One concern with preemergence herbicide use is timing of application. In a typical year, the earliest germinating crabgrass may be killed by subsequent frosts. However, in order to be effective, the preemergence herbicide must be applied before the first crabgrass that germinates following the last frost. It must also be activated by either rainwater or irrigation after application. Typically, they are applied two to three weeks earlier, when an indicator plant, such as Forsythia in the northern United States or dogwoods in South, bloom.
If applied too early, these products may dissipate prior to the end of the season, which will result in some late germination of annual grasses
Applying the herbicide as a split-application (half of the product prior to germination followed by the other half about six weeks later) has proven an effective strategy. This depends, however, on what product is being applied and in what part of the country. Always consult the label for specific usage recomendations.
Research conducted at Ohio State shows there is no difference in crabgrass control whether a preemergence herbicide is applied as a single or split application. What's more important is the amount that's applied. Herbicide applied at a higher rate takes longer to break down below what is minimally required to prevent germination. Because of this, we recommend that managers apply the maximum label rate in a single application in Ohio and other midwestern states.
In contrast, split applications are almost the norm and are effective in the southern United States where the crabgrass season is longer. Unfortunately, the dividing line for where split applications become more effective isn't well-understood.
Pre + postemergence herbicides
Perhaps the best option to combat the problem of achieving seasonlong control of crabgrass with a preemergence herbicide application is to use a product that allows for application later in the season. Three herbicides offer both pre and postemergence control of annual weeds. In addition, two products combine a preemergence herbicide with a postemergence herbicide.
Dithiopyr has been on the market for many years and was the first example of a product that offered both pre and postemergence control of crabgrass in cool-season turfgrass. Dithiopyr will provide excellent control of crabgrass preemergence for up to four months. Dithiopyr will also control emerged one to two-leaf crabgrass. This makes dithiopyr an excellent choice for application after it is too late to apply other preemergence herbicides, but also before crabgrass is easily treatable with a postemergence herbicide.