to evenly 
disperse in the soil before crabgrass germination starts.  Even better, 
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. 
Products that do require a follow-up application include pendimethalin 
(Scotts Halts) and Team (Hi-Yield Crabgrass Control).

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 15 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 who have many customers to service in the spring. Though 
Dimension is usually not 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-emergence 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-emergence 
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 some formulations can be applied as 
early as two weeks after the first sign of germination. However, read 
the label of the specific product you wish to use to ensure that this 
use is allowed. Lawns established in the fall can be safely treated with 
Dimension the following spring even if they have not been mowed.

Alicia Boor is an Agriculture and Natural Resources agent in the Cottonwood District (which includes Barton and Ellis counties) for K-State Research and Extension. You can contact her by e-mail at [email protected] or calling 620-793-1910.