Wide-bladed weedy grasses are an irritation to homeowners who enjoy a uniform, crisp-looking lawn. Crabgrass is a weedy species, whose only redeeming quality is that it’s an annual grass, meaning it must grow from seeds each spring, rather than having a perennial root system that survives winter, as quackgrass does.
Crabgrass sprouts each spring from seed that’s sitting in the soil, triggered to germinate when soil temperature reaches about 50 or 55 degrees Fahrenheit. Luckily there are crabgrass-preventer products that, when applied properly, kill newly sprouting crabgrass seed.
But timing is very important. The product must be applied before crabgrass seeds germinate, or it won’t be effective. It also can’t be applied too early, because the products lose effectiveness over time. How to tell when to apply? Monitor soil temperature, an apply the preventer right before soil reaches the 50 degree threshold. Soil thermometers are available from garden supply catalogs. Or follow the regional soil temperature reporting at sites such as https://ndawn.ndsu.nodak.edu/deep-soil-temperatures.html which gives daily soil temperatures at the 2-inch and 4-inch depths for many reporting cities.
North Dakota State University summarizes crabgrass prevention very well: “To be effective, pre-emergent herbicides must be in place before germination occurs. Preemergence treatments are preferred because they are generally more effective for crabgrass control and less injurious to the turfgrass than postemergence treatments. In general, preemergence herbicides should be applied before soil temperatures reach 50 to 55 degrees Fahrenheit. This will allow the pre-emergent herbicide to form a barrier before the crabgrass seedlings emerge. Preemergence herbicides work by inhibiting the growth of young seedlings. These products do not eliminate established plants and must be applied before germination begins. Applications made very early in the spring have potential to break down before the end of the germination window and are, for the most part, not recommended. This is a particular problem in the three out of 10 years when late-season conditions are conducive for a second germination flush. Delaying preemergence applications in a very cool or dry spring would provide better season-long control because crabgrass germination is also delayed under these situations.”
An important side note is to be certain the weedy grass we’re trying to prevent is crabgrass, and not quackgrass, as crabgrass preventer has no effect on established quackgrass. To distinguish between the two, the following post helps: