Although some pre-emergent chemicals are sprayed on in liquid form, most homeowners buy the granulated version. Typically, the mixture comes in a bag that includes suggested settings for most drop spreaders or broadcast spreaders. Maddox noted that these chemicals are activated by moisture, so it's best to apply them when the lawn is damp. He also suggests you take two steps that insure you're pre-emergent efforts aren't wasted. First, don't trim the lawn too closely when the grass begins to grow.
Longer grass grows longer roots, and longer, thicker turf does a better job of crowding out weeds.
Second, be ready to eliminate the weeds that survive exposure to the pre-emergent herbicide. Early next month some dandelions and other weeds will pop up. Again, the most popular forms are granulated mixtures you apply with a spreader. Here's one final turf suggestion. If you haven't sharpened your lawn mower blade yet, do it immediately.