Usually the middle to end of May is the prime time for putting down preemergence weed killer for crabgrass. In general, early in the month is appropriate for the southern one-third of Minnesota while later in May is fine for middle to northern sections of the state. But what if I miss the prime window of application, how do I know if it's too late to apply the product? That's a good question. For all practical purposes, once the crabgrass seedlings have emerged from the ground it is too late for a preemergence product to effectively be put down. There is one notable exception and that is the preemergence weed killer known as dithiopyr. It is known in the trade by its product name Dimension. It is a common ingredient in many homeowner formulations.