Use herbicides as a last resort - when nothing else works on a particular weed or when your lawn is completely overrun. And follow directions carefully. Used incorrectly, herbicides can injure or kill turf and other desirable plants. If you use an herbicide, choose one that's labeled as safe for the type of turf you're growing and effective against the weeds you've got. The label states when and in which conditions to use the product.
Herbicides fall into three major categories.
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. A drawback to these and most other preemergence herbicides is that they kill germinating lawn seed.