The real key to controlling crabgrass and other grassy weeds is to prevent seed germination early in the spring. Killing those seeds prior to germination will not only help prevent this season's weeds from emerging, but also greatly reduce the number of weed seeds present in the lawn next fall. For best results, then, you'll want to apply pre-emergent and post-emergent crabgrass preventer before last fall's seeds have a chance to sprout.

Crabgrass germinates at different times in different zones owing to variations in climate, but there is a handy way to identify the right time to apply crabgrass preventer. Just look for the bright yellow blooms of the forsythia bush. When these bushes are in full bloom, it's time!