Controlling weeds while they are small is easier and less expensive than trying to get rid of them once they have matured. One of the best ways to control them is to properly fertilize and water the yard to give lawn grass a competitive edge. In flower beds, maintaining a 3- or 4-inch layer of woodchip mulch is an attractive way to suppress weeds. However, in flower beds and lawns, pre-emergent herbicides should be your first choice to prevent weeds.
Fall and spring temperatures are ideal for weed seed development. Therefore, February and September are excellent times to apply these preventative herbicides in anticipation of weed development. The active ingredients of pre-emergents disrupt the growth of very young cells but have little effect on less vigorous cells.