Preemergence herbicides kill germinating seeds before seedlings break through the soil. Crabgrass is the primary target. The most common preemergence herbicides are synthetic. Natural, nontoxic preemergence herbicides made from corn gluten are safer, though you might have to apply them for several seasons for them to be fully effective. Three quality products are Concern Weed Prevention Plus, WOW! and WeedzSTOP. A drawback to these and most other preemergence herbicides is that they kill germinating lawn seed. Check product labels carefully.