Grass weeds are a difficult plant to get rid of by strictly cultural practices. Herbicides aid in the killing of tough grass when cultural practices fail. Preemergent herbicides kill grass seeds before they have the opportunity to develop, while postemergent herbicides are applied after the grass is sprouting from the grass. Used together, preemergent and postemergent herbicides, with the addition of some cultural practices will kill most grass in the lawn and prepare your lawn for replanting. Check the herbicide label before buying to ensure it will work on the grass you wish to remove.
Apply a preemergent herbicide before the grass has the chance to germinate, at least seven days before the germination period, states the Ohio State University Extension. Late March or early April is best for annual summer grasses, and late summer or early fall for cool season grasses.
Mow the grass as close to the lawn as possible to weaken grass. Healthy grass requires having 1/3 of the blade removed at a time, per the University of Missouri Extension. Cutting it more weakens the blade and prepares it for the postemergent herbicide. Do not water areas of grass that you wish to kill to weaken the grass further.
Apply the postemergent herbicide according to the instructions on the label once the grass has grown back to 2 to 3 inches in height. Apply when the air temperature is between 60 and 80 F and there is no wind or rain expected. Apply again after 20 to 30 days to ensure the grass is killed.