A thick, green lawn is not just beautiful to look at, it also signifies that the grass is healthy and well maintained. There are a few reasons why your lawn may be growing sparse. If your lawn started off looking thick in the spring and slowly thinned out over the summer, the problem may be heat and not enough watering. Insects can also cause a lawn to thin out. But your lawn may just be in need of some general maintenance such as fertilization or overseeding.
Figure out what is causing your lawn to thin out. Lawn that is under-watered will be slow to "spring back" when walked upon. You will see temporary foot impressions on the lawn as you walk across it. Under-fertilized turf will be slow to grow when provided with adequate water. You may also see the soil through the turf when looking down at it due to it becoming thin. Insects could be a problem if the sod lifts easily like a carpet.
Mow your lawn at a height of 2.5 inches or higher every four to seven days. Mowing shorter than this will remove too much leaf material, which hinders photosynthesis.
Dethatch your lawn. This will allow water and fertilizer to penetrate the soil more easily. It is recommended to dethatch in early spring or early fall.
Aerate. This will greatly improve the health of your lawn, especially in the high traffic areas. You should be able to rent a core aerator at a hardware store. Aerate in early spring or fall every couple of years.
Fertilize your lawn three times per year with a nitrogen fertilizer. Apply the first round of fertilizer in the spring with your broadcast spreader after your lawn begins growing. Follow the application instructions on the fertilizer bag. Apply the second round of fertilizer in late spring and the third round in the early fall.
Overseed especially sparse areas. Mow the grass down to 1 to 1.5 inches in height. Either rent a slit seeder or stir up the soil a bit with a rake before laying seed. Seed at a rate of 16 to 22 seeds per square inch.
Water your entire lawn at least once a week if you have not had a good rainfall. Grass needs about an inch of water per week.