Is your lawn composed of cool-season grasses? If so, it may well take a beating from the heat to which it is subjected in summer. After all, by definition, these grasses crave the cooler temperatures of spring and autumn. But there's something you can do to undo summer's assault. It's called "overseeding lawns."
"Overseeding lawns" is just what it sounds like. Namely, you're sowing seed over existing grass, in order to fill in the bare patches. 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 need to tear up the old lawn and start a new lawn from seed.
Note that overseeding lawns with warm-season grasses is also possible, although the rationale behind the operation is different in that case.
Preparations for Overseeding Lawns
Since you will be sowing seed not over an open stretch of soil, but rather over existing grass, take into consideration that the seed will be competing with that existing grass. If nothing else, the latter will rob the seed of some sunlight. To give the seed a better chance, mow the existing grass, cutting it shorter than you normally would. Normally, you should cut grass to a height of 3"-3.5". In this case, reduce that height to 1.5"-2". Also, bag or rake up the clippings in preparation for overseeding lawns, even if normally you don't. You want to give seeds the best chance of making good contact with the soil, and clippings would just get in the way.
Another step to take to promote contact between seeds and soil is core aeration, or "lawn aeration." This step will help reduce lawn thatch, which stands in the way between grass seeds and the soil they'd like to call home. Core aerators (or "lawn aerators") can be rented from local rental centers.
In severe cases, you may need to add a layer of topsoil before overseeding lawns. For instance, due to shallow tree roots popping up on the lawn, your topsoil layer may be too thin. Spread 1/4" of screened topsoil over such an area, and rake it in.
The bag of grass seed that you buy for overseeding lawns should have information on the back concerning recommended seeding rates and overseeding rates. Note the difference: you don't need to spread as much seed when overseeding lawns as when starting new lawns.
Grass seed is often sold in different "blends." Thus, even if you buy a bag of grass seed that says, "Kentucky blue grass" (one of the cool-season grasses), what you're buying may be a mixture of different grass seeds. This is why I am unable to state, point-blank, that the recommended rate for overseeding lawns for Kentucky blue grass is such and such: it would depend on the particular blend of Kentucky blue grass that you bought. For instance, for one blend of Kentucky blue grass, the recommended rate for overseeding lawns might be 1-2 pounds for every 1000 square feet that you need to cover. But for another, the rate might be 2-3 pounds.
For the operation proper of overseeding lawns, use a fertilizer spreader. Set the spreader to the recommended overseeding rate. At the same time, apply a starter fertilizer for better results.
The grass seeds must be watered properly, in order to germinate. Use just a fine spray, as you don't want to create a flood! The soil should be kept evenly moist, which may mean several waterings per day (depending on the weather), for several weeks.
After the grass blades sprout, you'll still need to water a couple of times per day. If you know your schedule won't permit this, the time to look into automatic irrigation systems is before overseeding lawns, not after.
Five weeks after the grass has sprouted, apply a quick-release nitrogen fertilizer at the recommended rate; repeat in another six weeks.
The best time for overseeding lawns that have cool-season grasses is in September; the second-best time for overseeding lawns is in March or April.