To reduce the amount of weeds growing in a lawn, encourage the grass to grow in thicker so it crowds out undesired plants. Watering often, applying fertilizer and mowing higher helps the grass become more dense. Remove any weeds by hand while they are still young.
To encourage deep roots, give the lawn at least 1 inch of water each week, or when the grass begins to wilt. Many weeds thrive in dry soil and can overtake shallowly rooted grass. Light watering encourages grass to have underdeveloped roots, so wet the soil to about a 5-inch depth.
Fertilize the lawn correctly by carefully following the directions on the package. Too much fertilizer will help weeds grow, while too little leaves the grass weak and undernourished. Warmer southern lawns may need fertilizer up to three times a year, while in the cooler climates, grass only needs fertilizing once or twice a year.
Set lawnmower blades to a height of 2 to 3 inches. Longer grass is better able to produce important nutrients. Some weeds, like crabgrass, need light to germinate, so taller, thicker grass produces more shade. Leave the clippings on the lawn to function as mulch and organic fertilizer.
To weed, dig out undesired plants with a weeding tool or sharp spade. This is easiest if the grass is wet. Carefully remove all of the roots, especially for weeds that have deep taproots, like dandelions.