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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.