Pre-emergent control means applying the chemical before the seeds have germinated. Most crabgrass control products are mixed with fertilizer so that you are able to apply the chemical safely through a broadcast spreader and fertilize at the same time. The chemical works by forming a protective blanket over the soil and preventing seedlings from forming. Many broadleaf weeds and most grasses will also be prevented from germinating with crabgrass control products. The chemical should not be used on newly seeded lawns or if you are seeding the area with wildflower seeds.
Timing is critical in crabgrass control.
If controls are applied too early, crabgrass that germinates late in the season will not be controlled. If controls are applied too late, some crabgrass will already have germinated and will escape the treatment. A new product has recently been introduced, called Dimension, which will lengthen the time period for effective control since it also kills crabgrass in the early seedling stage. Daisy Moore, 1998.