When it comes to dead grass, there are two strategies for encouraging new growth. First, you can try to resuscitate it and find out if it's really dead or whether it's simply malnourished. If this doesn't work, the grass is likely dead and will need to be re-seeded for continued growth. Keep in mind some annual bluegrass "dies" every summer and rejuvenates naturally come springtime. If this is what you've got, your best bet is to wait for spring or plant a new type of grass that doesn't have an annual hibernation period.
Fertilize the lawn with a mixture of slow- and fast-release nitrogen. Usually, the percentages of slow-release versus fast-release nitrogen are listed on the fertilizer bag. You want between 50 percent and 70 percent fast-release nitrogen for an immediate infusion, with 30 percent to 50 percent slow-release nitrogen for even growth over time.
Water the lawn with about an inch of water every week. This is usually enough to keep soil 6 to 8 inches below the surface moist, encouraging optimal growth. Water in the morning to avoid losing moisture to evaporation and allow the lawn plenty of time to dry. Lawns that stay wet overnight are breeding grounds for pests and disease.
Remove thatch buildup with a rake. Thatch is a gnarled layer of dead grass, roots, stems, leaves and other debris that can choke your lawn. It sits between the soil and the tips of the grass shoots, blocking water absorption.
Mow the lawn, leaving the clippings behind. Clippings contain nutrients such as nitrogen, phosphorus and potassium, all necessary for healthy lawn growth. Once the clippings decompose, those nutrients will return to the soil.
Weed the lawn with a hand-weeding tool or weed popper. It's better to do this by hand than with a chemical weed killer. All-purpose weed killers often douse the lawn with chemicals it doesn't need and, oftentimes, they're useless against crabgrass.
Dig up the dead grass with a shovel.
Mix the existing soil with an organic humus compound to make sure the soil has plenty of nutrients. You can buy bags of organic humus at garden shops and home improvement stores.
Plant new grass seed and top with fertilizer. Don't be afraid to over-seed—the denser the new grass grows, the fewer weeds will be able to take root.
Cover the seeded area with a light layer of peat moss to help seal in moisture.
Water the seeded area as directed above. Don't water unless the top 2 to 3 inches of soil are dry—over-watered lawns don't develop deep root systems, leaving them vulnerable to pests and heat stress.
Remove weeds by hand or with a weed-popping tool as soon as you spot them.
Limit foot or pet traffic over the seeded area until the grass reaches the desired height.