Weed control in the lawn requires good grass cultivation, along with regular fertilization to improve soil quality and ground cover. Mowing the lawn to the proper height according to grass variety also prevents bare patches in the grass, the condition in which weeds like to grow. If weeds are already a problem in the lawn, herbicides are available that kill them after they've matured and prevent new weeds from growing.
Run a mulching mower with a mulching bag attached to remove excess grass clippings and leaves from the lawn before applying any herbicides to the lawn suggests the University of Missouri Extension.
Apply a pre-emergent herbicide to the lawn two weeks before germination of weed seed, recommends University of Missouri Extension. Apply pre-emergent herbicide in the spring for summer annual weeds, and in the fall for winter annuals. Pre-emergent herbicides destroys the seeds' ability to germinate. Apply pre-emergent herbicides in areas where the weed has previously grown.
Spray post-emergent herbicides directly onto the growing weeds when the temperature is between 60 and 80 degrees Fahrenheit, there is no wind and there isn't any rain expected for a 48 hour period, recommends University of Minnesota Extension. Apply herbicides according to the instructions on the label.