"Blades are short, sparsely hairy and taper to a point. It forms dense, unsightly patches that smother the turf."
?definition of crabgrass,
Scotts Information Manual for Lawns.

Of all the weeds, crabgrass is the most notorious - and for a weed, that is a bad thing. In fact, 60 percent of respondents to a survey on Scotts.com indicated they had a problem with crabgrass in their lawns.

Let's face it. Crabgrass is downright ugly, and we don't want it growing in our lawns. But getting rid of it after it has established is more difficult than preventing it from germinating in the first place.

Like most grassy weeds, crabgrass is an annual grass - it lives for one growing season. However, while it is alive crabgrass will produce thousands of seeds that lie dormant over the winter until spring, just waiting to start a new crop. Research has shown that less than 50 percent of the crabgrass seed produced the previous year will germinate the following spring. When you consider that one single crabgrass plant can produce more than a thousand seeds, well that's a lot of crabgrass.

Having A Thick Lawn Helps

Seed germination depends mainly on soil temperature, and this is true of crabgrass seed as well. Researchers studying crabgrass at the University of Maryland and Cornell University have determined that turf density (the size of the gap between individual grass plants) plays an important role in the fluctuation of soil temperature. In other words, the thicker the lawn, the longer it takes to warm up, whereas a thin lawn will warm up much quicker.

The research conducted by these two universities also suggests that under conditions where the turf density is average (between thick and thin) the germination and emergence of crabgrass can occur for 10 to 12 weeks depending on the season.

They suggest that if you apply a pre-emergent crabgrass control too early in the spring the effectiveness of that material may dissipate and allow crabgrass to breakthrough areas of the lawn. On the other hand, if the pre-emergent is applied too late to a thin lawn, the seeds will have already germinated rendering the pre-emergent useless. (the active ingredient in ScottsŪ Turf BuilderŪ with HaltsŪ Crabgrass Preventer and ScottsŪ HaltsŪ Crabgrass Preventer is effective until the time that crabgrass first germinates).

What does this all mean? Simply that the timing of when the pre-emergent is applied is critical to the control of crabgrass in the lawn.