A lush, green, weed-free lawn is the dream of many homeowners. But keeping a lawn free of weeds is a process that begins when you're preparing the area to install grass. Grass on a new lawn is established from laying sod--sections of grass already embedded in a layer of soil--or by planting seed. To prevent weed problems on a new lawn, you must remove all vegetation and weeds from the planting area so they cannot grow back before the lawn is established. Begin the preparation process three months before adding the grass.
Water the area that is going to be the new lawn. Watering deeply forces as many weed seeds as possible to sprout. Water again every two or three days for two weeks, if there is no rain.
Spray the entire area with a herbicide containing glyphosate. Glyphosate is the active ingredient in Roundup. It is a herbicide that does not linger in the soil for more than two weeks. Glyphosate kills every plant it touches, so do not spray on a windy day when the spray can land on your other plants. Read the instructions and warnings on the herbicide container label carefully.
Allow the weeds to die, which takes about two weeks. During this two-week period, you may need to spot treat a few areas you missed on the first application. If a weed does not begin to decline within three days after the initial application, apply the weedkiller again. Once all the plant life is dead, use a shovel, hoe and rake to remove all the dead vegetation and rake the area smooth. Disturb the ground to a depth of 2 to 3 inches while you work. Disturbing the soil brings up old weed seeds that can still sprout. Rake the area smooth or grade as desired.
Spray water over the newly graded area, soaking the ground in the process. Continue watering every other day for two weeks. You will see another flush of weed seeds begin to sprout.
Spray the newly sprouted weeds with the glyphosate herbicide. This kills most of the weeds that will grow through your new sod or compete with the newly planted grass seed. After another two weeks, you should be able to plant your grass seed or lay sod. The small weeds that sprouted before the second application of herbicide will not be big enough to affect the grass planting. Keep in mind disturbing the ground again will bring another round of weed seeds to the surface.