If your lawn is composed of cool-season grasses, it might take a beating from the summer heat. After all, by definition these grasses crave the cooler temperatures of spring and autumn. But there's something you can do to rejuvenate your lawn from any summer damage: overseeding.
Overseeding lawns is just what it sounds like. You sow seed over existing grass to make your lawn thicker and lusher. Therefore, such a sowing makes sense only if the existing grass is healthy enough and abundant enough to be worth keeping. If you currently have only 50% coverage, then you likely should tear up the old lawn and start a new lawn from seed. The best time for overseeding lawns that have cool-season grasses is in September; the second-best time is in March or April.
Preparing for the Job
Because you will be sowing seed over existing grass rather than open soil, take into consideration that the seed will be competing with that grass. The grass can rob the seed of some necessary sunlight to grow.
So to give the seed a better chance, mow the existing grass slightly shorter than you normally would cut it. For example, normally you should cut grass to a height of 3 to 3.5 inches. But to prepare for overseeding, reduce that height to 1.5 to 2 inches. Also, bag or rake up the grass clippings in preparation for overseeding your lawn. This will give the seed a better chance at making contact with the soil.
Another step that's helpful to promote contact between seeds and soil is core aeration, or lawn aeration. This will reduce lawn thatch, which can stand in the way between grass seeds and the soil they would like to call home. Core aerators (or lawn aerators) can be rented from local garden centers.
You also might need to add a layer of topsoil before overseeding the lawn in specific problem areas. For instance, due to shallow tree roots popping up on the lawn, your topsoil layer might be too thin. So spread 1/4 inch of topsoil over such an area (or up to 2 inches in severe cases), and rake it in.
Furthermore, it's ideal to have your soil tested by your local county extension office in advance of overseeding. For all you know, the soil might be lacking in nutrients or have an incorrect soil pH, which can affect your grass growth. As part of the testing, the extension office will tell you what (if anything) you need to do to improve the soil.
Buying the Seed
Most garden centers will carry bags of grass seed suited to your area. The bag of seed you buy should have information on it about recommended seeding rates (for starting new lawns) and overseeding rates. Note the difference: You don't need to spread as much seed when overseeding lawns as when starting new lawns.
Also note that grass seed is often sold in different blends. Thus, even if you buy a bag of grass seed that says Kentucky blue grass (a common cool-season grass), what you are really getting might be a mixture of different grass seeds. This is why it's necessary to check your specific bag for the overseeding rate, rather than general recommendations for a certain type of grass.
Overseeding the Lawn
For the actual operation of overseeding the lawn, use a fertilizer spreader. Despite the name of this gadget, it is also meant for spreading grass seed. Set the spreader to the overseeding rate as recommended on the bag of grass seed. Then, load some seed into the hopper. You are now ready to go.
Simply push the fertilizer spreader, releasing seed as you go, across the whole lawn. Refill the hopper when necessary. Then, right after you are done overseeding, apply a starter fertilizer for best results.
The actual overseeding process should go fairly quickly, depending on your lawn size. It is the prep work and care of the seedlings afterward that take the most time.
Caring for the Seedlings
The grass seed must be watered properly starting right after it's spread for it to germinate. Use just a fine water spray, as you don't want to create a flood. The soil should be kept evenly moist, which might mean multiple waterings per day for several weeks depending on rainfall and your climate. Even after the seeds sprout, continue to keep the soil moist.
While it is OK to mow your lawn as you normally would after overseeding, avoid a lot of walking on the lawn until the new grass seedlings are 3 inches tall. Five weeks after the grass has sprouted, apply a quick-release nitrogen fertilizer at the label's recommended rate; repeat this in another six weeks. At that point, the new grass should be ready to be treated like the existing grass.