Reseeding a lawn improves the thickness of the grass and prevents a warm-season grass lawn from turning brown over winter. Late summer through early fall is the best time for reseeding -- grass seed sprouts well in cool, wet weather -- but you can also reseed in spring. For the best results, control weeds, improve the soil, apply fertilizer and water regularly when reseeding a lawn. If reseeding a warm-season lawn for winter color, you should reseed in fall.

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Removing Weeds

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Weeds should be removed before reseeding a lawn because they compete with grass for light, water and plant nutrients. Pull up or dig up small weeds with a narrow trowel. Apply a herbicide to a lawn that's heavily infested with weeds five to seven days before reseeding. Allow the lawn to grow about 1 to 2 inches longer than normal before applying the herbicide so the weeds develop a large surface area for absorbing herbicide.

Control broad-leaved weeds in most lawn grass types using a product that contains 0.1 percent quinclorac, 0.22 percent mecoprop-p and 0.22 percent dimethylamine salt. Put on long pants, a long-sleeved shirt, safety goggles and gloves, and spray the lawn on a dry, still day when temperatures are above 45 degrees Fahrenheit and below 90 degrees Fahrenheit. Lightly spray the product on the center of each weed.

Preparing a Lawn for Reseeding

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Dethatching, aerating and mowing a lawn prepares it for reseeding. Grass seed sprouts best in crumbly, well-drained soil, but a lawn often becomes covered in thatch and poorly draining over time. Thatch is dried grass clippings, moss and other plant debris.

Rake the lawn, or use a dethatcher, if the thatch layer is thicker than 1/3 inch. A dethatcher rakes thatch out of a lawn with vertical blades. Remove plugs of soil from the lawn with a lawn aerator. This helps break up compacted soil so that air and water can penetrate. Set your lawn mower on its lowest setting when mowing the lawn, so that the blades cut just above the soil surface.

Selecting Seed

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The best grass variety for reseeding a lawn and the sowing rate depend on the reason for reseeding and the growing conditions. For example, the cool-season grass annual ryegrass (Lolium multiflorum), which is hardy in U.S. Department of Agriculture plant hardiness zones 3 through 9, provides winter color in a warm-season lawn. Annual ryegrass is sown at a rate of 7 to 9 pounds per 1,000 square feet.

To reseed a high-traffic lawn, select a tough grass, such as perennial ryegrass (Lolium perenne), which grows in USDA zones 7 through 9. This is sown at a rate of 6 to 8 pounds per 1,000 square feet. Both annual and perennial ryegrass can be invasive.

Tall fescue (Festuca arundinacea, USDA zones 4 through 7) is a cool-season grass for partially shaded or sunny areas, and has a sowing rate of 8 to 10 pounds per 1,000 square feet. This grass provides a green lawn year-round within its hardiness zones and should be sown in fall.

Bermudagrass (Cynodon spp., USDA zones 7 through 10) is a warm-season grass that grows well in sunny spots and at high temperatures. The sowing rate for Bermudagrass is 1 pound per 1,000 square feet.

Another warm-season grass, buffalograss (Buchloe dactyloides) is highly drought-resistant. Sow buffalograss at a rate of 1 1/2 to 2 pounds per 1,000 square feet.

Sowing Seed

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Even sowing is the key to good results when reseeding a lawn. Sowing rates for grass usually state a low and a high amount for 1,000 square feet. Use the lowest sowing rate if the soil is rich and moist and the weather is wet. If the weather is dry, the soil is thin and dry, use the higher sowing rate.

Sow grass seed by scattering it, or use a drop or broadcast spreader. Divide the seed into two portions, and scatter or spread one portion up and down the lawn. Scatter or spread the second portion from side to side across the lawn. On bare patches, sow the grass seed two to three times more thickly.

Fertilizing and Irrigating

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To sprout well and grow thickly, grass seed needs fertilizer and regular watering. Before reseeding, if no soil test report is available, apply 1 pound of nitrogen, 2 pounds of potassium and 2 pounds of phosphorus per 1,000 square feet. After reseeding, work in the fertilizer and cover the grass seed by lightly raking the soil to a depth of 1/4 inch.

Use a soft spray attachment on a garden hose to water the lawn until the soil is moist to a depth of 4 inches. Whenever the soil surface is dry, water the lawn, so that the soil stays moist but is never soggy.

Apply 1/2 to 1 pound of nitrogen per 1,000 square feet to the lawn when the grass seedlings are 2 inches high.