Grasses germinate from a single leaf, and the veins on the leaf are parallel. They differ from broadleaf weeds because these germinate with multiple leaves, and the veins on the leaves branch. These and many other differences between grasses and broadleaf weeds make it possible to use selective herbicides on growing broadleaf weeds (postemergence) without hurting the grasses nearby. Preemergence herbicides (that kill germinating seeds) are not selective, but they don't harm grasses already growing.
If you use preemergence in an area, no seeds will grow there. Preemergence herbicides aren't nearly as selective as postemergence herbicides. Preemergence weed killers should be applied to the lawn or garden in fall, winter or very early spring to prevent weeds before the warm weather. Some preemergence herbicides will last many months in the soil, so you can apply them in plenty of time before spring. Some must be reapplied, usually in the summer for fall seeds.
Postemergence herbicides should be used only when the weeds are actively growing, especially during the spring or fall. In the hotter southern states, applying even selective herbicides during the summer might injure nearby grasses. The combination of heat and some of the chemicals found in broadleaf weed killers is not good for the grasses. Broadleaf weed killers designed for use while grasses are dormant are the only ones that should be used while the weather is cold.
The safest way to kill weeds is to put on garden gloves and apply some elbow grease. Many turfgrasses and ornamental grasses are hurt from misuse of broadleaf herbicides. For example, many home owners try to use broadleaf weed killers to rid of weeds in a bed of monkey grass (also known as mondo grass), only to find their bed starting to die because monkey grass is a liriope, susceptible to many broad-leaf herbicides.
Warm-weather grasses tend to go dormant from late fall to early spring. Overseed them with annual rye grass to keep weeds from invading. The rye grass will die back in warm weather and nourish the ground. If it managed to produce and spread seed, you don't need to reapply it next year. Cut or pull broadleaf weeds before they set heads, and keep your grasses irrigated to reduce broadleaf weed problems before they start.
There always seems to be one or two areas of a lawn where the grass won't grow in thick enough to prevent a bed of weeds from forming. Rather than constantly battling this area, convert it into a garden or flower bed. After all, if you have to take care of that particular spot at specific times during the season, your efforts may as well be repaid with blooms, fruits or vegetables.